Adaptations of an Avian 

Supertramp: Distribution, 

Ecology, and Life History of 

the Pearly-Eyed Thrasher 

(Margarops fuscatus)

Wayne J. Arendt

United States 

Department of 

Agriculture

Forest Service

International Institute of 

Tropical Forestry

General Technical 

Report

IITF-GTR-27

August 2006
Author

Wayne J. Arendt is a research wildlife biologist, USDA Forest Service, 

International Institute of Tropical Forestry, Sabana Research Field Station, 

HC 2 Box 6205, Luquillo, PR 00773.

Cover photograph: Male pearly-eyed thrasher (Margarops fuscatus) singing 

from its favorite song perch in an avocado tree. Photo taken by Jerry Bauer 

in May 2006 from the second-story balcony of the author’s home in Pitahaya 

Ward in northeastern Puerto Rico.

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Abstract

Arendt, Wayne J. 2006. Adaptations of an avian supertramp: distribution, 

ecology, and life history of the pearly-eyed thrasher (Margarops fuscatus). 

Gen. Tech. Rep. 27. San Juan, PR: U.S. Department of Agriculture, Forest 

Service, International Institute of Tropical Forestry. 404 p.

The pearly-eyed thrasher is a major nest predator and competitor for nest sites 

of the endangered Puerto Rican Parrot. In all aspects of its distribution and 

ecology, Margarops fuscatus is a classic example of an avian “supertramp.” It is a 

pugnacious, highly vagile species, i.e., a good disperser, with a propensity to fill 

vacant or underexploited niches at all elevations by adopting generalized nesting 

and foraging strategies, as well as a varied diet. Having evolved superior colonizing traits at the expense of strong competitive characters, it is confined to about 

80 generally small, often disturbed, species-poor islands and habitats throughout 

the Caribbean. The pearly-eye’s future depends on its ability to adapt to the everchanging conditions in the region’s natural and anthropogenic environments.

Keywords: Amazona vittata, life history, Margarops fuscatus, pearly-eyed 

thrasher, Puerto Rican parrot, supertramp.
Preface

This volume attempts to explain the current distribution of West Indian birds 

from an ecological perspective rather than from a physical or geological standpoint. Its primary objective is to present to the reader, for the first time, indepth 

information on the life history of a unique and widely distributed Caribbean 

mimid, one which exemplifies many of the ecological and reproductive traits 

recognized more than three decades ago by the human physiologist and avian 

ecologist Dr. Jared M. Diamond of the University of California in Los Angeles.
Contents

1 Chapter 1: Introduction

1 Study Area and Methods

3 Nest-Box Study

6 Natural Habitat Disturbances

9 The Effects of Dipteran Ectoparasitism

18 Supplemental Nest Sites and Thrasher Densities

20 Effects of Artificial Nest Sites (Boxes) on Study Results

22 Lack of Experimentation in This Study

23 Banding

23 Statistical Analyses and Graphics Software Programs

27 Chapter-Specific Information

34 Distribution of West Indian Landbirds

34 Subjectivity of West Indian Avifaunal Analyses

35 Chapter 2: Supertramps in the West Indian Avifauna

35 What Is an Avian Supertramp?

36 Incidence Functions

36 Intertaxonomic Analogues of Avian Tramps

37 Species-Area Curves and Incidence Functions of West Indian Birds

37 Comparison of Pacific and Caribbean Island Landbirds

41 Species-Specific Incidence Functions

42 Competitors Versus Colonizers Among Taxonomic Groups

48 The Pearly-Eyed Thrasher as a Classic Supertramp

50 Tramp Species Research

50 Summary: Supertramps in the West Indian Avifauna

51 Chapter 3: Origins, Classification, and Taxonomy

51 Colonization and Dispersal of West Indian Fauna

52 History of Classification, Affiliations, and Distribution of the Mimidae

53 Origin of the Genus Margarops

55 Taxonomy of the Genus Margarops

55 Pearly-Eyed Thrasher

57 Scaly-Breasted Thrasher

61 Summary: Origins, Classification, and Taxonomy

63 Chapter 4: Distribution and Abundance

63 Current Range

64 Historical Range

66 Range Contractions
67 Range Expansions

67 Bahamian Archipelago

68 Puerto Rico

69 Montserrat

69 Bonaire

73 History of Regional Abundance

73 Virgin Islands

74 Sombrero Island and Anguilla

74 Netherlands Antilles

75 St. Kitts and Nevis

75 Need for More Standardized Census Methods

75 Comparisons of Regional Abundance

75 Mist Netting

78 Variable-Width Line Transects

80 Point Counts

83 Density Compensation in Mona Island Birds

89 Extreme Temporal Fluctuations in Mona Island’s Thrasher Population

89 Seasonal Fluctuations in Mona Island’s Thrasher Population

91 Environmental Factors Influencing Range and Abundance

91 Climate

92 Habitat Destruction

93 Hunting

94 Ecological Correlates of Range and Abundance

94 Evaluation of Ecological Correlates Via Categorical Modeling

95 Island Size

95 Number of Species

98 Geographical Distribution of the Red-Legged Thrush

100 Island Size

101 Number of Species

101 Thrasher-Thrush Distribution Analysis

103 Competition Within West Indian Bird Communities

104 Summary: Distribution and Abundance

107 Chapter 5: Morphology

107 Statistical Analyses

108 Sources of Intraspecific Variation in Avian Body Size

108 Geographic Effects

110 Latitudinal Effects (Museum Specimens)
111 Landmass and Elevational Effects (Field Measurements)

112 Climatic Effects

114 Effects of Dispersal on Local Body Size

114 Effects of Investigator Bias, Genes, Age, Gender, and Season

124 Extent of Regional Sexual Dimorphism

132 Terrestrial Vs. Arboreal Adaptations (Museum Specimens)

134 Sources of Interspecific Variation in Avian Body Size

134 Effects of Competition on Body Size

135 Potential for Intrageneric Competition

136 Combination of Uni- and Multivariate Analyses (Museum Specimens)

139 Combination of Uni- and Multivariate Analyses (Field Measurements)

140 Intergeneric Competition

143 Conclusion

143 Summary: Morphology

145 Chapter 6: Survival and Dispersal

145 Annual Survival Rates (Rain-Forest Population)

146 Effects of a Major Habitat Disturbance on Adult Annual Survival

148 Effects of Dipteran Ectoparasitism on Adult Survival

151 Annual Survival Rates (Dry-Forest Population)

152 Annual Survival of North Temperate Vs. Tropical Birds

152 Pearly-Eyed Thrasher Longevity

152 Rain-Forest Population

153 Dry-Forest Population

154 Pearly-Eyed Thrasher Longevity Compared to Other Landbirds

156 Dispersal

158 Short-Range Natal Dispersal Within the Sierra de Luquillo

164 Factors Affecting Natal Dispersers Within the Sierra de Luquillo

171 Short-Range Natal Dispersal Within Puerto Rico

171 Long-Range Natal Dispersal Among Islands

172 Homing Experiments on Adult Males

174 Conclusions

175 Summary: Survival and Dispersal

177 Chapter 7: Reproduction

177 Return Rates of Resident Breeders

180 Nest-Site Persistence and Mate Fidelity

182 Pair Bonding and Survival Rates

182 Number of Mates
183 Courtship Behavior

184 Cuckoldry and Egg “Dumping”

185 Comparison of Survival, Nest-Site, and Mate Fidelity With Other Species

186 Reproductive Dispersal

187 Males Changing Nest Boxes

190 Females Changing Nest Boxes

190 Prehurricane Disturbance Nest-Box Changes

190 Propensity for Posthurricane Nest-Box Changes

193 Interseasonal Nest-Box Changes

193 Early-Season Final Nestings in Original Boxes

193 Late-Season Final Nestings in Original Boxes

194 Interseasonal Reproductive Success of First Nestings in Postmove Boxes

195 Intraseasonal Nest-Box Changes

195 Intraseasonal Reproductive Success of First Nestings in Postmove Boxes

196 Phenology of Reproduction

196 Length of Breeding Seasons

199 Length of Nonbreeding Seasons

199 Incubation Period in Relation to Adult Body Mass

200 Number and Timing of Clutches and Egg Deposition Dates

209 Age of Female Breeders and the Onset of Egg Laying

210 Reproductive Recycling

210 Comparison of Clutch Types

212 Temporal Aspects of Recycling (Clutch to Clutch)

214 Temporal Aspects of Recycling (Interclutch Recovery Period)

219 Extremities in Interclutch Recovery Periods

221 Females Recycling in Fewer Than Two Weeks

223 Females Recycling in Less Than a Week

223 Annual Reproductive Success and Influential Factors

224 Reproductive Yield

229 Egg Mortality

232 Nestling Mortality and Number of Fledglings

235 Egg Deposition Strategy and Reproductive Response to Hurricane Hugo

238 Lifetime Reproductive Success

239 Comparison of Lifetime Reproductive Success Parameters 

 Among Avian Taxa

240 Lifetime Reproductive Success Patterns Among Known- and 

 Minimum-Aged Pearly-Eyed Thrashers
240 Clutch Size and Number Per Season and Egg Volume

242 Body Size and Age of the Female

242 Total Number of Nestlings and Fledglings Produced

253 Recruitment Into the Breeding Population

253 Factors Influencing the Number and Fitness of Recruits

255 Production of Recruits and Potential Breeders

257 Influence of Nest Boxes on Fledglings and Recruits

259 Seasonal Influences on Fledglings and Recruits

259 Influence of Ectoparasites on Fledglings and Recruits

261 Lifespan as a Predictor of Lifetime Reproductive Success

263 Conclusions

264 Summary: Reproduction

267 Chapter 8: Impact on Other Vertebrates and Epilogue

267 Pugnacity

269 Predation on Adult Birds and Other Vertebrates

271 Nest Predation

274 Potential for Controlling Vertebrate Populations

274 Impact on Forest-Bird Populations in Puerto Rico

281 Comparison of Results With Other Studies

282 Comparison of Wet- and Dry-Forest Frugivore-Nectarivore Populations

284 Impact on Forest-Bird Populations in Montserrat

284 Impact on Other Caribbean Vertebrates

286 Impact on Endemic and Endangered Species

287 Interspecific Competition and the Supertramp Strategy

288 Attributes of Large Body Size in Birds

289 Generalized Food Habits and Diffuse Competition

290 The Pearly-Eye’s Future

291 Summary: Impact on Other Vertebrates and Epilogue

292 Acknowledgments

295 English Equivalents

295 References

358 Appendix 1: Competitive Strategies Classification, Distribution, and 

 Habitats of 224 Species of West Indian Landbirds

379 Appendix 2

385 Appendix 3

396 Appendix 4

403 Appendix 5

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Study Area and Methods

Data presented in this volume were obtained from 1978 to 2005. I assist in 

the efforts to restore the critically endangered Puerto Rican parrot (Amazona 

vittata), now limited to the Sierra de Luquillo Mountains (fig. 1.1), mostly within 

the confines of the Luquillo Experimental Forest (fig. 1.2). The Luquillo Experimental Forest is also known as the Caribbean National Forest, or simply “El 

Yunque,” the Spanish vernacular for a blacksmith’s anvil, which the Sierra de 

Luquillo reportedly resembles. I undertook this long-term study of the pearlyeyed thrasher because it is the Puerto Rican parrot’s principal nest predator and 

competitor for nest sites. The following section describes the thrasher study area 

Figure 1.1—Overview of the Sierra de Luquillo Mountains located in eastern Puerto Rico, an island mostly forested prior to European colonization. This picture is part of a landsat thematic mapper satellite image captured in 1985 showing the Luquillo Experimental Forest and the area to 

its east. (Photo courtesy of Dr. Eileen Helmer and Olga Ramos of the USDA Forest Service, International Institute of Tropical Forestry’s Global 

Imaging System/Geographic Positioning System Laboratory. The satellite image and its full description are available at the following Web site: 

http://luq.lternet.edu/data/spat-asc.html#satellite.)

Chapter 1: Introduction
GENERAL TECHNICAL REPORT IITF-GTR-27

and the methods used to obtain the data. One analysis covers a period of 32 years 

(1973–2005), another through 2004, and many more that include every year over 

a 21-year period from 1979 to 2000 (several figures); in other analyses, however, 

some years are excluded, e.g., 1983, 1984, and 1989 owing to incomplete data sets. 

Likewise, some analyses include shorter periods, e.g., 12 years—1979–91 (thrasher 

survival—a limitation of the model), 18 years—1979–97 (return rates), and 19 

years—1979–98 (various egg-laying and recycling data sets).

Methods used will be summarized chapter by chapter. To ensure consistency 

and standardization in citing references pertaining to Caribbean birds, I followed 

as closely as possible the format used by J.W. Wiley (2000) in his comprehensive 

bibliography of West Indian ornithology.

Figure 1.2—The 28 000-ha Luquillo Experimental Forest (noms. alt. Caribbean National Forest and “El Yunque”) located in northeastern 

Puerto Rico. The Taino Indians, early inhabitants of the area, revered the mountains believing they were the home of the “espiritu benefactor” 

(or benefactor spirit) who would bless them if they paid homage to him, usually through sacrifices. The word Luquillo is believed to be a corruption and contraction of “Yukaju” (the mythological benefactor spirit of the Taino Indians inhabiting Haiti, and the root word from which the 

Yokahú observation tower located at km 9 along highway 191 in the Luquillo Experimental Forest was named). “Yukaju” was used together with 

“Yuku,” resulting in the more simple term “Yukiyu.” The contemporary word “Luquillo” was adopted by the Spanish who thought that Yukiyu 

referred to a crazy Taino Chieftain who incessantly attacked the Spanish inhabitants of the area. “Loco” (or “loquillo”) in Spanish means crazy, 

from which was eventually derived the word “Luquillo,” which in addition to referring to the forest, is also a small nearby coastal city (preceding 

information taken, with permission, from Domínguez-Cristóbal 2000). (Photo courtesy of Gerald P. Bauer.)
Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Before presenting methodology summaries, I will first describe my long-term, 

life-history nest-box study (1979 to present), the software packages used to perform 

numerous parametric and nonparametric statistical tests, and those in the graphic 

display of the results, which include data gathered between 1979 and 2005.

Nest-Box Study

The pearly-eyed thrasher is a nonobligate, secondary cavity nester (Arendt 2004a). 

Although it often constructs its bulky open-cup nest on heavily foliaged tree limbs 

and within bamboo thickets and vine entanglements, it readily takes to existing 

cavities. However, it does not excavate its own cavity. Nor does it lay its clutch of 

one to four pale bluish-green eggs directly on the interior cavity substrate as do 

primary cavity nesters, most of which have unpigmented eggs (Arendt 2004b). 

Instead, the pearly-eye builds a definitive stick nest lined with aerial or, less often, 

terrestrial rootlets within natural cavities or, as in this study, within nest boxes (fig. 

1.3). In December, 1978, within the Luquillo Experimental Forest, I began monitoring 30 to 40 (avg. = 34 per year) modified wood-duck (Aix sponsa) nest boxes 

(fig. 1.4) known as “Kepler Boxes,” first used by C.B. Kepler as part of another 

Figure 1.3—The pearly-eye’s typical three-egg clutch laid in a bulky stick nest lined with aerial rootlets and 

placed within a Kepler box (gray shadow surrounding the nest). Even in the rain forest, not all pearly-eyes 

nested in cavities. Favorite noncavity nest sites included bamboo thickets, vine entanglements, and other 

vegetation associations with dense foliage to ensure that nests were not only hidden but also sheltered from 

the persistent heavy rains.

The pearly-eyed 

thrasher is a 

nonobligate, 

secondary 

cavity nester.
GENERAL TECHNICAL REPORT IITF-GTR-27

study related to the Puerto Rican Parrot. Boxes were numbered to enhance 

quality control of the data. Aboveground box height was irrelevant because 

breeders would readily accept boxes placed as low as 2 m and at any height above 

the forest floor, even in the upper story canopies of emergent trees. Because of rat 

predation, sheet metal was often nailed to the roof and walls of the box to reduce 

the rat’s traction and lower the probability of predation. With the invasion of the 

forest by the Africanized honeybee (Apis mellifera scutellata) in the mid-1990s, 

usurpation of nest boxes by swarming bees increased dramatically (Arendt 2000). 

By using both attractant and repellant baits as well as a viscous glue coating on 

the upper inner surfaces of boxes, the installation of bee traps helped to reduce 

the number and severity of invasions but did not successfully deter the ever more 

frequent swarms. Each box was placed about 0.1 km apart at elevations ranging 

from about 600 to 900 m primarily in palo colorado forest (fig. 1.5; see also Ewel 

and Whitmore 1973, Wadsworth 1951 for a description of the forest, including 

the four major forest types). 

Figure 1.4—Full view of the Kepler nest-box design used in this study. (see “Study Area and Methods” for a 

more detailed description of box placement and maintenance, and Snyder et al. 1987 for box specifications).


Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Figure 1.5—The colorado forest type. Classified as lower montane wet forest and located between 

roughly 600 and 900 m, colorado forest encompasses 3318 ha, or about 30 percent of the entire forest, 

and is the pearly-eyed thrasher’s prime habitat in the Sierra de Luquillo. The large palo colorado tree 

(Cyrilla racemiflora L.) shown in the foreground rarely exceeds heights of 18 m but may reach almost 3 m 

in diameter and can survive more than a thousand years (Weaver 1986). It is a favorite of cavity-nesters 

such as the Puerto Rican parrot (Amazona vittata) and the pearly-eyed thrasher because its heartwood 

decays, forming natural cavities, over which the parrot and thrasher vie for use as nest sites.
GENERAL TECHNICAL REPORT IITF-GTR-27

Box locations have been permanently recorded by using geographic information system (GIS) and global positioning system (GPS) technology (Trimble 

Pro XL®).1 Nest-box data were differentially corrected and plotted by using 

ArcView® software and overlaid on a forest map. 

Nest boxes were monitored every 2 to 4 days throughout the nesting period, 

and daily at critical times, e.g., egg-laying, hatching, and fledging periods. During 

the nonbreeding season, each box was checked every 1 to 2 weeks for signs of 

activity. In several analyses throughout this document, data were collected from 

known-aged pearly-eyed thrashers (individuals that hatched in nest boxes) and 

“minimum-aged” individuals (determined by tallying the number of years after 

banding). For example, an unknown-aged (unbanded) thrasher that was banded 

and then bred in a nest box at the onset of the study (1979) was considered a minimum-aged 1-year-old as it generally takes about 9 months for first-time breeders 

to nest. Each succeeding year, the minimum age of that breeder was incremented 

by 1.

Natural Habitat Disturbances

The effects of Hurricanes Hugo (1989) and Georges (1998) on much of the forest 

and its vegetation will be felt for decades. Fortunately, however, their impacts 

were less severe in the areas harboring thrasher nest-box trees included in this 

study. Most nest-box trees were located in the Icacos Valley (fig. 1.6) along a 

south-facing slope that, in general, did not suffer extensive damage from high 

winds and water-induced damage such as erosion, landslides, and occasional 

uprooting. Only three nest-box trees toppled during Hurricane Hugo, and an 

additional two nest boxes blew down. As a result of a more direct hit and the 

heavier rains associated with Hurricane Georges, six nest-box trees toppled 

owing to high winds (n = four boxes) and landslides (n = two boxes). In addition 

to the 6 boxes downed with their respective trees, 4 more boxes were destroyed as 

a direct result of Hurricane Georges. After each hurricane, boxes were replaced 

either in their previous trees, or as in the instances of felled trees, within 10 m of 

their previous locations.

To study various aspects of the pearly-eye’s life history, my assistants and 

I constructed observation blinds made of easily sewn and portable burlap 

(fig. 1.7), camouflaged canvas, fiberglass, and even palm fronds, which were 

used for the roof and walls. Bundled tightly together, palm fronds not only 

hid the observer from view, but also were adequate in keeping the observer 

1The use of trade or firm names in this publication is for reader information and does not imply 

endorsement by the U.S. Department of Agriculture of any product or service.

The effects of 

Hurricanes Hugo 

and Georges will be 

felt for decades.
Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Figure 1.6—Pearly-eyed thrasher research areas within the Luquillo Experimental Forest. Delineated are the main study 

sites and the dispersion of nest boxes along PR Hwy. 191 in the Icacos Valley, PR auxiliary 9930 (Loop Road), and PR 

auxiliary 930 (Molindero Loop). One box (not shown) was located near Quebrada Juan Diego at an elevation of 460 m 

at km 9.8 along PR Hwy 191. Map courtesy of Carlos Rodríguez of the USDA Forest Service, International Institute of 

Tropical Forestry’s Global Imaging System/Geographic Positioning System Laboratory.
GENERAL TECHNICAL REPORT IITF-GTR-27

(relatively) dry during even heavy downpours. Blinds were placed on the ground 

or in trees at distances of 3 to 30 m from nest boxes to observe thrasher behavior 

and activities such as (a) incubation and brooding schedules, (b) fledging, (c) rates 

of feeding and fecal sac removal, and (d) regurgitation of pellets containing the 

seeds of fruits and bones of animals too large to digest.

Adults were captured in traps attached to nest-box entrances during the 

breeding season, and in mist-nets outside of the reproductive period. Eggs and 

chicks were weighed and measured by using digital and dial calipers accurate to 

0.01 and 0.02 mm, respectively, and 50- to 300-g spring scales with increments of 

Figure 1.7—Nest-monitoring observation blind (or “hide”). In the background, the author has just placed 

a mounted thrasher on the roof of a nest box during the nonbreeding season to determine if the pearly-eye 

is territorial year-round, which it is. Mist and low clouds moving through caused the haze noticeable in the 

background. (December 1979 photo courtesy of Gerald P. Bauer.)


Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

0.5, 1, and 2 g (see also Arendt, 2005). Adults were also measured by using 

the same calipers and 300-g spring scales with 2-g increments (measurement 

techniques followed Baldwin et al. 1931, Pettingill 1970, Pyle et al. 1987).

For almost 10 years, my wife, Angela, and I conducted the bulk of the fieldwork. Then, beginning in 1987, after receiving adequate training in the use of the 

previously described equipment, parrot project volunteers and one to three field 

technicians began assisting in the collection of nest-box data. After 1990, most 

of the field data were collected by Roberto Díaz (1991 to 1997), Morris S. Ford 

(1997 to 2002), and Matthew Anderson (2002 to present). During the 1990s, I 

periodically assisted in collecting the data when circumstances required my 

participation, e.g., posthurricane bird-population and habitat assessments.

The Effects of Dipteran Ectoparasitism

Some of the information in the following paragraphs is taken from Arendt (2000). 

It merits repeating to ensure that the reader has an adequate introduction to 

Philornis ectoparasitism in Puerto Rico (see also Delannoy and Cruz 1991, 

Rivera Irizarry 1990) and within the Sierra de Luquillo because it is such an 

integral part of the forest ecosystem and this research.

The genus Philornis includes more than 50 species of parasitic flies that have 

radiated widely throughout the Neotropics, showing a wide host specificity and 

parasitizing more than 100 species of birds (for examples, see Couri 1999, Dodge 

and Aitken 1968, LaRue 1987, Teixeira 1999). Within the Luquillo Experimental 

Forest, philornid botflies (fig. 1.8) parasitize more than 20 avian species in diverse 

taxa including Falconiformes, Columbiformes, Psittaciformes, Cuculiformes, 

and several species of Passeriformes (Snyder et al. 1987; Pérez-Rivera 1993, in 

litt.; and pers. observ.).

Adult Philornis botflies (Diptera: Muscidae) resemble the common housefly 

(Musca domestica L.) in appearance, and both have a lapping proboscis. Body 

shape and wing venation are typically muscid. Wings are hyaline (transparent 

and colorless). Philornid botflies are identified by the prominent r-m and m-m 

(radial and medial) cross veins in their wings. Adults have black abdomens and 

red heads. Females have a fleshy telescopic ovipositor and average body length 

(9.17 mm) smaller than males (10.65 mm). Females can also be distinguished 

from males by the yellowish pleural (lateral) areas of their legs and their wider 

interorbital spaces, which are golden rather than silver as in males (Arendt 1983, 

LaRue 1987). LaRue (1987) found that adults were mostly crepuscular (active at 

twilight); and, although they extended their proboscises to honey water, adults 

did so more frequently when parrot and thrasher excrement was provided.

Philornis 

ectoparasitism 

is an integral part 

of this research.
10

GENERAL TECHNICAL REPORT IITF-GTR-27

Female flies in the genus 

Philornis may be oviparous 

or viviparous, depending on 

their adaptive reproductive 

strategies (see detailed 

discussion in Couri 1999, 

Skidmore 1985, Teixeira 

1999). Although to date 

in this forest, there is only 

circumstantial evidence, 

e.g., females with fleshy 

telescopic ovipositors, 

complex structural design of 

the egg encasement (fig. 1.9), 

sperm in the spermatheca, 

and chorionated (protective 

membrane) eggs in the 

median oviduct ready to 

be laid (LaRue 1987; and 

pers. obs.), it is believed that 

these botflies lay their eggs 

topically on their hosts. 

To my knowledge (and 

corroborated by J.E. Loye 

1998, pers. comm.), no one 

has yet documented in the literature seeing philornid botfly eggs on nestling birds, 

although I, as well as others, have observed the eggs of blowflies (Calliphoridae) 

laid on moribund thrasher nestlings that exhibit rapidly falling body temperatures 

and soon succumb to the effects of philornid ectoparasitism. The telltale raised 

bumps on the integument of recently hatched nestlings suggest that female botflies 

are depositing eggs topically on the hosts; the eggs most likely hatch quickly 

following deposition, and the larvae burrow into the skin almost immediately 

after hatching. Citing Arendt (1985b), Couri (1999) and Teixeira (1999) concluded 

that because so many larvae infest ventral areas of pearly-eye nestlings, it is 

more probable that the adults lay their eggs in the nest and the larvae then climb 

up onto the host. This hypothesis does not, however, account for the primarily 

dorsal larval implantation sites on young, especially recently hatched, nestlings 

(Arendt 1985a). Moreover, in 1987, from within a tree blind with a thrasher box 

Figure 1.8—Adult Philornis botfly (Diptera: Muscidae). Although 

they resemble the common housefly (Musca domestica), philornid 

botflies can be identified by the prominent r-m and m-m (radial 

and medial) wing cross veins, which are visible in the photograph.
11

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Figure 1.9—Scanning electron microscope image of a philornid botfly egg. Although it remains unknown 

whether or not the philornid flies inhabiting the Sierra de Luquillo are oviparous or viviparous (J.E. Loye 

1998, pers. comm.), the intricate honeycomb-within-honeycomb pattern of the respiratory pores and structural supports shown in this picture, which no doubt greatly strengthens the egg and increases its resistance 

to being crushed, lends support to the notion that the fly has adopted an oviparous deposition strategy. Eggs 

laid topically must be resilient and able to withstand external pressures such as the nestling pressing itself up 

against the wooden wall of the nest box or even brushing up against its siblings. (Photo courtesy of Robin 

Kennedy, University of Missouri-Columbia.)

mounted on one side, and through one-way glass, I observed adult philornid 

females buzzing over nestlings and alighting on their heads and dorsal areas. 

Unfortunately, no female was observed ovipositing, and the blind, box, and glass 

were destroyed soon thereafter in a windstorm, curtailing further observations. 

However, more recently, A.L. Rivera (2004, pers. comm.) and others have 

captured on video philornid flies entering Puerto Rican parrot nest cavities and 

alighting on the heads and dorsal areas of parrot chicks, thus strengthening the 

possibility of a topical (vs. nest) deposition of eggs. Clearly, much more research 

into the ecology of the ectoparasite is needed before definitive statements can be 

made regarding its egg/larvae deposition strategy.

Philornid botfly larvae infesting pearly-eyed thrashers, both nestlings and 

adults, live subcutaneously for about a week, situated in furuncles (cavernous 

lesions with cornified epithelial walls) with their caudal spiracles visible through 

the dermal openings (fig. 1.10). Botfly larvae are maggot-like in general appearance. The anterior end tapers to a rounded head segment, and the much broader 

posterior end is obliquely truncate. Larvae are off-white to dark brown. They 

consist of 12 segments and range in length from about 1 to 20 mm, with a width 
12

GENERAL TECHNICAL REPORT IITF-GTR-27

of 1.3 to 6 mm. Paired anterior 

and posterior respiratory spiracles 

are present on the 2nd and 12th

segments, respectively. Rear-facing and sharply pointed cuticular 

spines cover portions of the 

thoracic and abdominal segments 

and are used to anchor the larvae 

within the furuncle, thus making 

extraction by a preening bird much 

more difficult. The heavily sclerotized cephalopharyngeal skeleton 

of the first instar larvae is probably 

a secondary adaptation for cutting 

through the host’s integument 

(Nielsen 1911, Skidmore 1985). 

The puparial cuticular bands 

and caudal spiracles are the best 

taxonomic characters for the genus 

(Dodge and Aitken 1968).

Larval implantation strategies follow the advancement of the breeding season 

and the ontogeny and pterylae development of the thrasher nestlings. Early in the 

season, larvae usually appear first in dorsal areas of young, unfeathered nestlings 

(fig. 1.11), usually commencing in anterior (coronal and nucal) areas, and then 

along the capital tract beginning in the interscapular region. Larvae infest axillary 

and pelvic areas generally only after the remigial, humeral, and caudal feather 

tracts had begun to develop. Infesting larvae then concentrate ventrolaterally in 

older chicks to take advantage of the nutrient-rich blood supplied to the developing feathers of the ventrolateral and humeral tracts (fig. 1.12). Later in the season, 

however, owing to the exponential growth of the philornid population, there are 

so many flies vying for implantation sites that space is critically limited. As a 

result, early-season site specificity is abandoned and eggs are laid virtually over 

the host’s entire body, commonly observed even in recently hatched nestlings. 

Larval saturation is conducive to the host’s early demise because the period of 

most rapid growth in thrasher nestlings is within the first 1.5 weeks after hatching. 

Even if a heavily infested nestling survives, it is often critically compromised as 

evidenced in the nestling in figure 1.12. Soon after the larva that was implanted 

in its left auditory canal evacuated, the nestling’s external auditory orifice sealed 

Figure 1.10—Philornid botfly larva. Before evacuation 

and pupation, infesting larvae live subcutaneously in 

both nestlings and adults for about a week, situated in 

furuncles (cavernous lesions with cornified epithelial 

walls). Paired posterior respiratory spiracles are visible 

in the photo.

Larval implantation 

follows the advancement of the breeding 

season and the 

ontogeny and pterylae 

development of the 

nestlings.
13

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Figure 1.11—Four-day-old, still unfeathered pearly-eye nestling heavily infested with philornid larvae. 

Late in the breeding season, owing to the exponential growth of the botfly population, there are so many 

female flies vying for implantation sites that space becomes critically limited. As a result, the early-season 

site specificity (dorsal—nucal and coronal—implantations in young nestlings) is augmented by eggs being 

laid virtually over the host’s entire body as shown in this nestling hatched in June 1982.
14

GENERAL TECHNICAL REPORT IITF-GTR-27

Figure 1.12—Ten-day-old pearly-eye nestling infested with philornid larvae. Once the host’s feather 

development was well under way, larval implantation sites tended to shift from dorsal to ventral surfaces, 

especially in the throat and inferior neck areas, and along the ventrolateral feather tracts. Note the large 

larva protruding from the bird’s left auditory meatus.

over completely and it is assumed that this individual was left hearing impaired 

as a result of the ectoparasitism. In addition to its hearing problems, this nestling 

was unable to respire or feed normally owing to the numerous larvae implanted 

near its trachea and esophagus. As the larvae grew, they obstructed the passage 

of air and food.

The shift from dorsal sites in younger nestlings to ventral sites in older 

nestlings has been reported in another passerine, namely the redwing, Turdus 

iliacus (Bakkal 1980). Although nutrient-rich feather tracts and other trunk sites 

are preferred (Arendt 1985b: table 5), botfly maggots sometimes infest the host’s 

legs and feet, including footpads and intertarsal joints (fig. 1.13), especially when 

the more “preferred” corporal and brachial areas are saturated with larvae, 

particularly late in the breeding season. Heavy larval infestations in and around 

intertarsal joints sometimes had a crippling effect on the host. In one instance 
15

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

in 1982, a nestling thrasher having suffered from more than 20 larval implants 

on its legs could not stand firmly, and consequently did not fledge. It remained 

in the nest box 42 days before succumbing to what appeared to be malnutrition. 

The adults eventually stopped feeding the physically impaired chick. A similar 

instance involving a nestling of the critically endangered Puerto Rican parrot 

occurred in 1984. The nestling did not fledge because it was unable to climb up 

and out of its nest cavity owing to 12 philornid larvae infesting both tibiotarsi 

(Snyder et al. 1987).

Dissection of larval-infested thrasher nestlings that succumbed to philornid 

ectoparasitism suggested that the larvae were feeding on proximate subcutaneous 

blood supplies (fig. 1.14). Therefore, the pathenogenesis of the philornid myiasis 

was also investigated as part of this research (Uhazy and Arendt 1986). These 

Figure 1.13—Pearly-eye nestling infested with botfly larvae implanted in appendicular extremes. Late in the 

breeding season when the “preferred” corporal and brachial implantation sites were saturated, infesting 

larvae then became more concentrated along the tibiotarsi, tarsometatarsi, and footpads, where they were 

able to take advantage of the proximity of underlying blood supplies.
16

GENERAL TECHNICAL REPORT IITF-GTR-27

Figure 1.14—Dissected pearly-eye nestling. Thrasher nestlings that succumbed to the effects of philornid 

ectoparasitism were taken to the laboratory and necropsied to further study the effects of avian myiasis. 

Several small, first-instar larvae are shown here implanted along the nutrient-rich ventrolateral feather 

tracts. Note the protruding keel of the sternum, demonstrating the malnourishment of this individual as 

a result of its heavy infestation of botfly larvae.

investigations confirmed that larvae feed on their host’s rich adipose (“fatty”) 

tissues forming the base of the ventrolateral feather tracts, as well as erythrocytes, 

mononuclear cells, necrotic cellular debris, and body fluids (fig. 1.15). At pupation, via modified salivary glands, the larvae secrete a frothy cocoon (fig. 1.16) 

that anchors the puparia to twigs (fig. 1.17) under the female-warmed nest cup, 

which also assures relative safety from the brooding bird (see Couri 1984, 1985, 

1991, 1999; LaRue 1987; Teixeira 1999; Uhazy and Arendt 1986 for a more detailed 

history of the taxonomy and ecology of philornid botflies).

The prevalence (proportion of nestlings infested), incidence (total number 

of infesting larvae), and larval implantation sites were recorded on diagrams of 

thrasher nestlings and in field notebooks, noting the age, size, and feather development of each chick and the age (instar) and size of each larva.

Larvae feed on rich 

adipose tissues as 

well as erythrocytes, 

mononuclear cells, 

necrotic cellular debris, 

and body fluids.
17

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Figure 1.15—Photomicrograph of the cross section of a philornid larva showing the alimentary tract. 

Red blood cells (ghost-like circular to subcircular structures), mononuclear cells, and cellular debris 

are often found within the peritrophic membrane. The black dots represent pycnotic nuclei associated 

with degenerated necrotic cells (standard histological stain Haematoxylon and Eosin, at 610 power 

magnification) (Photo courtesy of Leslie S. Uhazy).

Figure 1.16—Philornid botfly maggot and cocoon. This maggot is secreting its frothy cocoon 

via modified salivary glands. The cocoon will soon adhere to twigs just below the thrasher’s nest 

cup (Arendt 1983). At pupation, the larval hindgut is purged, and the dense, black fluid smells 

strongly of ammonia; the elongated salivary glands lose their white color and thereafter appear 

translucent (LaRue 1987).
18

GENERAL TECHNICAL REPORT IITF-GTR-27

Supplemental Nest Sites and Thrasher Densities

Installation of artificial nest boxes is often used to achieve, maintain, and enhance 

forest-bird biodiversity and density (Blem and Blem 1991, Fleming and Petit 

1986, Twedt and Henne-Kerr 2001). Therefore, nest boxes are an integral part of 

the ever-growing arsenal of tools and techniques used by conservation biologists 

to manage forest-bird populations. Placement of artificial nest boxes in certain 

habitats such as small forest fragments can augment avian populations to desired 

levels, sometimes comparable to those in nearby natural forest stands (Báldi 1991). 

However, at the other end of the conservation spectrum, the occupancy of surplus 

nest sites by “undesirable” or “pest” species such as the house sparrow (Passer 

domesticus) and European starling (Sturnus vulgaris), which often results in the 

complete competitive exclusion of all other potential nesters, poses a potential 

threat to populations of several beneficial (and often already threatened or endangered) cavity-nesting birds, for whom the boxes were originally designed. Because 

the pearly-eye is often considered a “pest” species, a few concerned people have 

asked if supplemental nest boxes might have dangerously elevated this forest’s 

population of the pearly-eyed thrasher, and if the research results from this study 

may be biased as a consequence of supplemental nest sites and an artificially 

elevated population?

Thrasher densities are higher in my study area than in some other sections 

of the forest and within the other three forest types (see chapter 8). However, 

my study area is in prime thrasher habitat, where a high population density is 

expected, regardless of the presence or absence of supplemental nest sites. But, 

to test the relevance of this finding, thrasher populations on other islands were 

surveyed comparing different habitat types (see chapter 4). Both mist-net and 

Figure 1.17—Philornid puparium. Although viscous during construction of the cocoon, the glue-like 

adhesive rapidly dries, cementing the puparium to the nest-cup twigs. Adult botflies emerged from their 

puparia after about 2 weeks (14 to 17 days).
19

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

audio-visual (pointcount) census results revealed that thrasher numbers in the 

Luquillo Experimental Forest, and even in my study area, are not as high as on 

some other islands where no supplemental nest sites have been provided. In fact, 

pearly-eyed thrasher numbers in colorado forest, particularly within my Icacos 

Valley study site, declined significantly during the first 6 years following Hurricanes Hugo (Arendt 2000: fig. 2) and Georges. The same pattern was observed in 

tabonuco forest within the Puerto Rican parrot’s primary nesting valley during 

the first 6 years following Hurricane Georges (fig. 1.18). Furthermore, dipteran 

ectoparasitism is so intense within the study area that, after May of each breeding 

season, often 100 percent of the infested nestlings die before fledging. Thus the 

forest thrasher population is not being saturated with new potential breeders as has 

been suggested by some. As further examples, not a single thrasher-box nestling 

fledged during the 1998 breeding season owing to exceptionally heavy botfly 

infestations that year. Likewise, only four thrasher-box nestlings fledged during 

the 1999 breeding season owing to very limited breeding and heavy philornid 

ectoparasitism resulting from the effects of Hurricane Georges the previous 

year. Lastly, during a three-season span (2000–2002), owing to a series of egg 

experiments conducted by University of California Berkeley collaborators, only 

Figure 1.18—Population dynamics of the pearly-eyed thrasher between 1989 and 2004 in the Icacos Valley study area within its preferred palo 

colorado (Cyrilla racemiflora L.) forest habitat, and in tabonuco (Dacryodes excelsa Vahl.) forest within the main nesting valley of the critically 

endangered Puerto Rican parrot. In both forest types during this 15-year period, pearly-eyes reacted as do supertramps in general by increasing 

in number immediately following two major habitat disturbances, one each in September of 1989 and 1998, but then their numbers dwindled in 

both areas between disturbances. Note also the continued significant downward trend in thrasher populations in both areas during this period.
20

GENERAL TECHNICAL REPORT IITF-GTR-27

45 young pearly-eyes fledged from nest boxes. Thus, over a 5-year period, thrasher 

nest boxes produced only 49 fledglings (about 10 per year); certainly not enough to 

deleteriously impact the forest (see additional effects of ectoparasitism in chapters 

6 and 7).

On a closely related subject, to assuage any anxieties among readers that I am 

significantly increasing the population of parasitic botflies in my study area as a 

consequence of producing more thrasher nestlings each season, I simply point out 

that I have, on numerous occasions over the past two decades, found comparable 

parasite loads on nestling pearly-eyes from natural cavities and open-cup nests in 

areas of the forest far removed from the study area. Moreover, philornid botflies 

are known to parasitize some 100 species of forest birds throughout the Neotropics (LaRue 1987: app. 1), and at least 20 species within the Luquillo Experimental Forest (see chapter 6). Thus, unless a species is already stressed (e.g., 

Delannoy and Cruz 1991), no single species should be significantly affected by 

potentially small increases in the botfly population. Most importantly, however, as 

stated above, nestling mortality resulting from ectoparasitism is high throughout 

each breeding season, often with 100 percent mortality from May onward. Consequently, numerous nestling thrasher hosts die, often well before literally thousands 

of implanted larvae can develop sufficiently to successfully pupate.

A review of the literature on long-term, avian life-history studies (Newton 

1989, Sorace et al. 1993, and others) revealed that some researchers supply hundreds (Báldi and Csörgő 1994, Potti and Montalvo 1991) and even more than a 

thousand artificial nest sites in their study areas (e.g., Sternberg’s [1989] long-term 

study of the pied flycatcher [Ficedula hypoleuca]). I am confident that I have not 

significantly increased the Sierra de Luquillo thrasher or botfly population, or 

biased the research results by monitoring some 30 to 40 nest boxes per year. In 

reality, the botfly parasite and its thrasher host are acting as biological controls 

on each other’s populations within the Luquillo Experimental Forest.

Effects of Artificial Nest Sites (Boxes) on Study Results

Within about the last decade, much concern has arisen about the use of artificial 

nest structures, generally nest boxes, in avian reproductive research. Some, e.g., 

Møller (1989, 1992; Møller et al. 1990), fear that nest-box study results will be 

biased at best, or even erroneous at worst, because birds in nest boxes are safer 

from predators than they would be in natural cavities. Also, researchers who 

routinely clean nest boxes before each subsequent breeding season may be removing large quantities of nest parasites that, if left in situ, could potentially affect 

the health and longevity of the adult birds and lower their reproductive success 

The botfly parasite and 

its thrasher host are 

acting as biological 

controls on each 

other’s populations 

within the Luquillo 

Experimental Forest.
21

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

by causing developmental retardation, illness, or even death of the young. These 

fears are justified in some instances (Kedra et al. 1996, Kruszewicz 1995), especially in areas where nest ectoparasites are abundant and known to significantly 

impact reproductive success (e.g., Merino and Potti 1995, Rendell and Verbeek 

1996). However, many researchers have since shown that these biases and the 

parasites’ impact on their hosts in relation to “clean” (previous nests removed) 

vs. “dirty” (previous nests left intact) nest boxes differ greatly from species 

to species and region to region (e.g., see Christe et al. 1994, Davis et al. 1994, 

Johnson 1996, Pacejka and Thompson 1996, Purcell et al. 1997, Thompson and 

Neill 1991).

In this study of the pearly-eyed thrasher, the previously mentioned biases 

are not relevant. Regarding the reduced predation assumption, over the past 21 

years, the deaths of adult thrashers and reproductive losses within nest boxes 

have been substantial. Loss of breeding adults and nest contents have been 

caused by a diverse group of nest predators, competitors for nest sites, and ectoparasites, e.g., Puerto Rican screech-owls (Megascops nudipes) (fig. 1.19), black 

Figure 1.19—Puerto Rican screech-owl (Megascops nudipes) nesting in a thrasher box. This endemic owl is 

common in the Luquillo Experimental Forest. Owls prospecting for roosts and nest sites often eat the adult 

thrashers (usually females) and their young before usurping the box. Here, an incubating female is startled 

by the author during a routine nest inspection.
22

GENERAL TECHNICAL REPORT IITF-GTR-27

rats (Rattus rattus), other thrashers, philornid botflies (Philornis spp.) and, more 

recently, honey bees Apis mellifera ligustica and A. m. scutellata) (Arendt 2000). 

Moreover, as in other parts of the forest, losses increased following major habitat 

disturbances, namely Hurricane Hugo (1989), a class 4 hurricane (Arendt 2000) 

and Hurricane Georges (1998), a strong class 3 hurricane.

As for the concern about the removal of previous nesting material reducing 

the potential impact of ectoparasitism on subsequent nestings, to minimize as 

much as possible any potential nest-site manipulation biases, I did not clean old 

nest material from boxes, either within, or between, breeding seasons. I allowed 

thrasher nest pairs to make the choice of building over old nests or removing 

previously used nesting materials. Such behavior differed among nest pairs. Many 

males (first) and females (later) simply threw in small sticks and twigs forming 

a platform over old nest materials, upon which they built new nest cups. Others 

hollowed out depressions in the old nest substrates, which often consisted of a 

compacted mud-like surface composed of discarded seeds, frog, bird, and lizard 

skeletons, etc., all glued together by the nestlings’ excrement. The pairs (most 

often the females) would then construct open-cup nests in the excavated hollows. 

If the “ownership” of a box was not certain, prospecting individuals would sneak 

in and remove nest materials. Finally, some pairs actually pitched out old nests 

in their entirety before constructing new platforms and nest cups, especially if 

previous nests were elevated to the point that they were close to the entrance hole. 

I have observed similar behavior by thrasher nest pairs constructing open-cup 

nests and nests within natural cavities in areas of the forest free of nest boxes and 

in other forests and diverse habitats on other islands.

Lack of Experimentation in This Study

I am often asked why I conducted so few experiments while conducting thrasher 

research over so many years, as, after all, nest-box studies are conducive to the 

scientific experimental design with its quantitative and controlled manipulation 

of the variables, which is preferred by most researchers over the more traditional 

“field” studies, which are mostly limited to observations. Early on, I did attempt 

a few experimental studies such as clutch size manipulations and supplemental 

food provisioning to answer some of the more interesting biological and ecological questions about clutch and egg size, parental and offspring fitness, chick 

growth and development, and so on. Unfortunately, however, in this rain-forest 

population of the pearly-eyed thrasher, even over a 21-year period, the numbers 

of unparasitized nests and nestlings were so few that sample sizes in the various 

experimental categories were never large enough for statistical comparisons, thus 
23

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

rendering experiments impractical, at least in the nestling stage. However, some 

experimentation while nests were still in the egg stage was possible. Consequently, 

just prior to the 2000 breeding season, cooperative studies into the theory behind, 

and mechanisms (e.g., microbial pathogens) influencing, the onset of incubation in 

birds in general, and the pearly-eye in particular, began with researchers from the 

University of California at Berkeley (Cook et al. 2003, 2005).

Banding

Numbered U.S. Geological Service Biological Resources Division (formerly 

U.S. Fish and Wildlife Service) metal leg bands and two to five uniquely coded, 

colored leg bands were attached to adult and nestling thrashers for future identification. Over the 21-year period, 1,363 nestlings and 247 adult breeders (9 months 

and older) were banded. Many more nestlings were hatched and handled, but 

not all of them were given metal leg bands because of anticipated prefledging 

mortality resulting from heavy infestations of botfly larvae.

Statistical Analyses and Graphics Software Programs

With the exception of chapter 5 (see below), all statistical analyses were conducted 

by using SigmaStat® version 2 (Fox et al. 1995) for Windows 95® and SYSTAT®

version 9 for Windows 98® (SPSS 1998a, 1998b) statistical software packages for 

personal computers. A 95 percent confidence level (α = 0.05) was maintained in 

all of the analyses (except chapter 5, see below).

During exploratory analyses, all variables used in the statistical analyses were 

checked for normality (Kolmogorov-Smirnov test with Lilliefors’ correction) and 

equal variance (Levene Median Test). Deviations from Gaussian expectations for 

skewness and kurtosis, as well as the adherence to the assumptions underlying 

regression statistics, were examined by using various descriptive statistical 

procedures in the STATS module, and PPLOT (probability plot), STEM (stemand-leaf diagrams), and RESIDS (residuals) commands in SYGRAPH. However, 

throughout this volume, most graphs were made by using SigmaPlot® (SPSS, Inc., 

formerly Jandell Scientific), whereas free-hand line drawings (fig. 4.1 in chapter 4) 

were produced with an ACECAD® model ACECAT II graphics tablet.

Owing to the complex nature of the geographical, environmental, biological, 

and ecological variables and multiple interactions among them and those of the 

pearly-eye’s morphological characters, a special, more indepth statistical analysis was conducted by Dr. Michael R. Willig, a professor and biostatistical and 

environmental consultant at Texas Tech University, Lubbock, Texas. All of his 

statistical analyses were conducted by using SPSS (1990).

Over the 21-year 

period, 1,363 

nestlings and 

247 adult breeders 

were banded.
24

GENERAL TECHNICAL REPORT IITF-GTR-27

Unless otherwise specified, significance was recognized at five levels. Letters and symbols within brackets correspond to those appearing in figure 5.2 in 

chapter 5. “NS” denotes nonsignificance.

1. [NS] P > 0.10

2. [@] 0.10 > P > 0.05

3. [*] 0.05 ≥ P > 0.01

4. [**] 0.01 > P > 0.001

5. [***] 0.001 > P

Program ONEWAY was used to conduct a priori (SNK tests) and a posteriori 

analyses of univariate characters; in these cases, equality of variances was not 

assumed. Pearson Product Moment Correlation (PPMC) was used to compare the 

pearly-eye’s morphological data with geographical information such as latitude 

and landmass. Program MANOVA was used to conduct multivariate analyses 

of variance (MANOVA), whereas Datadesk (Data Description 1995) was used to 

generate Principal Components Analyses (PCA) based on variance-covariance 

matrices.

MANOVA and MANCOVA testing allowed for the determination and 

assessment of group differences in morphology with respect to combinations 

of dependent and independent variables. Through PCA, covarying patterns 

of variation in morphometric data are summarized to produce independent 

composite variables that are interpreted as size (PC1) and shape (PC2) axes 

(Bookstein et al. 1985, Pimentel 1979). The first axis is usually a unipolar size 

axis and is therefore a multivariate measure of size. All original morphological 

variables are positively correlated to the axis, and the correlations are more 

or less of the same magnitude. The first axis usually explains a substantial 

proportion of the variation in the original morphological data set and, 

oftentimes, the first two axes explain most of the variation.

Museum specimens and free-flying thrashers were measured and included 

in the various analyses. However, at no time were field and museum specimen 

data mixed. Some studies have shown that mensural variation in the population 

is often far greater than pre- vs. postmortem differences (Herremans 1985). Yet, 

several other studies (Bjordal 1984, Haftorn 1982, Winker 1993, and their respective references) have shown that the discordance in pre- and postmortem data can 

significantly affect analyses and results.

Four data sets were used in the morphometrics analyses: (1) ELEVCOMP 

(elevational comparison); (2) MUSEUMPET (pearly-eye museum specimens); 

(3) ELYUNQUE (live-caught pearly-eyes from the nest-box study); and (4) 

FIELDPET (all other live-caught pearly-eyes from around the Caribbean). 
25

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

For each of the four data sets, the general analytical strategy was to conduct 

multivariate analyses to assess hypotheses of group differences in morphology with respect to season (see the 4 [“ELEVCOMP,” “MUSEUMPET,” 

“ELYUNQUE,” “FIELDPET”] analytical rationales below), sex (gender), 

SBTH (presence or absence of the scaly-breasted thrasher), or elevation (in 

meters), as appropriate. A suite of univariate tests on analogous treatment 

groups, modified by Bonferroni’s Sequential Adjustment, was used to determine 

the robustness of multivariate conclusions with regard to deviations from 

assumptions. In the case of multivariate significance, significant characters 

after adjustment by Bonferroni’s method (Beal and Khamis 1991) identify the 

univariate characters that strongly contribute to those differences. A priori and 

a posteriori contrasts were conducted in univariate settings as appropriate. When 

covariates were measured as well, corresponding MANCOVAs and ANCOVAs 

were conducted. To visualize differences in multivariate space for each of the 

four data sets, the PCA on log-transformed characters was conduced with groups 

defined by combinations of levels of treatment factors from the multivariate 

analyses. For heuristic purposes, if a treatment was significant in MANOVAs, 

differences between pairs of groups were interpreted as significant if confidence 

ellipses (1 standard error = ± SE) did not overlap.

“ELEVCOMP” (elevational comparison) data set analytical rationale—

Two general approaches were undertaken to assess the effects of elevation (Mona, 

Guánica, El Yunque mid elevation, and El Yunque high elevation) and season 

(March to October vs. November to February) on morphological variation in the 

Pearly-eyed thrasher. In the first approach, an explicitly multivariate procedure 

was performed by using a two-way MANOVA based on culmen length from 

the feathered base (exposed culmen), tarsus, wing, and body mass (in lieu of 

“weight”—see Chardine 1986). Two characters (bill length from nares and tail) 

were deleted from these analyses because they had missing values for individuals 

at elevation = 1 (Mona Island), and would have prevented analyses of morphological differentiation that included that elevation. Because of concerns about 

conforming to the assumptions of MANOVA, and because an analysis based on 

all characters was desirable, a univariate approach was undertaken as well. In the 

second approach, univariate analyses were conducted by using separate two-way 

ANOVAs based on each of the six morphological characters separately, followed 

by Bonferroni’s Sequential Adjustment on the p-values for the treatment groups 

(cells based on combinations of elevation and season). If at least one character 

was significant after Bonferroni’s Sequential Adjustment, then a multivariate 

difference owing to treatment levels could be inferred. Moreover, the contribution 
26

GENERAL TECHNICAL REPORT IITF-GTR-27

of each morphological character to overall differences was deduced based on the 

significance of those characters after adjustment. A priori and a posteriori analyses identified the level at which differences became significant owing to treatment 

effects. The PCA on log-transformed characters was used to graphically visualize 

the extent to which discrete groups of individuals corresponded to categories 

based on season and elevation.

“MUSEUMPET” (pearly-eye museum specimens) analytical rationale—

Both univariate and multivariate approaches were undertaken to assess the effects 

of season (March–June vs. July–October vs. November–February), sex, and 

SBTH on a suite of morphometric characters, i.e., exposed culmen, bill length 

from nares, tarsus, wing, and tail. In both cases, analyses were conducted before

(MANOVA and ANOVAs) and after (MANCOVA and ANCOVAs) controlling 

for the effects of covariates, i.e., SPPNET (mist-netted, potential competitors 

in the immediate area), SPPISLA (island-wide, potential avian competitors), 

elevation (m), latitude (°N), and area (km2). Rationale for use of Bonferroni’s 

Sequential Adjustment of univariate results to corroborate multivariate analyses 

appears above in the “ELEVCOMP analytical rationale” section. The PCA on 

log-transformed characters was used to graphically visualize the extent to which 

discrete groups of individuals corresponded to categories based on season, sex, 

and SBTH.

“ELYUNQUE” (live-caught pearly-eyes from the nest-box study) 

analytical rationale—

After controlling for the effect of age, both univariate and multivariate 

approaches were undertaken to assess the effects of season and sex on 

morphometric variation in a suite of characters (length of exposed culmen, 

bill length from the nares, tarsus, wing, tail, body mass, and primary 9). 

Statistical analyses were performed on log-transformed data so that they 

corresponded to results from the PCA. The PCA was used to graphically 

visualize the extent to which discrete groups of individuals corresponded to 

categories based on season and sex. Rationale for use of Bonferroni’s 

Sequential Adjustment of univariate results to corroborate multivariate 

analyses appears above in the “ELEVCOMP analytical rationale” section.

“FIELDPET” (all other live-caught pearly-eyes from around the Caribbean)

analytical rationale—

Both univariate and multivariate approaches were undertaken to assess the effects 

of season (March–October vs. November–February) and SBTH on morphometric 
27

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

variation in the same set of morphometric characters named in “MUSEUMPET” above. The rationale behind the use of Bonferroni’s Sequential Adjustment 

of univariate results to corroborate multivariate analyses appears under “ELEVCOMP analytical rationale.” The PCA on log-transformed characters was used 

to graphically visualize the extent to which discrete groups of individuals corresponded to categories based on season and SBTH.

Chapter-Specific Information

Chapter 1—

Species-specific information on the distribution and ecology of 224 species of 

landbirds inhabiting islands in the Greater Caribbean Basin rests on field data 

gathered over almost 30 years, augmented by information obtained from hundreds of published references. Nonetheless, pertinent information is incomplete 

for some species and must await the continued efforts of others in the field. 

Recent taxonomical classifications have altered the total number (224) of species 

under discussion but have not affected my major results and conclusions.

Chapter 2—

Background information on the supertramp theory was obtained through interviews with Dr. Jared Diamond and extensive literature reviews, and by literature 

searches of computerized biological databases. I used Citation Index to determine 

who was citing supertramp-related publications written by Jared Diamond, and 

to determine in what context and how broad an audience he had captured.

Species analyses—In my analyses of the West Indian avifauna, I incorporated 

the results of almost 30 years of research in the region (1976–2005). The information I present includes a synthesis of thousands of observations, numerous 

census results (e.g., Arendt 1990, 1995; Arendt et al. 1999; Faaborg and Arendt 

1985; Faaborg et al. 2004), and unpublished data and field notes summarizing the distribution, abundance, and habitats of landbirds on several islands 

throughout the Caribbean. In addition, I spent almost 2 years reviewing standard reference texts (e.g., AOU 1998, Clements 2000), field guides (e.g., Bond 

1979a, Evans 1990, Raffaele 1983, Raffaele et al. 1998), and the many publications on West Indian birds written by museum researchers, field ornithologists, 

regional experts, and early (1700–1800s) historians and naturalists (see Wiley 

2000) to determine the range and habitat(s) of each species. Because most 

reference texts do not include habitat use, I continue to correspond with 

numerous local and regional authorities, many of whom have reviewed 

appendix 1 (see “Acknowledgments”).

Information on the 

distribution and 

ecology of 224 species 

of landbirds inhabiting 

islands in the Greater 

Caribbean Basin rests 

on field data gathered 

over almost 30 years.
28

GENERAL TECHNICAL REPORT IITF-GTR-27

Species-by-island matrix—I constructed a large matrix to delineate the range 

of each of the 224 resident landbirds, and to double-check the concordance 

of the various references, which often were contradictory to one another. My 

determination of the “true” range of each species was established by relying 

on (a) personal experience, (b) the most authoritative and complete reference 

texts, and (c) concurrence by several references for the current range of a given 

species. For example, many standard reference texts use broad statements, e.g., 

“throughout the Lesser Antilles...” Thus, I had to determine the specific and 

total number of islands on which each species is found.

Incidence functions—To construct Diamond’s (1975a) species incidence function curves for each species of Caribbean landbird, I conducted an extensive 

literature search to determine species richness (number of breeding species) for 

as many islands as possible for inclusion in the analyses. I obtained information pertaining to the number of breeding landbirds for 177 islands. To establish that the pearly-eye generally inhabits small islands, I consulted various 

reference texts, atlases, maps, and special reports to obtain the most recent 

estimations of the landmass (in square kilometers) of each of the 177 islands. 

It is noteworthy that the figures for total area (landmass) in appendix 2 differ 

greatly from one reference text to the next. Therefore, when possible, landmass 

figures in appendix 2 were taken from World of Information (1986).

While calculating “J” (the incidence of occurrence), I found that five categories (<12, 13 to 24, 25 to 36, 37 to 48, and >48 breeding species) best fit the data. 

Caribbean supertramps are found exclusively on small islands and, rarely, on 

larger islands in species-poor habitats. D-Tramps, besides colonizing small, 

species-poor islands in addition to large, species-rich islands, often are found 

on 80 to 100 percent of the islands in the various S-classes (determined by the 

number of breeding landbirds per island). C-Tramps are found on 40 to 60 

percent of the islands in the different S-classes. B- and A-Tramps, although they 

do not generally colonize the small, species-poor islands, are often found on 

40 to 60 percent of the larger, species-rich islands. High-S species are generally 

confined to 40 percent or fewer of the larger, species-rich islands.

Chapter 3—

To present a brief history of the origins, classification, and taxonomy of the 

pearly-eye and its congener, I performed numerous literature searches and 

visited several North American museums as well as the British Museum to 

speak with regional experts and to examine and measure specimens, including 

type specimens.
29

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Chapter 4—

The background information for chapter 4 required years to compile. The 

historical accounts of the distribution and abundance of the pearly-eye 

presented in chapter 4 and appendix 3 are the results of numerous, extensive 

literature searches, personal observations, and correspondence with local and 

regional authorities. In gathering information on the distribution and ecology 

of mimids, numerous ornithological journals, reference texts, field guides, and 

life-history accounts were reviewed.

The sections on pearly-eye abundance in the Guánica Commonwealth 

Forest and Biosphere Reserve and abundance and density compensation on 

Mona Island were greatly enhanced by the timely completion of a comprehensive doctoral thesis written by Enrique Nicolás Hernández-Prieto (1993). 

Hernández-Prieto studied Mona’s and Guánica’s avian communities by using 

aural-visual census techniques. He compared avian species diversity and abundance, among many other parameters, on a year-round basis and over several 

years (1986–1990), and clearly demonstrated ecological release and density 

compensation on Mona Island.

Susúa pearly-eyed thrasher data were collected by Mark R. Ryan and 

his students from the University of Missouri-Columbia.

Census techniques—I obtained quantitative, regional distribution and abundance information on the pearly-eye by using a combination of three capturerecapture and standardized aural-visual census methods.

Mist-netting—Standardized mist-netting procedures followed Terborgh and 

Faaborg (1973): 16 contiguous nets, each made of 30- to 36-mm mesh and 

measuring 12 m long by 2.5 m high. Nets were operated from dawn to dusk 

for 3 consecutive days.

Variable-width line transects—I used Emlen’s (1971) method to complement 

mist-net censuses. I established 1-km transects at least 100 m from edge in 

various habitats on several islands. By using this procedure, counts of all 

detections (visual and aural) were made to the limit of detectability. Counts 

were then tallied, and the count for each species was then multiplied by a 

conversion factor (coefficient of detectability), which represents the percentage of the population that is normally detected by these procedures.

Fixed-radius point counts—To compare results presented by others, I 

conducted fixed-radius point counts (Hutto et al. 1986) in Puerto Rico, 

Montserrat (Arendt 1990), and tiny Guana Island in the British Virgin 
30

GENERAL TECHNICAL REPORT IITF-GTR-27

Islands (Arendt 1994). I established points every 100 m for a distance of 1 km 

in various habitats and at different elevations. All points were placed at least 

50 m from edge. At each point, all detections (visual and aural) of birds ≤25 m 

were recorded. Difference in abundance results were tested statistically as 

recommended by Hutto et al. (1986) by using a Kruskal-Wallis one-way ANOVA 

on ranks procedure (e.g., chapter 4, table 4.3). In chapter 8, the abundance of 

six species of frugivorous species inhabiting three forest types (plantation, 

colorado, and cloud forest) was compared by using the mean number of 

detections per each of thirty 25-m-radius point counts per forest type. In total, 

396 point counts (132 monthly counts × 3 forest types) were conducted between 

January 1990 and December 2000—4 months (September through December 

1989) following Hurricane Hugo were excluded to achieve a balanced design 

(12 months in each of 11 years). For comparisons of monthly averages between 

two paired years resulting in a significant difference between their medians, 

a Bonferroni all pair-wise multiple-comparison test was performed. Not only 

does the Bonferroni test for significant differences between all paired-year 

combinations, it also controls for the likelihood of committing Type I errors. 

That is, it minimizes the possibility of falsely declaring a significant difference 

between paired years simply by chance as a result of conducting several 

statistical comparisons (Rice 1989). Each Bonferroni test included 55 possible 

paired-year comparisons (1990 vs. 1991, 1990 vs. 1992...1999 vs. 2000).

Distance sampling (Buckland et al. 1993) was used to calculate the Centre 

Hills’ (Montserrat) pearly-eyed thrasher population estimate reported in 

Arendt et al. (1999).

Categorical modeling—Categorical modeling was used to evaluate the ecological correlates related to the pearly-eye’s distribution, abundance, and potential 

avian competitors. Categorical data modeling (Agresti 1990) is a statistical technique for fitting generalized linear models to categorical data, or data consisting of counts of observations classified according to one or more categories, 

e.g., contingency tables. The procedure involves fitting linear models to sample 

frequencies, and may be thought of as the analog of analysis-of-variance techniques for categorical dependent (response) variables.

Chapter 5—

For the analyses of the pearly-eye’s nine morphological characters treated in 

chapter 5, thousands of measurements were taken by me, John Faaborg, and 

our associates on live birds from 34 islands from around the Caribbean (Arendt 

et al. 2004; see app. 2 for specific islands visited). In addition, I also made about 
31

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

4,000 measurements on 584 specimens of pearly-eyes collected on some 40 islands 

and deposited in five North American museums (American and National Natural 

History Museums, Museum of Comparative Zoology at Harvard University, 

Natural Science Museum at Louisiana State University, including the Schwartz 

collection, and the Chicago Field Museum) and the British Museum at Tring. 

Owing to the complexity of the morphological data, a special, indepth statistical 

analysis was conducted by Dr. Michael R. Willig, a professor and biostatistical 

and environmental consultant at Texas Tech University, Lubbock, Texas. I extend 

to him my sincerest gratitude and indebtedness.

Measurement techniques follow those cited above as well as those presented 

in the North American Bird Banding Manual II prepared jointly by the U.S. 

Fish and Wildlife Service and the Canadian Wildlife Service (Canadian Wildlife 

Service 1991).

Chapter 6—

Jolly-Seber Capture-Recapture Models (described below; see also Jolly [1965] and 

Seber [1982]) were used by Kate Dugger (Oregon State University, Corvallis) to 

analyze pearly-eyed thrasher survival data. The survival data used in the JollySeber models include resident breeding adults (9 months or older) at thrasher 

nest boxes, captured between 1979 and 1991, and adult individuals of both sexes 

captured in mist nets periodically throughout their lives, but not included in 

the nest-box study. Dugger then compared survival results from the rain-forest 

thrasher population to those of a dry-forest thrasher population in southwestern 

Puerto Rico (see Faaborg and Arendt 1995).

Jolly-Seber capture-recapture models—The program JOLLY (ver. 01/24/91) offers 

five capture-recapture models for estimating rates of avian survival, each with its 

own unique set of parameters and underlying assumptions. All five models were 

explored to evaluate the “goodness of fit” of each model to the thrasher data 

(Pollock et al. 1985). The best fit was generated by model A, the standard JollySeber model for open populations assuming births, deaths, emigration, and 

immigration in the sampled population. The close fit of model A to the thrasher 

survival data is evidenced by very small coefficients of variation (standard error 

divided by the square root of the mean) and the standard errors of the annual 

survival estimates. Both parameters suggest that these data meet all of the 

underlying assumptions of model A.

Four thousand 

measurements 

were made on 

584 specimens of 

pearly-eyes collected 

on some 40 islands.
32

GENERAL TECHNICAL REPORT IITF-GTR-27

Nest-box observations—Once nestlings had fledged, nest boxes were visited 

on alternate days to monitor subsequent nestings. During such visits, it was 

possible to determine the location of postfledged young by listening for their 

food-begging calls, and to note the number of days since fledging, their distance 

from the nest box, and other site-related information, including behavior. 

Within the first 17 years of the study, 648 observations (about 50 per year) were 

made (some from observation blinds) involving recently-fledged young in the 

vicinity of their nest boxes. Length of parental care for a brood once fledged was 

determined, especially in relation to subsequent nesting efforts by the adults.

Biotelemetry studies—Over the course of the study, 24 fledglings were radio 

tagged: 5 in 1982, 11 in 1987, 6 in 1990, and 2 in 2000. Nestlings were fitted with 

low-drain, one-stage transmitters weighing 4 to 6 g. All but the 2000 season 

transmitters, which were collar mounts, were “back-pack” mounts (fig. 1.20). 

Transmitters plus harnesses averaged 5 percent (SE = 0.34; range: 3.3—7.7 

percent) of the fledgling’s body mass (avg. = 104.4 g; SE = 2.88; range: 86.4—

118 g). In 1987, an interscapular glue-mount technique was tested, but adults 

removed the transmitters and carried them 20 to 30 m from the nest box 

before discarding them.

Figure 1.20—A radioed pearly-eye nestling showing the trailing antenna and harness-mount transmitter 

design. Dummy transmitters were placed on nestlings about a week before fledging so the young could 

habituate to them, and also because adults occasionally removed both dummy and actual transmitters and 

carried them as far as 30 m from the nest boxes before discarding them. Although the transmitter shown 

here is still obviously burdensome, this nestling will reach its asymptotic body mass of about 100 g before 

fledging, at which time the transmitter, which weighs less than 5 g will be at the recommended ≤5 percent 

of the thrasher’s body mass.
33

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Radio-tagged fledglings were tracked by using hand-held receiving systems 

and folding 3-element Yagi® antennas (from Wildlife Materials, Inc.). Encounters 

of radio-tagged young were recorded in field notebooks and on forest maps. An 

encounter consisted of either visual contact or electronic location (formation of 

minimum-area convex polygons) of radio-tagged fledglings via triangulation 

(for descriptions and evaluations of this technique, see Hupp and Ratti 1983, 

Kenwood 1987, Lee et al. 1985). Thrasher fledglings were tracked by following 

them on foot or by monitoring them from elevated sites, which included towers 

and vantage points on ridgetops throughout the forest (see Kenwood 1987 for 

further information on the telemetry techniques used in this study).

Recaptures and sightings of banded thrasher dispersers—To supplement the 

observational and biotelemetry studies, various analyses were performed 

on the dispersal data obtained from recaptures of banded pearly-eyes and 

sightings of banded thrasher dispersers.

Chapter 7—

Most of the data collected for the various topics covered in chapter 7 (return rates, 

site and mate tenacity, and numerous reproductive parameters) were collected as 

part of the nest-box study outlined above, and occasionally augmented by supplemental data from other areas of the forest and other islands. Under the discussion 

of the various egg fates, undoubtedly some addled (designated “infertile”) eggs 

contained embryos too small for detection without a microscope. Most embryos 

die at early stages of development (Beissinger et al. 2005).

Data analyses and their graphic presentations (with the exception of two 

graphs, explained below) are straightforward and self-explanatory. For example, 

I compared adult breeder return rates by using the parametric Student’s t-test 

when the assumptions of normality and equal variance were met. Otherwise, 

I used the nonparametric Mann-Whitney rank sum test (M-W R S). The two 

exceptions involve figures 7.4 and 7.6 (both include dates of egg deposition). 

Data were analyzed by using Julian dates (based on a 365-day calendar) for 

egg deposition, and then converted to the more conventional 12 months and 

28 to 31 days, which are depicted on the ordinates in the two graphs.

Chapter 8—

The only methods used in chapter 8, other than literature searches and reliance 

on personal experience and fieldwork, involved the previously described audiovisual techniques used in conducting monthly point counts in three forest types 

within the Luquillo Experimental Forest.
34

GENERAL TECHNICAL REPORT IITF-GTR-27

Distribution of West Indian Landbirds

For centuries, ornithologists and ecologists have been fascinated with the distribution of landbirds among the literally thousands of islands and cays that make up 

the West Indian avifaunal region (boundary delineated by Bond 1934). How does 

one go about studying such an inviting, yet so complex and multifaceted, subject? 

First, we must identify the natural and anthropic mechanisms that influence 

insular bird distributions.

The present-day distribution of West Indian birds is the result of regional 

geomorphic plate tectonics, climate, natural and human-induced vegetational 

changes, in addition to all of the consequential selective pressures, physiological 

changes, and mutations within avian taxa that have taken place over several millennia as a result of these environmental and ecological influences. The processes 

that govern these events and organismic changes are dynamic by nature. Thus, 

landbird distributions have varied greatly over time and will continue to change at 

an even more accelerated pace in the future, commensurate with regional development, the ever-changing global environment, and the dynamic state of the diverse 

and unique ecosystems found among Caribbean islands.

In addition to the ultimate mechanisms shaping West Indian bird distributions, at least some of the contemporary distributions can be attributed to the 

varied, and often distinctive, life-history strategies previously recognized by 

Diamond (1974).

Subjectivity of West Indian Avifaunal Analyses

By design, the analysis of the West Indian avifauna following Diamond’s classification scheme is subjective. Thus, the reader may question the placement of some 

of the species into their designated categories. Although someone may rightfully 

reassign some species to other categories, the overall results and conclusions will 

be little affected. Moreover, although I believe that some additional species may 

be considered supertramps, e.g., thick-billed vireo (Vireo crassirostris) and Caribbean elaenia (Elaenia martinica), their limited distributions and other speciesspecific characteristics suggest to me that they do not fit the supertramp mold 

as well as they might and, therefore, they were placed in other more appropriate categories. Even if more Caribbean supertramps are designated, it is highly 

unlikely that they will compete with the pearly-eyed thrasher (Margarops fuscatus) 

for the title of being the region’s premier example.

The present-day 

distribution of West 

Indian birds is the 

result of regional 

geomorphic plate 

tectonics, climate, 

natural and humaninduced vegetational 

changes.
35

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

What Is an Avian Supertramp?

After studying the distributional patterns of South Pacific birds (Diamond 

1970a, 1970b; Diamond 1972), Jared M. Diamond (1974) identified two distinct 

and mutually exclusive competitive strategies in insular landbirds: colonizers and 

competitors. He noted that certain species, which he coined “tramp species,” 

are exceptional colonizers but poor competitors. The epitome of tramp species 

is the supertramp, i.e., a species that has developed superior colonizing traits at 

the expense of competitive characters. At the other extreme is what Diamond 

termed a high-S species, i.e., a species that has evolved strong competitive traits 

to establish itself and maintain its place in species-rich avian communities. 

Good competitors are generally K-selected,1 sedentary, habitat specialists 

with limited geographic and ecological ranges. They are typically endemic 

and confined to larger, species-rich islands. In contrast, superior colonizers 

are generally r-selected2 species that have evolved good dispersal abilities and 

a high reproductive potential featuring rapid and extended breeding and large 

clutch sizes (for further discussion, see Horn and Rubenstein 1986). Moreover, 

superior colonizers have the ability to fill vacant or underexploited niches 

(MacArthur 1968) and are capable of undergoing marked shifts in altitudinal 

range, habitat type, diet, foraging techniques, and morphology (Baldwin 1953; 

Diamond 1970a, 1978, 1979; Diamond and Case 1986; Diamond and Marshall 

1977a, 1977b; Diamond et al. 1989; Grant 1972a, 1986; Keast 1970; MacArthur 

et al. 1972; Terborgh et al. 1978; Traylor 1950; Voous 1955). This ecological and 

morphological plasticity permits successful establishment and rapid population 

growth by colonizers in areas less suitable, and thus more sparsely populated 

by the more competitive species already established on an island (Abbott 

1974a, 1981; Diamond 1970b). Having evolved a superior colonizing ability 

at the expense of a strong competitive ability, however, tramps are forced out 

of species-rich communities by more specialized K-selected competitors and 

diffuse (interspecific) competition (Schoener 1982). As a result, the extreme 

supertramps become vagile (highly mobile) species that are typically confined 

to small, remote, or recently disturbed islands or habitats with few competing 

species (Diamond 1974, 1975a, 1982; Diamond and Marshall 1977a, 1977b).

1K-selected species are those with a superior competitive ability in stable predictable environments, in which rapid population growth is unimportant as the population is maintained at or 

near the carrying capacity of the habitat (Lincoln et al. 1983).

2The r-selected species are characteristic of variable or unpredictable environments; typically 

show rapid development, high innate capacity for increase (r), early reproduction, and small 

body size (Lincoln et al. 1983).

Chapter 2: Supertramps in the 

West Indian Avifauna

“Tramp species” are 

exceptional colonizers 

but poor competitors. 

The epitome of 

tramp species is 

the supertramp, i.e., 

a species that has 

developed superior 

colonizing traits 

at the expense 

of competitive 

characters.
36

GENERAL TECHNICAL REPORT IITF-GTR-27

Incidence Functions

To further quantify his theory of mutually exclusive competitive strategies in 

insular birds, while studying bird distribution in the Bismarck Archipelago in 

New Guinea, Diamond (1975a) noted differences in how the incidence of occurrence (J) of a particular species on an island was related to the number of breeding 

landbird species (S) on that island. He defined five species types based on their 

life-history traits, dispersal capabilities, and reproductive potentials. He placed 

each species in one of six distinct categories according to the shape of its incidence function: competitive high-S species and five categories of more generalist, 

K- and r-selected tramp species (A-tramps, B-tramps, C-tramps, D-tramps, and 

supertramps). A-tramps are most similar to high-S species, and thus tend to be 

K-selected specialists confined to very few islands, whereas D-tramps are most 

akin to supertramps, the champion r-selected opportunists.

Intertaxonomic Analogues of Avian Tramps

The discovery of tramp species among many nonavian taxa adds credence to 

Diamond’s original concept. Tramp species have been described among terrestrial 

arthropods (Green 1979), land snails (Bauer and Bengtsson 1987), fresh-water 

invertebrates (Ball and Glucksman 1978), marine invertebrates (Green 1979), and 

zooplankton (Hayward and McGowan 1979). However, in communities of sessile 

marine organisms, especially where predator-prey relations appear to be more 

important than interspecific competition in determining colonization success 

and community structure, the tramp concept is less applicable (Keough 1984). 

Tramp species have been identified also in reptiles (Case et al. 1979) and mammals 

(Glazier 1980). All of these studies show that the traits of a tramp species include 

(1) superior dispersal ability, (2) high degree of plasticity and ability to broaden 

their ecological niches, (3) regional abundance, (4) competitive exclusion by 

superior resource exploiters, (5) the ability to inhabit marginal habitats, and 

(6) high reproductive potential.

The concept of tramp species has even been applied to plants. K-selected, 

late-successional plants are analogous to Diamond’s competitive high-S species. 

Ruderals (“weedy” species that are often r-selected exotics confined to marginal 

habitats) are analogous to Diamond’s supertramps. Fugitive species (often native 

pioneer species with life-history traits and dispersal abilities intermediate between 

those of high-S and supertramp species) were considered analogous to Diamond’s 

A- through D-tramp species (Platt 1975). From a theoretical perspective, three 

primary reproductive strategies in plants are equated to the r- and K-selection 

continuum presented by MacArthur and Wilson (1967) and expanded by Pianka 
37

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

(1970). C (competition)-selected plants have evolved to be highly competitive in 

productive, relatively undisturbed areas. S (stress)-selected plants, although having 

reduced vegetative and reproductive vigor, have evolved to endure continuously 

unproductive conditions arising from environmental stress, severe resource depletion by the vegetation, or the combined effect of the two. R (Ruderal)-selected 

plants are generally short-lived, prolific species that have evolved in severely 

disturbed but potentially productive environments (Grime 1977).

Species-Area Curves and Incidence Functions of 

West Indian Birds

A linear logarithmic relation exists between the numbers of most species of flora 

and fauna and island size (area). In general, island species’ numbers represent an 

equilibrium between extinction and immigration, larger islands reaching equilibrium at more species because of larger populations, lower extinction rates, and 

greater habitat diversity (MacArthur and Wilson 1967, Schoener 1976). West Indian 

islands and their respective avifaunal populations (Slud 1976) are exemplary of this 

well-known species-area relationship (fig. 2.1). However, a recent study (Ricklefs 

and Lovette 1999) showed that bird species richness in the Lesser Antilles responds 

independently to both habitat diversity and area. These authors concluded that the 

species-area relationship is most likely generated by island-size-dependent extinction. Because many West Indian islands are small, the number of breeding landbird 

species on most islands is consequentially limited (fig. 2.2). Only on the larger 

islands of the Greater Antilles are there more than 50 species of resident landbirds.

Comparison of Pacific and Caribbean Island Landbirds

To illustrate the use of incidence functions in classifying island birds in terms of 

their competitive strategies, and to compare Pacific birds with Caribbean birds in 

terms of Diamond’s classification scheme, I offer a classification of West Indian 

landbirds (Falconiformes, and Columbiformes through Passeriformes) (app. 1). 

Following the American Ornithologists’ Union Checklist (AOU 1998), I categorized 

the region’s 2243 landbird species into seven categories: Diamond’s original six, 

plus an additional category of anomalous (introduced or aberrant) species, based 

on their (a) incidence of occurrence on islands with varying numbers of breeding 

species, (b) degree of endemism, (c) dispersal capabilities, and (d) habitat use 

(table 2.1 and app. 1). 

3Although the total has changed following recent taxonomical revisions, the overall results and 

conclusions from the original analyses have not been significantly affected. Therefore, some 

genera and species changes have not been included in the text, tables, and figures.
38

GENERAL TECHNICAL REPORT IITF-GTR-27

Figure 2.1—Species-area curve for West Indian landbirds. Note that several Bahamian islands fall well below the curve, most likely because 

they, like all of the islands in the Bahamian Archipelago, are very flat, with few habitat types (discussed in chapter 4). Island codes: ac (Acklins) 

ad (Andros) ag (Anguilla) am (Big Ambergris Cay) an (Anegada) at (Antigua) ba (Barbados) bb (Buck-BVI*) bc (Booby Cay) be (Beef) bf 

(Big Flat Cay) bh (Big Hans-Lollick) bi (Beata Island) bk (Buck-US) bl (Bellamy Cay) bm (Biminis) bq (Bequia) br (Bird Rock) bs (Big Sand 

Cay) bu (Barbuda) bv (Bovoni Cay) by (Bay Cay) ca (Catalina) cb (Cockroach-BVI) ce (Culebra) cg (Congo Cay) ci (Crooked) ck (CockroachUSVI) cl (Castle) cm (Cayman Brac) cn (Cinnamon Cay) co (Cocoloba Cay) cp (Cooper) cr (Carriacou) cs (Cay Sal) csc (Cas Cay) ct (Cat) ctc 

(Cotton Cay) cu (Cuba) cv (Carval) cy (Cayemites) dc (Dutch Cap) de (la Desirade) dg (Dog) dl (Dellis Cay) do (Dominica) ds (Desecheo) dt 

(Dead Chest) ea (East Cay) ec (East Caicos) ed (East Seal Dog) el (Eleuthera) es (East Six Hill) eu (Eustatia) fc (French Cap) fg (Ft. George) 

fi (Fish Cay) fo (Fortune) fr (French Cay) ga (Great Abaco) gb (Grand Bahama) gbv (Green Cay-BVI) gc (Guana Cay) gce (Great Camanoe) 

gcy (Grand Cayman) gd (George Dog) ge (Great Exuma) gg (Great Dog) gi (Great Inagua) gj (Great St. James) gn (Ginger) go (Gonâve) gp 

(Guadeloupe) gr (Grenada) gs (Grass Cay) gt (Grand Turk) gto (Great Tobago) gu (Guana) gus (Green Cay (USVI) gy (Gibbs Cay) ha (Hassel) 

hc (Henley Cay) hi (Hispaniola) hr (Harbour) ib (Inner Brass Cay) is (Îslès les Saintes) iv (Île-à-vache) ja (Jamaica) ju (Juventud) jv (Jost Van 

Dyke) kc (Kalkun Cay) la (Little Abaco) lam (Little Ambergris Cay) lc (Little Cayman) le (Little Exuma) lg (Long) lh (Little Hans-Lollick) 

li (Little Inagua) lj (Little Jost Van) lm (Little Camanoe) lt (Long Cay-T) lv (Lovango Cay) ly (Long Cay-C) mc (Middle Caicos) mg (Marie 

Galante) mn (Mingo Cay) mo (Mona) mq (Martinique) mr (Marina Cay) ms (Mosquito) mt (Montserrat) my (Mayaguana) nc (North Caicos) 

nk (Necker) no (Norman) np (New Providence) nv (Nevis) ob (Outer Brass Cay) pa (Parrot Cay) pc (Pear Cay) pd (Providencia) pe (Peter) pi 

(Pine Cay) pk (Perkins Cay) pl (Plana Cay) pn (Penniston Cay) pp (Prickly Pear) pr (Puerto Rico) pt (Patricia Cay) pv (Providenciales) pw 

(Plana Cay West) ra (Ragged Islands) rg (Ramgoat Cay) ro (Rotto Cay) ru (Rum Cay) sa (Saba Cay) sav (Savanna) sb (St. Barthelemy) sd (San 

Andrés) se (St. Eustatius) si (South Caicos) sj (St. John) sk (St. Kitts) sl (St. Lucia) slb (Salt Cay-BVI) slt (Salt Cay-T) slu (Salt Cay-USVI) sm 

(St. Martin) smc (Samana Cay) sn (Sandy Cay) so (Saona) sp (Sheep Cay) sr (Shark Rock) ss (San Salvador) st (St. Thomas) ste (Steven Cay) stu 

(Stubbs Cay) su (Scrub) sv (St. Vincent) sw (Swan) sx (St. Croix) sy (Saba Cay) th (Thatch Cay) to (Tortola) tr (Trunk Cay) tt (Tortue) tu (Turtledove Cay) un (Union) vg (Virgin Gorda) vq (Vieques) wa (Water Cay-C) wc (West Caicos) wd (West Dog) we (West Cay (USVI) wh (Whistling 

Cay) ws (West Seal Dog) wu (Water Cay-USVI) (*See app. 2 for Island political affiliates).
39

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Although West Indian birds reflect incidences of occurrence and distributional patterns similar to those in the Bismarck Archipelago (table 2.1), they show 

two striking differences at both extremes of the classification scheme. About 

one-third (37 percent) of the birds in the Bismarck Archipelago are high-S species 

(fig. 2.3), whereas more than half (59 percent) of the West Indian birds are high-S 

species, possibly because of the two archipelagoes’ disparate frequency distributions of island areas (J. Diamond 1994, in litt.). The Bismarck Archipelago has one 

giant island (New Britain) that is five times larger than any of the other islands. 

Moreover, New Ireland, the second largest island, is several times the area of the 

next largest island, followed by a graded distribution of islands from 2590 km2

downwards. The West Indies differs in having several large islands. At the opposite extreme, whereas 9 percent of the Bismarck birds are supertramps, only one 

species (<1 percent) of the West Indian birds merits supertramp status (fig. 2.3). It 

is noteworthy, however, that after this analysis was completed and, following the 

passage of two severe hurricanes that greatly affected the Caribbean, the explosive 

and invasive nature of the Caribbean elaenia (Elaenia martinica) observed on 

several islands and among several forest types within islands makes it a strong 

contender for supertramp status (see also Voous 1955).

Figure 2.2—Frequency distributions of islands compared to the number of breeding landbird species on 

each. The high percentage of islands with fewer than five species reflects the large number of islets and cays 

included in the analysis. Several small- to medium-size islands in the Lesser Antilles account for the range 

from 10 to 25 resident breeding species. All five islands with more than 50 permanent resident species are 

among the larger islands found in the Greater Antilles.

Thirty-seven percent 

of the birds in the 

Bismarck Archipelago 

are high-S species, 

whereas more than 

half (59 percent) of the 

West Indian birds are 

high-S species.
40

GENERAL TECHNICAL REPORT IITF-GTR-27

Table 2.1—Distribution of birds in the West Indian and Bismarck Archipelagos 

following Diamond’s competitive strategy classification scheme

Degree of endemism

Incidence

function category Species Islands None Subspecies Species

Number

High-S 132 (52)a 1–6 (1–5) 0 (9) 50 (14) 82 (8)

A-Tramp 22 (26) 2–11 (3–9) 0 (8) 19 (14) 3 (1)

B-Tramp 19 (17) 4–14 (10–14) 0 (4) 15 (12) 4 (0)

C-Tramp 29 (19) 10–73 (15–19) 0 (8) 26 (10) 3 (0)

D-Tramp 16 (14) 23–148 (20–35) 3 (9) 13 (5) 3 (0)

Supertramp 1 (13) 80+ (2–33) 0 (2) 1 (8) 0 (1)

Anomalousb 5 (0) 1–5 (0) 0 (0) 5 (0) 0 (0)

 Total 224 (141)c 177 (50) 31 (40) 29 (63) 95 (10)

aParenthetical numbers are from Diamond (1975a).

bAnomalous species are those thought to be introduced by man, relict species that did not radiate 

following isolation resulting from past, dramatic climatic changes, or mainland species naturally 

occurring on landbridge or extralimital islands near continental source populations.

cThe total number of species in columns 4 through 6 is 113 because an additional 28 species (for the 

correct total of 141) were categorized as “Semispecies” (allospecies of a superspecies) in Diamond’s 

analysis (1975a: table 1), but not treated in this comparison. 

Figure 2.3—Comparison of West Indian birds with those of the Bismarck Archipelago by following Diamond’s (1975) competitive strategy scheme. Fifty Pacific islands and 141 avian species were 

included in Diamond’s analysis, whereas 177 Caribbean islands and 224 species were treated in this 

analysis. The y-axis constitutes the number of species in each of the six categories presented as a 

percentage of the total number of species. The two most obvious differences between these widely 

separated geographical regions are the disparate numbers of supertramps and the greater extent of 

endemism (reflected by the large number of High-S species) attained in the West Indies.
41

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

The above-noted differences between the Pacific and Caribbean landbird 

avifauna notwithstanding, the forms of the various incidence function curves 

for the 224 West Indian species combined are quite similar to those obtained by 

Diamond (1975a) for Pacific landbirds (fig. 2.4). As predicted by the supertramp 

theory, with one exception, the Caribbean’s single supertramp species is found 

on only the smallest species-poor islands in which suitable habitat is found, but 

its occurrence drops off precipitously as island size and species-richness increase. 

Although found on the relatively large island of Puerto Rico, the pearly-eyed 

thrasher (Margarops fuscatus) abounds only in species-poor habitats. The 

Caribbean’s wide ranging C- and D-tramps are found on 40 to 100 percent of 

all islands, regardless of the number of breeders. In contrast, B- and A-tramps, 

although occurring on all of the larger islands, are absent from smaller islands 

with fewer than 25 resident landbird species. High-S species are confined to the 

largest islands with more than 48 resident landbird species.

Species-Specific Incidence Functions

To further explain contemporary species’ distributions, incidence function curves 

for each of the 224 resident landbird species were plotted (fig. 2.5a–d). Criteria 

used to classify each species into Diamond’s six colonization categories included 

elevation, habitat and vegetation type, and structure (app. 1). It is not obvious 

Figure 2.4—Incidence functions for all 224 species of West Indian landbirds combined. “J” is the incidence 

of occurrence, or the proportion (percentage) of islands on which a species is found plotted against “S,” the 

number of breeding landbirds per island (see “Study Area and Methods” for a more indepth description).
42

GENERAL TECHNICAL REPORT IITF-GTR-27

from the illustrations that there are any clearly defined patterns, e.g., certain 

taxa, range of body sizes, or ecological groups, that help to explain today’s 

species’ distributions within a competition vs. colonization context. For example, 

not all wide-ranging birds (colonizers) are large, highly mobile species such as 

raptors and pigeons found at widely ranging elevations or among several habitat 

types. Likewise, at the opposite extreme, not all high-S species (competitors) are 

limited to a single taxon or habitat type. Examples of species ranging from very 

small to very large occur at all elevations and among most habitats and within 

several vegetation associations, and thus are found in all of the six distribution 

categories (fig. 2.5a–d). As a further example, whereas some hummingbirds are 

high-S species confined to large, species-rich islands inhabiting single (or very 

few) habitats, others are C- and D-tramps found among several habitats along 

a wide range of island-sizes and geographical expanse. At the other extreme, 

although some large raptors are found over much of the region on islands of all 

sizes and among many habitats, others are high-S, habitat specialists confined to 

single islands and few habitats. Thus, it is safer to conclude that distribution, in all 

respects (e.g., geographical and ecological), is highly species dependent. Indeed, 

resulting from recent advances in molecular biology, by determining levels of 

mitochondrial DNA (mtDNA) divergence among populations of island birds, 

researchers (e.g., Lovette et al. 1999) are now able to trace not only the source 

islands for the various species but can also ascertain the chronological sequence 

of colonization for each species. They are finding that insular bird communities 

are composed of species with different invasion histories, and that community 

composition changes repeatedly over time. In effect, they are using a new tool 

to corroborate what ecologists and ornithologists have known for many years 

through direct observation.

Competitors Versus Colonizers Among 

Taxonomic Groups

Although the incidence function curves for individual species are informative and 

helpful when comparing one or more species, it is difficult to see how the species 

are distributed among the seven competitive strategy categories and to discover 

possible trends. To facilitate the interpretation and evaluation of the individual 

species-specific incidence function curves, West Indian landbirds were placed in 

32 broad taxonomic groups (hawks and columbids through finches) and sorted 

into Diamond’s six competitive strategy categories (table 2.2). Not obvious from 

the illustrations, but immediately evident from the table, is the fact that most 

high-S through B-tramp species confined to single or few islands and one or 

(text continues on page 47)
43

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Figure 2.5 (A–D)—Incidence functions (J) for individual species of West Indian landbirds. Species names are coded (see app. 1). The 

number of islands on which each species is found is in parentheses following the species code. Histograms are arranged sequentially into 

seven categories (including anomalous species) from extreme colonizers to extreme competitors: Supertramps (ST), D-Tramps (DT), 

C-Tramps (CT), B-Tramps (BT), A-Tramps (AT), and High-S (HS) species. Histograms of anomalous species difficult to classify are 

included for illustrative purposes. Some High-S species such as the Hispaniolan endemics (e.g., RIHA, HIPT, GCPT, and PALM) appear 

to be better candidates for the A- and B-Tramp categories. This is due to their occurrence on nearby satellite islands (e.g., Saona, Beata, 

Cayemites, Ile-à-vache, Tortue, and Gonâve). Satellite islands act as land-bridge islands, thus confounding the classification results.

A
44

GENERAL TECHNICAL REPORT IITF-GTR-27

B

Figure 2.5—Incidence functions (J) for individual species of West Indian landbirds (continued).
45

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

C

Figure 2.5—Incidence functions (J) for individual species of West Indian landbirds (continued).
46

GENERAL TECHNICAL REPORT IITF-GTR-27

D

Figure 2.5—Incidence functions (J) for individual species of West Indian landbirds (continued).
47

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

very few habitat types constitute more primitive and/or aberrant avian taxa. Even 

among the remaining primitive taxa with a broader distribution (i.e., including 

C- and D-tramps), most species (often at least half) are still classified as high-S to 

B-tramps. The fast flying, mobile columbids and more highly developed passerine 

groups are the best dispersers and thus are more evenly spread out among categories. 

The best colonizers (from owls onward) have at least as many dispersers (A- through 

supertramps) as specialists (high-S species). Flycatchers, pigeons, and thrashers 

Table 2.2—Generalized taxonomic groups of West Indian birds following Diamond’s (1975a) competitive 

strategy classification scheme (in order of decreasing competiveness)

Number of species in respective categories

Taxonomic group High-S A-Tramp B-Tramp C-Tramp D-Tramp Supertramp Anomalousa

Competitors:

Parrots 11 1

Woodpeckers 9 2

Nightjars 6 1

Todies 5

Crows 3 1

Trogons 2

Becards 1

Caracaras 1

Honeycreepers 1

Kingfishers 1

Nuthatches 1

Palmchats 1

Wrens 1 1

Gnatcatchers 1 1

Kites 1 1

Vultures 1

Colonizers:

Tanagers 5 1 2

Thrushes 5 2 1 1

Swifts 3 2 1

Finches 12 1 3 1 1

Warblers 11 3 1 1

Blackbirds 9 3 1 1 1

Hummingbirds 8 1 1 4 1 1

Flycatchers 6 4 6 1 1

Vireos 6 1 1 1

Cuckoos 5 1 1 2

Owls 4 1 1 1 1

Hawks 4 3 1

Swallows 3 1 1

Pigeons 5 1 3 4 4

Bananaquits 1

Thrashers 1 5 1

 Total 132 22 19 29 16 1 5

aAnomalous species are those thought to be introduced by man, relict species that did not radiate following isolation resulting from past, 

dramatic climatic changes, or mainland species naturally occurring on landbridge and extralimital islands near continental source populations.
48

GENERAL TECHNICAL REPORT IITF-GTR-27

have at least twice as many dispersers as specialists. Of all the groups, owing to 

their small size, high metabolism, and food specialization, probably the least likely 

to be considered good dispersers would be the hummingbirds. Yet, reminiscent of 

a few migratory species within north temperate genera (e.g., Archilochus and 

Selasphorus), almost as many Caribbean hummingbirds (seven species—not 

counting anomalous species) are dispersers as are specialists (eight species).

Caribbean mimids in general are tramp species. They constitute the only 

avian taxonomic group to decrease in species richness as island size increases 

(Terborgh 1973: fig. 6). Thrashers in particular constitute the only avian group in 

the West Indies whose members are primarily good to excellent colonizers (six of 

seven species). Other groups, e.g., pigeons and doves, include vagile species, but 

have similar (and usually many more) numbers of habitat specialists. Instead of 

being dubbed a supertramp, the ubiquitous bananaquit (Coereba flaveola) was 

classified as a D-Tramp. Although it is an excellent disperser and is found on 131 

(about 75 percent) of the 177 islands analyzed, possibly because it is a food specialist (nectar robber), the bananaquit is absent from some of the region’s smaller and 

larger islands. On Cuba, the region’s largest island, there is a competing honeycreeper, which probably hinders the bananaquit’s attempts to colonize.

The Pearly-Eyed Thrasher as a Classic Supertramp

From these analyses, the pearly-eyed thrasher (fig. 2.6) emerges as the only West 

Indian species that fits virtually all of the criteria of a supertramp. Two additional 

species, the Caribbean elaenia and thick-billed vireo (Vireo crassirostris), were 

strong candidates for the supertramp classification but were discarded because they 

have not dispersed throughout the region, although the elaenia continues to extend 

its range to the north and westward in the Greater Antilles (app. 1). The pearlyeyed thrasher, however, clearly satisfies the supertramp distributional requisite of 

generally being found only on small, species-poor islands throughout the region. 

It inhabits some 80 islands in both the Greater and Lesser Antilles, including 

islands in the extreme southern Caribbean Sea (apps. 1 and 2). And, until relatively 

recently, the pearly-eye was even found on extralimital islands off the north coast 

of Venezuela (see chapter 4, and app. 3). Although found on the large, species-rich 

island of Puerto Rico, the pearly-eye is scarce or absent in species-rich communities, but often super abundant (an example of density compensation4

 in speciespoor habitats where competitors are few or absent) (see also chapter 4). Diamond 

4Density compensation is a higher abundance of insular species compensating for the absence of 

mainland species; it usually results in habitat expansions, wider ranges of vertical foraging strata, 

and decreased morphological variability, among other adaptations (MacArthur et al. 1972).

Caribbean mimids 

in general are tramp 

species. They 

constitute the only 

avian taxonomic 

group to decrease in 

species richness as 

island size increases.
49

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Figure 2.6—The pearly-eyed thrasher (Margarops fuscatus): the Caribbean’s prime example of an 

avian supertramp. It is found widely throughout the Greater Caribbean Basin on more than 80 

islands and cays. Although restricted to small islands and species-depauperate communities, it 

occupies a full complement of habitats ranging from urban settings and coastal, littoral vegetation, 

to montane cloud forest. It is especially abundant in edge and disturbance-prone habitats. (Photo 

courtesy of Gerald P. Bauer.)

(1982) has documented a similar phenomenon among three archipelagoes in 

the Pacific Ocean. Supertramp species, confined to small islands in species-rich 

archipelagoes, expand to occupy larger islands in archipelagoes where the 

competitors are few or absent. 

In summary, as with other mimids, the pearly-eye has evolved superior 

colonizing abilities and other life-history traits typical of all avian tramps. It is 

a strong flier, shows excellent dispersal among islands and habitats (see below 

and chapters 4 and 6), and is wide-ranging in the Caribbean region (AOU 1998, 

Bond 1979a, Evans 1990, Lack 1976, Raffaele et al. 1998). Within islands, it is 

often found at all elevations and in every major terrestrial habitat. It is omnivorous, exhibiting much variability in its diet and foraging techniques. It also 

shows morphological disparity among sexes and among individuals in allopatric 

(nonoverlapping) populations (chapter 5). Even more germane to the supertramp 

concept, the pearly-eye has evolved a reproductive strategy characterized by (1) 

extended breeding seasons, (2) multiple broods, (3) rapid recycling within breeding 

seasons, (4) asynchronous hatching, and (5) a highly adaptive resiliency to major 

habitat disturbances (chapter 7). Following natural disasters such as droughts and 

hurricanes, the pearly-eye can significantly alter its reproductive parameters. By 

adjusting its clutch size, and onset and length of breeding season, it compensates 

for the various physical and ecological stresses (e.g., damaged habitat and scarcity 

of adequate food resources) that severely lower the reproductive success of many 

other species. In addition to its superior reproductive performance, the pearlyeye often reaches high population densities in many, often disturbed or altered, 

habitats on several Caribbean islands (chapter 4).
50

GENERAL TECHNICAL REPORT IITF-GTR-27

Tramp Species Research

Previous authors have classified various species as supertramps usually on the 

basis of (a) incidence functions; (b) general impressions about their dispersal, 

colonization potential, and lack of competitive ability in species-rich communities; and (c) a few reproductive parameters, e.g., clutch size, length of 

nesting periods, and number of clutches per season (summarized by Diamond 

1975a: 379–380). However, there have been no comprehensive studies of the 

ecology and life history of any supertramp species. By presenting the results of 

an ongoing (1979 to present) ecological study of the pearly-eyed thrasher, this 

report takes the first indepth look at the life history and ecology of an avian 

supertramp.

Summary: Supertramps in the West Indian Avifauna

At least two distinctive competitive strategies exist among insular landbirds. 

Island birds tend to be either superior colonizers or strong competitors. Colonizers are generally r-selected species that have evolved good dispersal abilities 

and a high reproductive potential featuring rapid and extended breeding and 

large clutch sizes; they fill vacant or underexploited niches by making marked 

shifts in altitudinal range, habitat type, diet, foraging techniques, and morphology. However, colonizers are forced out of species-rich communities by more 

specialized K-selected competitors and diffuse competition. Supertramps 

are those species that have evolved extreme colonizing traits. They are highly 

vagile species that are typically confined to small, remote, or recently disturbed 

islands and habitats with few species. Tramp species have been described 

among terrestrial arthropods, land snails, freshwater and marine invertebrates, 

zooplankton, reptiles, mammals, and plants. In the Caribbean, the pearly-eyed 

thrasher best fits all the criteria of a true avian supertramp. It has evolved 

superior colonizing abilities, is a strong flier, shows excellent dispersal among 

islands and habitats, and is wide-ranging in the region. It is resident on some 

80 islands and cays. Within islands, the pearly-eye is often found at all elevations and in every major terrestrial habitat type. It is omnivorous, exhibiting 

a diverse diet and numerous foraging techniques. It is long lived and shows 

morphological disparity among sexes and among individuals in allopatric 

populations. Most germane to the supertramp concept, the pearly-eye has 

evolved a reproductive strategy characterized by extended breeding seasons, 

multiple broods, rapid recycling within seasons, asynchronous hatching, and 

a highly adaptive resiliency to major habitat disturbances.
51

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Before presenting the available information on the origins, classification, and 

taxonomy of the pearly-eyed thrasher, a brief introduction to the possible mechanisms governing the current distribution of all West Indian birds is presented.

Colonization and Dispersal of West Indian Fauna

There are two main schools of thought regarding the mechanisms behind the 

origin, diversity, and distribution of the present-day West Indian fauna: (1) dispersal and (2) vicariance (allopatric speciation) (see Williams 1989 for a comprehensive review). These two partially conflicting theories notwithstanding, many 

biogeographers concur that the West Indian fauna, especially Greater Antillean 

species, are derived from North and Middle American taxa (Bond 1979b, 1982b; 

Darlington 1938; Pregill et al. 1994; Rosen 1976). For example, most West Indian 

bird genera arose from Middle American stock and are predominantly of tropical North American ancestry (Bond 1934, 1963a, 1966a, 1978a; Mayr 1946; S.L. 

Olson 1994, in litt.). Only two avian families with examples in South America, i.e., 

Trochilidae and Thraupidae, are represented by endemic genera (Bond 1979b).

Why should more ancestors of the West Indian fauna come from North and 

Middle America as opposed to South America? The Caribbean plate is known 

to be very old (more than 80 million years), and its history is as complex as it is 

ancient (Perfit and Williams 1989). Thus, to answer the question of the origins of 

West Indian progenitors, a plausible explanation is a geophysical theory based 

on plate tectonics and global shifts of prominent landmasses (continental drift) 

throughout Earth’s early history. Rosen (1976) pointed out that the geophysical 

theory is “...strongly consistent with the distributional evidence.” and West Indian 

islands were once “...part of an early lower Middle American archipelago that was 

later replaced by the tectonically and topographically similar, present-day, lower 

Middle American land.” More recently, Perfit and Williams (1989) summarized 

the present state of geological knowledge of the Caribbean region in terms of four 

periods covering a large expanse of geological time, i.e., from the Late Cretaceous 

to the present. These authors showed the strengths and weaknesses of both major 

colonization hypotheses attempting to explain West Indian biogeography and 

concluded that caution and new evidence is needed for a more comprehensive 

knowledge of the region’s biogeographical history. In support of the dispersal 

hypothesis, Donnelly (1989), using pelagic oceanic sedimentation records, showed 

that corridors or filter bridges (sensu Simpson 1940), so conducive for islandhopping organisms, existed as early as the early Miocene (see Keith et al. 2003 

for a detailed summary of the Zoogeography of the region, including more 

recent studies).

Chapter 3: Origins, Classification, and Taxonomy

There are two mechanisms 

behind the origin, diversity, 

and distribution of the 

present-day West Indian 

fauna, dispersal and 

vicariance.
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GENERAL TECHNICAL REPORT IITF-GTR-27

History of Classification, Affiliations, and 

Distribution of the Mimidae

The pearly-eyed thrasher is a member of the avian family Mimidae. To put the 

pearly-eye in proper perspective, a brief discussion of the classification of the 

“mimic thrushes” follows.

The Mimidae (mockingbirds, thrashers, and allies) comprise a small family 

of New World birds, probably from Middle American stock (Bond 1978b, 1979b; 

Mayr 1946). For many years systematists thought mimids were most closely 

related to the thrushes and wrens and placed the group in an intermediate phylogenetic position between them, e.g., Lucas (1888 cited in Ridgway 1907). Sclater 

(1859) proposed that the genera Margarops and Cichlherminia be transitional 

from Turdus to the “mock-birds.” He noted that, like the turdid thrushes, species 

in the genera Margarops and Cichlherminia exhibit a well-developed first spurious primary and prominent scutella on the anterior crest of the tarsi. Some early 

authorities believed mimids are thrushes (e.g., Bonaparte 1850, Coues 1884, Gray 

1869), and some more recent authors continued to place mimids with thrushes 

(e.g., Beecher 1953, Gulledge 1975, and Morioka 1967). Thereafter, the Mimidae 

were placed between the subfamily Turdinae (true thrushes) and Prunellidae 

(accentors) (AOU 1998). However, from a series of DNA-DNA hybridization tests, 

with substantiating results from morphological and serological evidence (Sibley 

and Ahlquist 1984) and analyses of nucleotide sequences (Barker et al. 2002; 

Ericson and Johansson 2003; Ericson et al. 2002a,b), mockingbirds were shown 

to be more closely related to starlings (Sturnidae) than to any other living taxon, 

and both groups have been placed by some authorities as sister tribes (Mimini 

and Sturnini) within the family Sturnidae, superfamily Muscicapoidea, parvorder 

Passerida, and suborder Passeres (C.G. Sibley 1992, in litt.). The mimids’ next 

nearest relatives are members of the Turdidae, including typical thrushes, erithacine chats, and muscicapine flycatchers (Sibley and Ahlquist 1984). These authors 

suggested that the sturnine-mimine dichotomy occurred in the late Oligocene or 

early Miocene, about 23 to 28 million years B.P. At that time, there were emergent 

land connections between Europe and North America, thus allowing the progenitor of the starlings and mockingbirds to spread over the Northern Hemisphere at 

a time when the increasingly severe boreal climate was pushing the two populations southward and out of contact.

Currently, most avian taxonomists recognize at least 11 extant genera and 

some 34 species of mimids (Clements 2000, Davis and Miller 1960). Most mimids 

are found in North America from Canada through the United States to Middle 
53

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

America and the West Indies. Seven of North America’s 10 genera and 17 species 

are found in Mexico, where mimids reach their greatest diversity. Only Mimus is 

found in South America (Howard and Moore 2003). Nesomimus (= Mimus) comprises four species of mockingbirds inhabiting various Galápagos islands. An 11th

monotypic genus, Donacobius, Donacobius atricapillus (black-capped donacobius, 

or mockingthrush), was shown to be a wren (Clench et al. 1982) and was moved 

from Mimidae to the Troglodytidae (AOU 1998).

Systematists currently recognize 4 to 5 mimid genera and 9 to 11 species 

inhabiting the West Indies: (1) Mimus (mockingbirds, 4 species); (2) Cinclocerthia

(tremblers—2 species now recognized by the AOU 1998 and Storer 1989. However, Hunt et al. (2001) proposed three distinct species, and Olson et al. (2005) 

suggest as many as four distinct trembler species on molecular genetic grounds); 

(3) Ramphocinclus brachyurus (white-breasted thrasher); and (4) Margarops—currently 2 species, the pearly-eyed thrasher and scaly-breasted thrasher; however, 

Hunt et al. (2001) found that “Margarops fuscatus displayed a moderate level of 

haplotype variation that lacked any apparent geographic pattern.” That is, M. 

fuscatus remains as one species. The same authors proposed two species of M. 

fuscus that also merit separation from the genus Margarops and potential replacement in a fifth genus, Allenia. With the exception of Mimus (widespread in North 

and South America), the remaining three genera are found only in the Caribbean 

region. Cinclocerthia and Ramphocinclus occur solely in the Lesser Antilles and 

historically, along with other avian genera such as Todus, Priotelus, Dulus, Loxigilla, Spindalis, Euneornis, Teretistris, and Coccyzus (formerly Saurothera), were 

believed to have originated in the southern region.

Origin of the Genus Margarops

The contemporary genus Margarops (Sclater 1859) is derived from the Greek 

μáργaρos et wψ, translated as “margaros” (S.L. Olson 2005, in litt.) or “pearl” 

and “ops” (eye) in English (Jobling 1991). The genus is presently known only from 

the greater Caribbean basin and comprises two species, the pearly-eyed thrasher 

(Margarops fuscatus) (= “fuscata,” Latin for dark; or, “fuscare” to darken) (fig. 

2.6), and the scaly-breasted thrasher (M. fuscus) (“fuscus” also connotes dark 

or brown in Latin) (fig. 3.1). Neither Margarops species shows close affinities to 

continental (Toxostoma spp.) mimids (Bond 1963a, 1978a; S.L. Olson 1994, in litt.).

Historically, several avian taxonomists and paleontologists hypothesized that 

whereas some West Indian birds, e.g., many Jamaican species, are autochthons, 

i.e., endogenous to the region (Pregill and Olson 1981), both Margarops species 

derived from progenitors that arrived in the Caribbean region during the middle 
54

GENERAL TECHNICAL REPORT IITF-GTR-27

Tertiary from Middle America (Bond 1978a), with M. fuscus preceding M. fuscatus (Bond 1934, 1970, 1979b). Bond (1934) postulated that during the Middle 

Tertiary, the West Indian landmass was much larger than it is currently, and 

probably was connected to Middle America. However, more recently, through 

mitochondrial DNA analyses, the three endemic Mimidae genera Margarops, 

Ramphocinclus, and Cinclocerthia are now thought to have originated 4 to 5 

million years ago, i.e., as early as the beginning of the Pliocene (Ricklefs and 

Bermingham 1997), and may be the best plausible example of an autochthonous 

radiation in the Lesser Antilles (Hunt et al. 2001). These authors found that “Phylogenetic analyses of species-level Mimidae relationships based on mtDNA and 

nuclear sequences provide strong support for the monophyly and Antillean origin 

of a clade consisting of the tremblers, pearly-eyed thrasher, and scaly-breasted 

thrasher.” As examples, although now considered rare or vagrant on St. Vincent 

(Evans 1990, Raffaele et al. 1998), the scaly-breasted thrasher inhabited that island 

long enough for subspeciation (Kratter and Garrido 1996). Moreover, Hunt et al. 

(2001) concluded that “...the history of the endemic thrasher lineage in the West 

Indies extends back about 4 million years, and the three distinct clades of tremblers split about 2 million years ago.”

The Lesser Antillean Provenance is very distinct from that of the Greater 

Antilles, from which it is separated by the Anegada Channel. Cogently, the eggs 

of the Margarops species (along with those of Cinclocerthia and Ramphocinclus) 

are immaculate greenish blue like those of North and Middle American mimids 

(Melanotis, Melanoptila, and Dumetella), and unlike any mimid species currently 

inhabiting South America. Long before DNA-DNA hybridization analyses were 

Figure 3.1—Scaly-breasted thrasher (Margarops fuscus), the pearly-eye’s only congener. Recent advances in molecular biology based on 

mitochondrial DNA and nuclear sequencing have prompted contemporary researchers to advocate that M. fuscus be returned to its former 

genus, Allenia.
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

used to link mimids to starlings, Voous (1957), who also believed that Margarops 

fuscatus arose in the West Indies, made an insightful observation stating that he 

thought the song of the pearly-eye closely resembles that of the European starling, 

Sturnus vulgaris (see also Sibley and Ahlquist 1984, Sibley et al. 1988). The recent 

molecular results (Ericson and Johansson 2003, Hunt et al. 2001, Olson 2005) 

corroborate Voous’ (1957) early hypothesis.

At the intraspecific level, very little mitochondrial differentiation was 

found between M. fuscatus populations on Puerto Rico, Barbuda, Montserrat, 

Guadeloupe, and Dominica (J.S. Hunt 2000, in litt.). This is less surprising, 

now that there is proof of interisland dispersal in the pearly-eye (e.g., Barbuda 

to Guadeloupe—see chapter 6).

Taxonomy of the Genus Margarops

As with mimids in general, confusion as to the true affinities of both Margarops

species, i.e., whether they are thrashers or thrushes, was evident early in their 

classification. L.J.P. Vieillot (1807; corrected date 1808—Browning and Monroe 

1991) collected the type specimen of the pearly-eyed thrasher in Puerto Rico and 

placed the species in the genus Turdus, naming it the pearly-eyed thrush Turdus 

fuscatus (The type remains in the Vieillot collection). More than 50 years later, 

Sclater (1859) assigned the pearly-eye to the present-day mimid genus Margarops. 

Before being transferred to the genus Margarops, the pearly-eyed “mocking 

thrush” (sensu Ridgway 1907) was placed in four separate genera with three 

distinct species names, e.g., the shrike-thrushes of the Australian region: Colluricincla, as C. fusca (Gould 1836); the New World mockingbirds: Mimus, as M. 

fuscatus (Bonaparte 1854); a defunct genus: Cichlalopia, as C. fuscatus (Ridgway 

1907); the forest thrushes: Cichlherminia, as C. fuscata (Newton and Newton 

1859), and an additional defunct thrush genus: Merula (John Gould, The Birds of 

Australia Supplement, 1851–1865, as M. fuscata [Cassin 1861]). An overview of the 

history of the collection of specimens and taxonomic classification of the pearlyeyed thrasher is presented in table 3.1 (see Arendt 1993: 50–60, for a more detailed 

history).

Pearly-Eyed Thrasher

Distribution of taxonomically distinct species—

Two distinct species of the pearly-eyed thrasher were recognized by taxonomists 

for many years, a light-colored species inhabiting the Caribbean’s northern (dry) 

islands (M. fuscatus) (Sclater 1859) and a darker species inhabiting the southern 

(humid) islands (M. densirostris) (Sclater 1859). The pearly-eye is a good example 

Two distinct species 

of the pearly-eyed 

thrasher were 

recognized by 

taxonomists for 

many years.
56

GENERAL TECHNICAL REPORT IITF-GTR-27

of Gloger’s Rule, i.e., “The generalization that among warm-blooded animals 

those races living in warm and humid areas are more heavily pigmented than 

those in dry areas; pigments are typically black in warm humid environments...” 

(Lincoln et al. 1983); see Burtt and Ichida (2004) for a causative mechanism, i.e., 

feather-degrading bacteria.

The species name densirostris is derived from Latin “densus” (thick or dense) 

and “rostris” (“billed”) (fig. 3.2). Today, one species and four subspecies of pearlyeye are generally recognized: (M. f. fuscatus (Greater Antilles and northern 

Leeward Islands); M. f. densirostris and M. f. klinikowskii (Lesser Antilles and 

southern Leeward Islands); and M. f. bonariensis (Bonaire and, previously, La 

Horquilla, Los Hermanos Islands, Venezuela) (Howard and Moore 2003, Meyer 

de Schauensee and Phelps, Jr. 1978, Phelps and Phelps 1948, Yepez Tamayo 1964). 

The subspecies, M. f. klinikowskii was named just recently. Garrido and Remsen 

(1996) proposed that the pearly-eyed thrasher population on St. Lucia warrants 

subspecific classification, basing their decision on morphological and plumage 

characteristics. St. Lucian pearly-eyes are definitely much larger and show distinct 

plumage differences from individuals inhabiting other islands (see chapter 5 for 

further discussion).

Table 3.1—History of specimen collections and taxonomic classificationa of the pearly-eyed thrasher

Genus, species, and subspecies Year Taxonomist Reference

Turdus fuscatus 1807 Vieillot Histoire Naturelle des Oiseaux de l’Amérique Septen-

 trionale, ii, p. 1, pl. 57 bis (Santo Domingo, Puerto Rico)

Turdus densirostris 1818 Vieillot Nouveau Dictionnaire d’ Histoire Naturelle, xx, p. 232 

 (Martinique)

Colluricincla fusca 1836 Gould Proceedings of the Zoological Society of London 4, p. 6

Mimus fuscatus 1854 Bonaparte Comptes Rendus, xxxviii, p. 2, footnote

Cichlherminia densirostris 1854 Bonaparte Comptes Rendus, xxxviii, p. 2, footnote

Cichlalopia fuscatus 1857 Bonaparte Revue et Magasin de Zoologie Pure et Applique Series 2, 

 vol. 9, p. 205

Cichlherminia fuscata 1859 Newton (A. & E.) Ibis, pp. 141–142 (St. Croix, St. Thomas); pl. 12, fig. 8 

 (St. Croix)

Margarops fuscatus 1859 Sclater Proceedings of the Zoological Society of London 27, 

 p. 335 (monogr. St. Croix, St. Thomas)

Margarops densirostris 1859 Sclater Proceedings of the Zoological Society of London 27, 

 p. 336 (monogr. Guadeloupe, Martinique)

Merula fuscata 1861 Cassin Proceedings of the Academy of Natural Sciences of 

 Philadelphia 12, p. 376 (St. Thomas)

Margarops f. bonariensis 1948 Phelps and Phelps Proceedings of the Biological Society of Washington 61, 

 p. 171 (Fontein, Bonaire)

aOnly major generic and species name changes are included, along with the respective years, taxonomists, reference sources, and occasional 

collection localities. For a more comprehensive summary of specimen collections and the numerous designations given by many early 

taxonomists, see Arendt (1993).
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Vernacular names—

Numerous vernaculars (local names) have been used to refer to Margarops fuscatus on various Caribbean Islands (table 3.2). No doubt, many more exist. Within 

the five major ethnic groups of the region (1) English; (2) Spanish; (3) French; 

(4) Dutch, including Papiamento; and (5) Patois, including Creole, the most commonly used local names are, respectively, (1) thrush (used as a general descriptor); 

(2) zorzal (Spanish general translation of thrush, e.g., zorzal pardo—“brown 

thrush” in Puerto Rico); (3) moqueur (French general translation of thrush, e.g., 

le moqueur corossol—soursop bird in Guadeloupe; (4) witoogspotlijster (Dutch 

translation of white-eyed mockingbird); palabrua (Papiamento translation of 

black magic bird); (5) grieve (pronounced grief), the general term used for thrush 

(e.g., grosse grieve, or “large thrush”) in the Patois and Creole dialects spoken in 

the French West Indies and Dominica, St. Lucia, and St. Vincent.

Scaly-Breasted Thrasher

Taxonomy and distribution of distinct subspecies—

Like the pearly-eye, the affinities of its congener, the scaly-breasted thrasher 

(fig. 3.1), were obscure during the genesis of its discovery and classification. It, 

too, was thought to be a thrush (Ridgway 1907). Consequently, the scaly-breast 

was placed in several thrush genera (some of which remain in the present-day 

turdinae) and was assigned various species names before finally being classified as 

a mimid. P.L.S. Müller (1776) was the first to describe the scaly-breasted thrasher 

from a specimen collected in Martinique, placing the species together with the 

Figure 3.2—Pearly-eye profile emphasizing its formidable beak. Despite early taxonomists’ reference specifically to Margarops f. densirostris as 

having a prominent bill, all four subspecies, including this Puerto Rican individual of M. f. fuscatus, possess quite an intimidating, predacious 

beak. The damage inflicted by the beak, in combination with its sharp, piercing claws, is what makes the pearly-eye such an intrepid predator (see 

chapter 8), and which has prompted such vernaculars as “insidious thrasher” (Snyder et al. 1987), “rapacious (or “predatory”) passerine” (several 

authors), and “intrepid poacher” (Seaman 1952).
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GENERAL TECHNICAL REPORT IITF-GTR-27

Table 3.2—Most common vernacular names given to the pearly-eyed thrasher by people from five ethnic 

backgrounds scattered throughout the Caribbean

Englisha Non-Englisha Islandsb Referencesc

Black magic bird (1, 3) Palabrua (2) 4, 5, 8 7

Black thrasher (1, 3) Zorzal negro (5) 2, 12 10, 14

Brown thrush Grive brune (1, 4) 7, 9, 13, 16 1, 11

Drab, dull, or brown thrush Zorzal pardo (5) 12 2

Jack bird (1, 3) Pío-Juan, Pío-Hyab (5) 2, 12 2, 10

Large thrush Grosse (grive) or grieve (1, 4) 7, 9, 13, 16 2, 9

Mango bird (1, 3) Ave que come mangó (5) 11, 12 10

Mango bird (1, 3) Le Moqueur que manage l’ fruit (4) 7, 9 10

Palm thrush (1,3) Zorzal de palmares (5) 12 14

Papaya birdd (1, 3) Ave que come lechosa (5) 12 14

Paw-paw birdd (1, 3) Ave que come lechosa (5) 2, 12, 14, 18 10, 14

(also paw-paw fruit thrush)d (1, 3) Ave que come lechosa (5) 2, 12, 19 6

Pearly-eyed thrush (1, 3) Zorzal con ojos de perla (5) 14 8

Soursop bird (1, 3) Ave que come guanabana (5) 11, 12 10, 14

Soursop bird (1, 3) Grisee (or Moqueur) corossol (2, 4) 2, 7, 12 9, 10, 11

Spanish mockingbird (3) Chuchubi spanjof g (3) 4, 5, 8 7

Spanish mockingbird (3) Tjutjubi spagnolf (var. spaño) (2) 4, 5 4, 7, 12

Strong beak Boca dura (2, 5) 4, 5, 8 7

Thrasher (1, 3) Louis jo (1) 11 10

Thrasher (1, 3) Pie voleuse (1, 4) 13 10

Thrush and thrushee (1, 3) Zorzal, Truche, Chucho (2, 4, 5) 1, 3, 10, 12, 15, 17, 19 2, 3, 5, 10, 11, 14

Wall-eyed (thrush) thrasher (1, 3) Zorzal ojiblanco (5) 12, 19 5, 10

White-eyed mockingbird (3) Witoogspotlijsterf h (2) 4 12, 13

(also, white-eyed mocking-thrush)

White-throated fruit thrush (3) Zorzal de las frutas con la 6, 7, 9, 10, 16 6

 garganta blanca (5) 

aNative language: (1) Creole: English and French Creole (including patois [pronounced “patwa”]) Antigua, Bahamas, Barbados, Barbuda, 

British Virgin Islands, Caicos Islands, Dominica, Jamaica, Montserrat, Nevis, St. Kitts, St. Lucia, St. Vincent and the Grenadines, Turks 

Islands, and U.S. Virgin Islands; (2) Dutch (including Papiamento, the local dialect spoken on Aruba, Bonaire, and Curaçao) Saba, St. 

Eustatius, and St. Maarten; (3) English: Nevis, Puerto Rico (small percentage), St. Kitts, St. Lucia, St. Vincent and the Grenadines, Turks 

Islands, and U.S. Virgin Islands (St. Croix, St. John, St. Thomas); (4) French (Guadeloupe and its satellites, e.g., la Désirade and Marie 

Galante, Martinique, St. Barthélemy, and St. Martin); and (5) Spanish (also Spanish Creole), Cuba, Dominican Republic and Puerto Rico.

b Island: (1) Antigua, (2) Bahamas, (3) Barbados, (4) Bonaire, (5) Curaçao, (6) Dominica, (7) Guadeloupe, (8) La Horquilla, (9) Martinique, 

(10) Montserrat, (11) Nevis, (12) Puerto Rico and Satellites, (13) St. Barthélemy, (14) St. Croix, (15) St. Eustatius, (16) St. Lucia, (17) St. Martin, 

(18) St. Thomas; (19) U.S. Virgin Islands.

cReferences: (1) Bond 1979a; (2) Danforth 1926, 1930a, 1930b, 1931, 1934, 1935a, 1935b, 1935c, 1939a, 1939b, 1939c; (3) Feilden 1889; (4) Hartert 

1893b; (5) Leopold 1963; (6) Maynard 1898–1903; (7) Meyer de Schauensee and Phelps 1978; (8) Newton and Newton 1859; (9) Noble 1916; (10) 

Raffaele et al. 1998; (11) Vieillot 1807; (12) Voous 1983; (13) University of the Virgin Islands 1988; (14) Wetmore 1916a, 1927.

dPaw-paw (var. papaw) is synonymous with custard apple in North America; in the Caribbean, however, it is often used to refer to the 

papaya fruit; paw-paw is thought to have been derived from the Spanish word papaya.

eGrise (= thrush); “corossol” in patois refers to the fruit of the soursop.

f

Translated by (the late) Frater Candidus van der Linden.

g “Chuchubi” by itself is the vernacular for mockingbird, e.g., tropical mockingbird (Mimus gilvus).

hWit = white, oog = eye, spotlijster also denotes “mockingbird” (the late Frater Candidus van der Linden 1992, in litt.).
59

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Old World flycatchers, Muscicapidae (Muscicapa fusca) (the type specimen is now 

in the Harvard Museum of Comparative Zoology, Cambridge, Massachusetts). 

More than a century later, Cory (1891) assigned the scaly-breast to the monotypic 

genus Allenia, resulting in four separate species recognized between 1891 and 

1945: A. montana (Cory 1891, Verrill 1892); A. albiventris and A. apicalis (Riley 

1905a cited in Ridgway 1907); and A. fusca (Bangs 1930, Bangs and Penard 1921, 

Bond 1928b). A brief overview of the history of the collection of specimens, 

taxonomic classification, and distribution among islands is given in table 3.3 

(for more detailed information, see Arendt 1993: 64–69). As with the pearly-eye, 

the plumage of the scaly-breast becomes progressively darker southward in the 

Lesser Antilles, the trend not appearing to be seasonal or sexual (Noble 1916). 

Almost 70 years after Cory (1891) assigned the scaly-breast to the genus Allenia

and, following early authors, Bond (1959), among others, proposed that it be 

considered congeneric with Margarops fuscatus, and assigned a fifth species name, 

viz., M. fuscus. In describing the geographical variation of the scaly-breasted 

Table 3.3—History of the collection of specimens and taxonomic classificationa of the scaly-breasted thrasher

Genus, species, and subspecies Year Taxonomist Reference

Muscicapa fusca ? Daubenton Enlarged plate 586, fig. 2 (Martinique) (plate cited in Bangs and Penard 1921)

Muscicapa fusca 1776 P.L.S. Müller Natursystems Supplements, p. 170 (Martinique)

Turdus apicalis 1854 Lichtenstein Nom. Av. Museum Berol., p. 27 (cited in Ridgway 1907)

Margarops montanus 1859 Sclater Proceedings of the Zoological Society of London 2, 

 p. 336 (Guadeloupe, monogr.)

Margarops montanus 1880 Lister Ibis Series 4: 38-44

Cichlherminia montana 1881 Sharpe Catalog of the Birds of the British Museum VI, p. 330 

 (Lesser Antilles monogr.) (cited in Ridgway 1907)

Margarops albiventris 1887 Lawrence Annals of the New York Academy of Science IV, p. 23 (Grenada) (cited in Ridgway 1907)

Margarops albiventris 1888a Cory Auk V, p. 157 (Grenada)

Margarops rufus 1888b Cory Auk V, p. 47 (Dominica)

Allenia montana 1891 Cory Auk 8, pp. 46-47 (Antigua, St. Eustatius)

Margarops albiventris 1902 Wells Auk xix, p. 349 (Carriacou; from St. Vincent hurricane of 1898)

Allenia albiventris 1905b Riley Smithsonian Miscellaneous Collections 47, p. 288 (Barbuda and Antigua)

Allenia apicalis 1905 Riley Proceedings of the Biological Society of Washington 18, 

 p. 186 (cited in Ridgway 1907)

Allenia fusca 1921 Bangs and Penard Bulletin of the Museum of Comparative Zoology 64, 

 p. 395

Allenia fusca 1945 Bond Check-list, 2nd ed., Academy of Natural Sciences of 

 Philadelphia

Allenia fusca 1961 Deignan Type specimens of birds in the U.S. National Museum

aOnly major generic and species name changes are included, along with the respective years, taxonomists, reference sources, and occasional 

collection localities. For a more comprehensive summary of specimen collections and the numerous designations given by many early 

taxonomists, see Arendt (1993).
60

GENERAL TECHNICAL REPORT IITF-GTR-27

thrasher, Buden (1993) assigned three new subspecies. Despite the current practice of placing the scaly-breast in the genus Margarops, some contemporary 

taxonomists (e.g., Howard and Moore 2003) continue to place the species in the 

monotypic genus Allenia. Indeed, based on the results of genetic testing, Hunt 

et al. (2001) not only reject the monophyly of the genus Margarops, they also 

propose distinct southern (St. Lucia and Martinique) and northern (Dominica 

to Montserrat) mtDNA lineages of the scaly-breasted thrasher, i.e., two clades, 

which translates into two separate species.

Vernacular names—

Possibly owing to the scaly-breast’s more restricted range, not as many vernacular 

names have been published for it as for the pearly-eye (table 3.4). In the more 

northern portions of its range, the scaly-breast is referred to locally as the blackbilled thrush or simply “thrush” on St. Martin, St. Eustatius, St. Kitts, Nevis, 

Antigua, and Montserrat (Danforth 1930b, 1934, 1936a, 1939b, 1939c). In the more 

southern Leeward Islands, it is known as the spotted thrush, e.g., on Carriacou 

(Wells 1902). A few variations of “grieve” are used to describe M. fuscus in the 

French Departments (e.g., Guadeloupe, Martinique, and respective satellites) 

and on former French and English colonies (e.g., St. Lucia, Dominica, and St. 

Vincent). On these islands, the scaly-breast is known as the “grevotte,” “grivotte,” 

“grive (variation of “grieve”) fine” (small thrush), “grive cendree” (gray or ashy 

thrush), and “spotted grieve” (Danforth 1935b, Devas 1970, Noble 1916).

Table 3.4—Most common vernacular names given to the scaly-breasted thrasher by people from five ethnic 

backgrounds scattered throughout the Caribbean

Englisha Non-Englisha Islandsb Referencesc

Black-billed thrush (1, 3) Zorzal de pico negro (5) 7, 8, 9, 10, 11, 14 1, 5

Spotted thrush (1, 3) Zorzal manchado (5) 2 7

Ash-colored thrushd (1) Grieve cendree (1) 5 4

Thrush, small thrush (Spotted) grieve, grivotte, grive fine (1, 4) 1, 6, 10, 12, 13, 14 1, 2, 5, 6

Thrush, small thrush Moqueur grivotte, grive fine (1, 4) 5, 6 5

Mountain fruit thrush (3) Le Moqueur de l’ montagneque que manage 3, 4, 5, 6, 12, 14 3

 l’ fruit (1, 4)

Lawrence’s fruit thrush (3) Le Moqueur de Lawrence que manage l’ fruit (1, 4) 4, 12 3

(also Cory’s fruit thrush) (3) 

aNative language: (1) Creole (also English and French patois), (2) Dutch (also Papiamento), (3) English, (4) French, and (5) Spanish 

(also Spanish patois).

b Islands: (1) Antigua, (2) Carriacou, (3) Dominica, (4) Grenada, (5) Guadeloupe, (6) Martinique, (7) Montserrat, (8) Nevis, (9) Saba, 

(10) St. Eustatius, (11) St. Kitts, (12) St. Lucia, (13) St. Martin (also Maartin), (14) St. Vincent.

cReferences: (1) Danforth 1930a, 1930b, 1934, 1935a, 1935b, 1935c, 1936, 1939a, 1939b, 1939c; (2) Lawrence 1879b; (3) Maynard 1898–1903; 

(4) Noble 1916; (5) Raffaele et al. 1998; (6) Sclater 1871; (7) Wells 1902.

dThe patois vernacular “Grieve cendree” (Ash-colored thrush) pertains not to the scaly-breasted thrasher but rather to the gray trembler 

(Cinclocerthia gutturalis) (reclassified by Storer 1989).
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Summary: Origins, Classification, and Taxonomy

Four genera and nine species of the exclusively New World avian subfamily 

Miminae (mockingbirds, thrashers, and allies) are found in the West Indies: 

Mimus (mockingbirds: four species), Cinclocerthia (tremblers: two to four species are recognized by different taxonomists), Ramphocinclus (white-breasted 

thrasher), and Margarops (two species). Only two genera, Cinclocerthia and 

Ramphocinclus, occur solely in the Lesser Antilles.

Historically, systematists thought mimids were most closely related to the 

thrushes and wrens. However, recent genetic research using mitochondrial DNA 

sequencing has shown that Lesser Antillean mimids in the genera Margarops, 

Ramphocinclus, and Cinclocerthia form a single set of lineages (or clade), along 

with the gray catbird (Dumetella carolinensis) of North America. Neither Margarops species shows close affinities to continental mimids. Moreover, recent 

evidence suggests instead an autochthonous radiation in the Lesser Antilles by 

both species. The most recent DNA-derived evidence even brings into question 

the phylogenetic relatedness and, thus, the congeneric classification of the two 

Margarops species.

For many years, taxonomists recognized two distinct species of the pearlyeyed thrasher: a light-colored species inhabiting the Caribbean’s northern low 

and dry islands (Margarops fuscatus), and a darker species inhabiting the southern high relief and humid islands (M. densirostris). Today, one species and four 

subspecies of pearly-eye are generally recognized: M. f. fuscatus (Greater Antilles 

and northern Leeward Islands); M. f. densirostris and M. f. klinikowskii (southern 

Leeward Islands); and M. f. bonariensis (Bonaire and, previously, La Horquilla 

Island, Los Hermanos Archipelago, Venezuela).

Neither Margarops 

species shows 

close affinities to 

continental mimids. 

Recent evidence 

suggests instead 

an autochthonous 

radiation in the 

Lesser Antilles by 

both species.
62

GENERAL TECHNICAL REPORT IITF-GTR-27
63

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Although confined to species-poor islands and habitats, a supertramp, by 

definition, possesses the ability to disperse over a wide geographic area. That 

the pearly-eyed thrasher (Margarops fuscatus) is the West Indies’ premier avian 

supertramp becomes obvious when its distribution and abundance are considered. 

However, no species’ current distribution or abundance is static, but rather is in 

a constant state of flux. In this chapter, the pearly-eye’s current and historical 

ranges are presented, followed by various examples of documented, historical 

and present-day range and abundance contractions and expansions. Some of the 

environmental factors and ecological correlates influencing its distribution and 

abundance are also discussed.

Current Range

Margarops fuscatus inhabits some 80 islands and cays throughout the Greater 

Caribbean Basin, from Rum Cay and San Salvador in the Southern Bahamas 

south to Bonaire, Netherlands Antilles, over a north-south geographical range of 

almost 3000 km (apps. 1, 2, and 3). The Bahamian Archipelago is the major island 

group in the northwestern extreme of the pearly-eye’s range. It encompasses about 

30 major islands and thousands of smaller cays, and has a combined landmass 

of 13 864 km2 (World of Information 1986). Elevations seldom exceed 5 to 10 m, 

except for a few ridges and hills that reach 30 to 60 m; the highest (67 m) is Mount 

Alvernia on Cat Island (Buden 1987a, 1987c). Although the pearly-eyed thrasher 

inhabits many of the smaller, species-poor islands and cays of the Southern Bahamas, it is curiously absent on the small islands and cays in the Cay Sal Bank and 

Ragged Islands, none of which has more than 11 landbird species (Buden 1987c, 

Buden and Schwartz 1986). Lack of habitat is no doubt a major factor explaining 

the pearly-eye’s absence on some of the smaller islets and cays, which are no more 

than barren rocks jutting out of the sea. Still, some of the larger cays and islets are 

scrub covered or even partially wooded, a few with patches of mangrove forest 

(Buden 1987c). Intuitively, one would think that pearly-eyes would have colonized 

these islands. It is possible that, historically, the thrasher did in fact inhabit at 

least some of the larger islets and cays, but there is no such evidence today.

Little San Salvador (24°34 ́ N. 75°56 ́ W.) is a small, uninhabited island in the 

northern Bahamas located about 15 km east of the southern tip of Eleuthera and 

20 km west of the northwestern tip of Cat Island. It has about eight species of resident landbirds (Norton 1993) and is less than 200 km northwest of San Salvador 

(= Watling Island), a potential source island on which the pearly-eye is a common 

resident (Murphy et al. 1998), and from which it could readily receive dispersing 

individuals. Yet, there are no reports of the pearly-eye from Little San Salvador 

Chapter 4: Distribution and Abundance

Margarops fuscatus 

inhabits some 80 

islands and cays 

over a north-south 

geographical range of 

almost 3000 km.
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GENERAL TECHNICAL REPORT IITF-GTR-27

Island, although ornithologists have visited it as recently as 1981 (Norton 1993). 

Little San Salvador, however, is inhabited by the Bahama mockingbird (Mimus 

gundlachii), a potential confamilial competitor that may be keeping the pearly-eye 

from successfully colonizing the island.

The pearly-eye is sparsely distributed among the Turks and Caicos Islands 

(combined landmass of 430 km2). Buden (1987c) suggested that the pearly-eye is 

possibly a recent invader to the Turks Bank, where it has been observed only on 

Grand Turk.

With the exception of Puerto Rico, pearly-eyes are noticeably absent from the 

larger, species-rich islands of the Greater Antilles such as Cuba, Hispaniola, and 

Jamaica, inhabiting only species-poor satellite islands in the region. Among the 

much smaller, species-poor Leeward and Windward Islands of the Lesser Antilles, 

pearly-eyes are more evenly distributed. They inhabit numerous islands, islets, 

and cays throughout the area but are absent from presumably habitable islands of 

the southernmost Lesser Antilles, e.g., St. Vincent, the Grenadines, and Grenada 

(Evans 1990, Raffaele et al. 1998).

Historical Range

The present-day distribution of the pearly-eyed thrasher is well known. But, has 

its geographical range changed significantly over time? One must seek answers 

to these questions from a paleogeological perspective. A complete prehistorical 

account of the radiation of the genus Margarops throughout the Caribbean is 

impossible, owing largely to the dearth of paleontological records (Olson 1978, 

Pregill et al. 1994). However, a few discoveries of prehistoric bones of both 

Margarops species from the late Quaternary derived from cultural (archaeological) excavations as well as paleontological sites (mostly owl pellets deposited in 

limestone caves and sinkholes) shed some light on the history of the genus in the 

region (see also the discussion based on results from genetic studies in chapter 3). 

Dating back some 2000+ years, the first evidence of the prehistoric occurrence 

of both species currently constituting the genus Margarops in the Caribbean 

comes from an archaeological site (recently destroyed by volcanism) located 

near Trant’s Bay, Montserrat, in the Lesser Antilles (Steadman et al. 1984b). The 

bones at the Trant’s archeological site represent the first prehistoric record for M. 

fuscus anywhere in the Caribbean, and the first regional record for M. fuscatus. 

Moreover, this may also constitute the first prehistoric record for M. fuscatus in 

the Caribbean, as the two previous reports by Wetmore (1922, 1937) of fossils from 

the Bahamas and Puerto Rico thought to pertain to the pearly-eye have been 

shown to be erroneous. Instead, the bones discovered in the Greater Antilles are 

First evidence of the 

prehistoric occurrence 

of both species of 

Margarops comes from 

an archaeological site 

located near Trant’s 

Bay, Montserrat, in the 

Lesser Antilles.
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

now thought to pertain to the Bahama mockingbird and a large species of Turdus

thrush (Olson and Hilgartner 1982, Olson in Snyder et al. 1987: 199–200). These 

results led Snyder et al. (1987: 200) to question an early arrival of the pearly-eye in 

Puerto Rico, citing the intriguing absence of pearly-eyed thrasher bones in cave 

deposits and failure by early ornithologists to report the species even after extensive fieldwork throughout the island. However, the fact that fossilized bones of the 

pearly-eye have been found in late Holocene deposits from the Burma Quarry, 

Antigua, Lesser Antilles (Pregill et al. 1988, Steadman et al. 1984a), does at least 

suggest that the species could have inhabited nearby Puerto Rico much earlier 

than previously thought.

Bones of the pearly-eyed thrasher from the late Quaternary have been found 

on Anguilla, St. Eustatius, Barbuda, Antigua, and Montserrat, whereas bones of 

the scaly-breasted thrasher, corresponding to the same period, have been found 

only in Barbuda and Montserrat (Pregill et al. 1994; and app. 1). More recently, 

Elizabeth Wing (1994, pers. comm.), Robert Chandler, and their associates at the 

University of Florida identified a bone as pertaining to the pearly-eye from an 

archaeological site on the Hope Estate, St. Martin. The bone has been dated to 

about 120 B.C. (2070 B.P.), a time when St. Martin was inhabited by ceramicproducing Amerindians represented by the Soladoid ceramic series.

The recent discoveries of the prehistoric presence of the pearly-eye on several 

islands from Anguilla to Montserrat notwithstanding, not all authors ascribe to 

the belief that the pearly-eye or even a proposed progenitor colonized the Caribbean islands from the west. Moreover, there is one integral piece of the puzzle 

missing. To date, no bones of Margarops spp. have been found anywhere in the 

Greater Antilles. To explain this enigma, Pregill and Olson (1981) postulated 

that Margarops’ ancestors arrived in the Lesser Antilles prior to the Wisconsin 

glaciation, spreading to Puerto Rico and the Bahamas quite recently in geological 

time. Likewise, Lack (1976, fig. 21: 144–145), after considering the contemporary 

range of M. fuscatus, also deduced that the pearly-eye spread northward from the 

Lesser Antilles. However, taking an alternative, and apparently correct, point of 

view, Voous (1957) assumed Margarops to be an autochthonic (indigenous) genus 

originating in the Lesser Antilles. More than 40 years later, Voous’ (1957) hypothesis was substantiated through genetic studies (Hunt et al. 2001).

In summary, a more complete history of the pearly-eye’s colonization and 

radiation throughout the Caribbean islands, including further discoveries of 

fossilized bones and ensuing interpretations to augment, and compare with, the 

more recent genetic studies, is desirable. However, whether the genus Margarops

originated in the Lesser Antilles, or instead its progenitor arrived in the region 
66

GENERAL TECHNICAL REPORT IITF-GTR-27

early in geological time is not as important as the apparent fact that the genus 

first radiated throughout the southern islands and then spread northward into the 

Greater Antilles.

Range Contractions

Written accounts by early ornithologists, taxonomists, specimen collectors, and 

others suggest that the pearly-eye once had a more extensive range throughout the 

Greater Caribbean Basin. However, like most insular birds, thrashers undergo 

population and range fluctuations throughout the region (app. 3). Bond (1979b) 

wrote that “Formerly this thrasher was...more widespread in the Greater Antilles, 

for the type locality of the nominate race was given as Hispaniola and Puerto 

Rico (Vieillot 1807; corrected date 1808-Browning and Monroe, 1991), and a 

specimen was collected at Spanish Town, Jamaica, on July 22, 1865.” Similarly, 

Wetmore and Swales (1931) noted that Vieillot (1807/1808) wrote “La grive brune 

[brown thrush] se trouve dans les grandes iles Antilles et particulierement a 

Porto-Ricco et a Saint-Domingue [Hispaniola].” Although Vieillot could have 

been referring to Beata Island specimens, it seems doubtful that he would have 

been so general in his locality description for a few records from a single location, 

especially a satellite island off the main coast of Hispaniola.

The pearly-eye experiences severe population oscillations in the southern 

extreme of its range. On Barbados, the most isolated of the Lesser Antillean 

islands, as early as 1750 at least two authors cited the presence of two species of 

“brown thrushes” (app. 3). They were most likely referring to both Margarops

species. A specimen of M. fuscatus was collected on Barbados by C.J. Manning 

in 1889 (see chapter 5 and app. 3), suggesting that it may have been resident prior 

to the 20th century. However, after failing to encounter “thrushes” of any species 

(and aware of Manning’s specimen), Feilden (1889) concluded that M. fuscatus

was no more than a straggler to the island. Both species of Margarops are now 

very rare (Evans 1990) or extirpated (if once resident) on Barbados as none was 

observed by the present author in 1984 or by D. W. Buden in 1991 (Buden 1992a, 

pers. comm.).

There is little information documenting the history of the pearly-eyed 

thrasher on St. Vincent. However, a few published accounts suggest that the 

species may once have been resident but is now extirpated. Unfortunately, the few 

published records that do exist are often ambiguous or even contradictory as to 

the thrasher’s historical status on St. Vincent. In his early Check-lists of the Birds 

of the West Indies, Bond (1945, 1950, 1956a) did not list the pearly-eyed thrasher 

as occurring on St. Vincent, either as a resident or vagrant. Later, though, Bond 
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

(1957) reported a specimen from St. Vincent (April 23, 1890) examined in the British Museum by D.W. Smith. Ironically, Bond (1960, 1971a) then listed the thrasher 

as resident on St. Vincent in two early editions of his field guide to West Indian 

birds. Some years later, Bond (1977: 2) specifically stated that he considered M.

fuscatus as a vagrant on St. Vincent. Unaccountably, however, in the same supplement (1977: 11) in a discussion of how thrasher numbers had been fluctuating 

among various islands for the past 50 years, he stated that the thrasher “...may be 

extirpated on St. Vincent...” thus connoting that it had been resident at one time. 

Later still, Bond (1979a) listed the pearly-eye as resident south to St. Vincent, and 

accidental (vagrant) on Barbados. Devas (1970), however, in his book of the birds 

of Grenada, St. Vincent, and the Grenadines, did not list the pearly-eye from any 

island in that group. Lack et al. (1973) did not observe the pearly-eyed thrasher 

in St. Vincent during their visits, which included transect censuses in March and 

July 1973, usually periods of peak pearly-eye activity. A decade later, Faaborg 

and Arendt (1985) did not observe a single pearly-eye during transect censuses 

in July 1983. Nor was a single individual captured during banding operations in 

March and April 1984, although some 300 birds were captured at two montane 

forest sites. Presently, the AOU’s (1998) Checklist of North American Birds cites 

the pearly-eye’s range as south to St. Lucia, with a disjunct population inhabiting islands north of Venezuela from Bonaire east to Los Hermanos Islands. It 

certainly is not obvious from published accounts whether or not the pearly-eyed 

thrasher has ever been resident on St. Vincent.

The pearly-eyed thrasher has been extirpated on La Horquilla Island in 

Los Hermanos Archipelago, Venezuela (Meyer de Schauensee and Phelps 

1978, Paynter 1982, Phelps and Phelps 1963). It is noteworthy that there is some 

confusion regarding the identity of La Horquilla Island. It is sometimes spelled 

“Orquilla” and should not be confused with Orchila Island just east of Los 

Roques group about 117 km north of Cape Codera, Venezuela (Cory 1909).

Range Expansions

Bahamian Archipelago

The pearly-eyed thrasher has undergone population declines on some islands, 

resulting in notable geographical contractions in several areas of its traditional 

range. In the northern portion of its range, however, there have been increased 

sightings of dispersing individuals on the larger Bahamian islands (documented 

by several observers—see app. 3). This has led some authors, e.g., Brudenell-Bruce 

(1975), Buden (1987c; 1990; 1988, D.W. Buden 1988, in litt.), and Paulson (1966), to 

conclude that the pearly-eye is spreading its range northward. Undoubtedly, some 

The pearly-eye is 

spreading its range 

northward and 

increasing in number 

in the south.
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GENERAL TECHNICAL REPORT IITF-GTR-27

of the more recent sightings in the northern Bahamas may be due simply to an 

increase in the number of observers. However, two long-time residents and a third 

regional specialist have assured me that during the past couple of decades they 

have been observing thrashers more frequently in the northern islands. In addition, Christmas bird counts have been conducted in the Northern Bahamas for 

many decades without sightings of pearly-eyes in the early years.

Like other mimids, e.g., northern mockingbird Mimus polyglottos (see for 

example David et al. 1990), some populations of the pearly-eyed thrasher have 

increased in response to clearings in forest cover, expanding agriculture, and 

urban development. Pearly-eyes take advantage of the seeds, fruits, and associated insects of a wide variety of native, orchard, garden, and ornamental plants 

in secondary forests and a wide variety of altered (e.g., urban) habitats. Historical 

records show that in Puerto Rico, Montserrat, and Bonaire the pearly-eye has 

undergone significant intraisland population increases within the last 30 to 60 

years (app. 3).

Puerto Rico

In Puerto Rico, for more than a century, pearly-eyes were scarce and only patchily 

distributed in the lowlands and hills. However, by the 1920s, resident biologists 

noted a steady increase in thrasher numbers throughout the island. Struthers 

(1923) reported the pearly-eye as “common” in montane forest at Maricao, and 

Wetmore (1927) listed the species as “quite plentiful” in montane forest in the 

upper Toro Negro area. By 1930, thrashers were reported up to 600 m in the Sierra 

de Luquillo (Danforth 1931). By the early 1950s, pearly-eyes became abundant in 

the Sierra de Luquillo, particularly within the confines of the Caribbean National 

Forest, also known as the Luquillo Experimental Forest (Snyder et al. 1987). It 

is noteworthy that the “explosion” (sensu Snyder et al. 1987) of pearly-eyes in the 

Sierra de Luquillo in the early 1950s closely followed Operation Bootstrap, a government-initiated program implemented to create more jobs on the outskirts of 

larger cities. This program culminated in the immigration of thousands of people 

to larger cities and subsequent urban sprawl. It is possible that urban development 

augmented thrasher populations by creating more food resources and nest sites. 

Dispersing young then could have easily emigrated to the nearby, species-poor 

Sierra de Luquillo, which might have acted as an ecological sink for dispersing 

individuals. Today, the pearly-eye is a common resident throughout the island 

(Raffaele 1983, Raffaele et al. 1998).
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Montserrat

In Montserrat, pearly-eyes were considered common for the first three-quarters 

of the 20th century (Bond 1945, Danforth 1939b, Terborgh and Faaborg 1980). 

However, thrasher populations greatly increased in the late 1970s and early 1980s 

and have continued to increase ever since (f. Fred Payne, forest guard; and Franklin Margetson, Montserrat National Trust, 1984, pers. comm.). The pearly-eye’s 

recent, marked increase in Montserrat led Steadman et al. (1984b) to consider it 

as the island’s most common forest bird. This finding has been corroborated by 

subsequent researchers (e.g., Arendt 1990, Arendt et al. 1999, Faaborg and Arendt 

1985), all of whom also considered the pearly-eye as one of Montserrat’s most 

abundant passerines. With the possible exception of the bananaquit (Coereba 

flaveola), the pearly-eye may well be the island’s most abundant landbird, 

even more so than the zenaida dove (Zenaida aurita) or common ground-dove 

(Columbina passerina), both of which are generally confined to lowland forest, 

agricultural lands, coastal, and residential areas. It is noteworthy that populations 

of the usually abundant and ubiquitous bananaquit are noticeably small in some 

habitats on Montserrat (pers. obs.), often resulting from pearly-eye nest predation 

(discussed further in chapter 8).

Bonaire

Historical accounts of the pearly-eyed thrasher on Bonaire suggest that populations, once quite restricted in number and range, have recently increased and 

spread throughout the island (app. 3). The first author to mention the thrasher’s 

status on Bonaire was Hartert (1893a; specimen collected). He reported that 

the species was restricted to the Fontein Plantation. More than 50 years later, 

Phelps and Phelps (1948), although having collected specimens of other species 

throughout the island, found the pearly-eye still restricted to the Fontein Plantation (Voous 1983). However, a contemporary resident, (the late) Frater (Brother) 

Candidus van der Linden (1988, and C.V.D. Linden 1992, in litt.) reported the 

pearly-eye as “...not abundant, but rather evenly dispersed throughout the hilly, 

rocky parts of the island.” (fig. 4.1). Although he concedes that the pearly-eye 

may nest in cavities and construct open-stick nests in Bonaire as it does on other 

islands, in his more than 20 years of observations, he has found them nesting only 

in “steep rock walls” at four locations (see fig. 4.1).

To update the pearly-eye’s status on Bonaire, I visited the island for a week in 

July (25–31) 1997. The pearly-eye’s status had not changed from that observed in 

the early 1990s. It was still common within the confines of the Fontein Plantation 

near water, but uncommon and evenly dispersed over most of the remainder of 
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GENERAL TECHNICAL REPORT IITF-GTR-27

the island, much of which is quite arid. Fontein was too small for an adequate 

point-count census, so a mist-net sample was taken. Pearly-eyes constituted 19 

percent of the 68 captures of 15 species. Pearly-eyes, and many other species 

as well, are abundant in the Fontein Plantation because the owner maintains a 

year-round watering fountain for his livestock. That many birds of several species 

are attracted to the permanent water source is evident from the cumulative species 

curve presented in figure 4.2. After 100 net-hours, the number of new species continued to increase despite the fact that almost 70 individuals had been captured. 

Figure 4.1—Contemporary distribution of the pearly-eyed thrasher on Bonaire, Netherlands Antilles, with documented records of nesting outside 

of the Fontein Plantation. Thrashers are especially common in and around human settlement, where they take advantage of additional food, 

water, and nesting sites. The original sketch and observations were supplied by (the late) Frater Candidus van der Linden, a long-time resident, 

author, and authority on Bonaire’s birds; he also had a broad knowledge of the island’s flora and other fauna.
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Another attribute of the plantation was the tall stature of many of the trees in 

the immediate vicinity of the fountain. Of 10 trees sampled, the mean height was 

14.9 m (SE = + 2.6; range: 8.4 to 21). Tree height was considerably greater at the 

fountain than in the Washington-Slagbaai National Park (see discussion below).

To compare the pearly-eye’s abundance in a more natural setting, the author 

conducted a point-count census within the Washington-Slagbaai National Park 

(fig. 4.1). In total, 40 points were established throughout the park, and each was 

visited once. Tree height within a 25-m radius around each point was measured. 

Average tree height (n = 160; 4 trees/point) was 5.6 m (SE = + 0.28; range: 2.5 to 9). 

Thus, in general, tree height within the park was much lower than that within the 

Fontein Plantation as discussed above.

Of 295 detections involving 26 species of birds, 8 species comprising 215 (73 

percent) of the detections were common enough to compare species’ numbers. As 

suspected, the pearly-eye was not at all common in the park. Only six (3 percent) 

of the 215 individuals detected were pearly-eyes (fig. 4.3a). Whereas all but two 

species (smooth flycatcher, Sublegatus, gen. nov. arenarum, syn. modestus), and 

troupial, Icterus icterus) averaged almost 1 to 2 individuals per point, the mean 

number of pearly-eyes per point was only 0.13 (fig. 4.3b). Moreover, whereas half 

Figure 4.2—Cumulative species curve resulting from 100 net-hours of banding at a water trough on the 

Fontein Plantation, Bonaire, Netherlands Antilles. That an upper asymptote was not reached, even after 

the capture of almost 70 individuals, attests to the importance of the site for local birds.
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GENERAL TECHNICAL REPORT IITF-GTR-27

Figure 4.3—Comparative abundance of the pearly-eyed thrasher (PETH) and the seven most common 

species found within the Washington-Slagbaai National Park, Bonaire. Few thrashers were observed and 

had a limited, patchy distribution within the park. All pearly-eye sightings were at, or near, the park’s two 

permanent sweet-water wells (Pos Mangel and Put Bronswinkel). Because Bonaire is not part of the West 

Indian faunal region per se, the smooth (or northern scrub) flycatcher (Sublegatus, gen. nov. arenarum, 

synonym. modestus) (SMFL) and the Caribbean parakeet (CAPA) are not listed in appendix 1 as are the 

remaining species in this figure. The parakeet is classified as Aratinga pertinax and is also known as the 

brown-throated parakeet (AOU 1998). In graph 4.3C, “f (25 m)” is the frequency of occurrence at ≤25 m.
73

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

(4) of the other 7 species were detected at 40 percent or more of the 40 points, 

pearly-eyes were detected at only five (13 percent) of the 40 points (fig. 4.3c).

Information gathered through interviews with local residents living in urban 

areas suggests that pearly-eyes are attracted to the more settled areas, taking 

advantage of the increased human-supplied water and food sources. Those 

interviewed mentioned seeing thrashers drink from seasonal standing water on 

concrete surfaces or from other human-made structures such as open drains. 

Thrashers are often observed eating the fruits of citrus and ornamental trees, and 

they readily prey on the increased populations of vertebrates and invertebrates 

associated with such human-induced food sources. Frater van der Linden (1992, 

in litt.) also mentioned that pearly-eyes congregate in and around residential areas 

especially during the dry season (see discussion under “Pugnacity” in chapter 8). 

As on other Caribbean islands, it is highly probable that increased human settlement is a major cause for the pearly-eye’s population expansion on Bonaire.

Concluding from the examples outlined above, it appears that the pearly-eye is 

increasing and extending its numbers at both the northern and southern extremes 

of its current range.

History of Regional Abundance

Virgin Islands

The pearly-eyed thrasher inhabits numerous islands and cays, all varying in size 

and topography, along the full length of the Caribbean Islands Archipelago. 

Thrasher abundance differs significantly throughout the region. For many years, 

the status of thrasher populations on various islands has been summarized by 

using general terminology, e.g., rare, uncommon, common, or abundant. For 

example, in the U.S. Virgin Islands, thrashers are relatively abundant on all 

islands (University of the U.S. Virgin Islands 1988). Robertson (1962) reported the 

pearly-eye as “...probably the most abundant bird on St. John and certainly the 

most conspicuous land bird.” He found it from the coast to above 360 m on Bordeaux Mountain and at every locality visited except in the dense stands of cactus 

around Concordia Bay and Ram Head. Today, the pearly-eyed thrasher is the 

most abundant landbird in the U.S. Virgin Islands (Norton 1980). In the neighboring British Virgin Islands (BVIs), pearly-eyes are most abundant on Tortola and 

Virgin Gorda. On Tortola, the pearly-eye is most abundant in coconut plantations 

and at mid elevations, where it is the most numerous landbird in the taller, denser 

vegetation (Mirecki et al. 1977). Its preference for taller vegetation may account 

for the thrasher’s scarcity or even absence on the smaller, flatter, windswept 

islands of the BVI group (Mirecki et al. 1977).

Thrasher abundance 

differs significantly 

throughout the region.
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GENERAL TECHNICAL REPORT IITF-GTR-27

Sombrero Island and Anguilla

Status and abundance of the pearly-eye appear uncertain in the BVI’s two most 

pelagic islands, Anegada and Sombrero. Nichols (1943) reported pearly-eyes from 

the BVIs including Anegada. LaBastille and Richmond (1973) reported a single 

individual near a freshwater pond in May 1970. However, the species’ residency on 

Anegada is doubtful. More recently (7 to 8 October 1994), not a single individual 

was observed during a 2-day census (Arendt 1995, unpubl. rep.). It is possible that 

pearly-eye sightings on Anegada constitute dispersing individuals that apparently 

cannot establish a resident population.

There is but a single record of the pearly-eye from Sombrero Island. Lawrence 

(1867) reported that A. Julien collected a specimen from the island on 28 September 1863 (see also Ogden et al. 1985). On nearby Anguilla, by the late 1800s pearlyeyes were becoming rare, possibly owing to the island’s low relief and conversion 

of much of the land to cattle grazing (Sclater 1892). Although the literature is 

scant regarding recent avian studies from Anguilla, Raffaele et al. (1998) considered the pearly-eye common there. More recently, as part of the Important Bird 

Areas initiative, Julian Hughes visited Anguilla during January and February 

2000. On behalf of the Anguilla National Trust, Hughes produced the country’s 

first checklist (for more details, see: http://www.birdtours.co.uk/tripreports/caribbean/anguilla/index.htm. Hughes reported the pearly-eyed thrasher as “Locally 

common throughout Anguilla, in scrub and trees around human settlements. The 

north and west of the island appears to contain the highest numbers.” Thus, as on 

other islands, the pearly-eye has recovered by taking advantage of human-altered 

habitats and the increased food and nesting resources accompanying continued 

development.

Netherlands Antilles

Among the northern Netherlands Antilles (combined landmass of 993 km2), the 

pearly-eyed thrasher is more common in dense vegetation at higher elevations. On 

St. Martin, Danforth (1930b) reported thrashers locally common in the denser, 

brushy woods remaining on the higher peaks (max. elev. 424 m). Voous (1983) 

reported pearly-eyes common on this species-poor island, stating that the only 

common landbirds besides thrashers are bananaquits, gray kingbirds (Tyrannus 

dominicensis), black-faced grassquits (Tiaris bicolor), and common ground-doves. 

On Saba, Danforth (1938) reported the pearly-eye as the most common landbird, 

found from sea level to the highlands (max. elev. 870 m). It was found even in 

cloud forest at the highest elevations. On nearby St. Eustatius, Voous (1983) 

reported pearly-eyes as less common in the desert scrub, and more common at 
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

higher elevations of the Little Mountains, especially in the remnant rain forest 

within the cone of the extinct Quill volcano (600 m elev. at its upper rim).

St. Kitts and Nevis

Pearly-eyes have been reported as common on St. Kitts and Nevis, the two 

remaining high relief islands of the Leewards. On St. Kitts, Danforth (1936a) 

reported the pearly-eyed thrasher as “...abundant in the high forest on the 

Crater....” On Nevis, Danforth (1936a) reported the thrasher as the only common 

bird in montane forest. Lawrence (1879a) reported that pearly-eyed thrashers were 

becoming less common on Antigua by the end of the 1800s, being found only in 

the valleys among the southern hills where rivulets and trees remained.

Need for More Standardized Census Methods

As informative as the preceding historical and contemporary accounts are 

concerning the pearly-eye’s numbers on various islands, the terms used by the 

different authors such as “common” and “abundant” are relative and differ from 

person to person. Clearly, there is a need for a quantitative means of assessing 

thrasher populations. However, arriving at such a goal is not an easy matter 

(review methodologies and techniques in Ralph and Scott 1981). I have assessed 

thrasher populations quantitatively by using a combination of capture-recapture 

and standardized aural-visual census methods (see Arendt 1993 for methodology).

Comparisons of Regional Abundance

Mist Netting

Following standardized mist-netting procedures and regimes used as early as 1972 

by John Terborgh and his students from Princeton University (Terborgh 1973, 

Terborgh et al. 1978), I began sampling Caribbean birds in 1979. For the present 

study, netting was used through 2000 to census the relative abundance (total number of captures relative to all captures) of resident forest birds, including pearlyeyes, in wet- and dry-forest habitat on 20 West Indian islands (for particulars, see 

app. 2, Faaborg 1985, Faaborg and Arendt 1985, Faaborg et al. 2000). Results 

of dry vs. wet-forest comparisons for 21 sites on 12 of the 20 sampled islands 

harboring resident thrasher populations are presented in figure 4.4. Numbers of 

pearly-eyes captured ranged from 1 individual (Antigua, Guadeloupe, and Marie 

Galante) to 174 individuals (Mona—discussed under “Density Compensation” 

below) per banding session in dry forest, and from 0 individuals (Dominica and 

Guadeloupe) to 82 individuals (Jubilee Heights, Montserrat) per site during each 

session in mesic and wet forest. Capture rates of all individuals of every species 
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GENERAL TECHNICAL REPORT IITF-GTR-27

per 3-day netting session ranged from 66 (Cas-en-Bas, St. Lucia) to 474 individuals (trail to Darby Cave, Barbuda) per site in dry forest, and from 25 (Chances 

Peak, Montserrat) to 300 individuals (Quilesse, St. Lucia) per site in wet forest.

Excluding density-compensation islands such as Mona, in the absence of 

ecological stressors, e.g., prolonged droughts (see below), relative abundance 

of pearly-eyed thrashers ranges from <1 to 45 percent in dry forest and <1 to 

59 percent in wet forest. To date, dry forest (Guana Island) is the habitat type 

in which I have found the highest relative abundance of thrashers (see “Point 

Counts” section). Nonetheless, on a regional scale, pearly-eyes tend to be 

consistently more abundant in wet-forest habitats.

The pearly-eye’s abundance in wet-forest habitat, especially at mid elevations, has been observed in the past, e.g., Kepler and Kepler (1970) in Puerto 

Figure 4.4—Capture rates (number of individuals) and relative abundance (percentage of total captures) of pearly-eyed thrashers at 21 sites on 

12 West Indian islands in which the pearly-eye breeds. Results are based on a standardized mist-net sampling technique (see “Study Area and 

Methods” for details). In dry forests, pearly-eyes reached their highest relative abundance on islands in which, because of ecological release, it 

undergoes density compensation (e.g., Mona and Montserrat). In wet forests, pearly-eyes can constitute as much as 60 percent of the total captures. 

Respectively, island and forest type numeric and alpha codes follow appendix 2: 79a Basse Terre; 131b Caribbean National Forest; 112b Chances 

Peak; 112c Dannenborg Estate; 156d Forestière; 112e Garibaldi Hill; 93 Grand Anse; 131c Guánica; 112g Jubilee Heights; 112h Killiekrankie; 155a 

Lodge’s Estate; 48a Morne Plaisance; 11 NE near prison; 156g Pitón Flore; 111 Plateau; 105 Pointe des Basses; 156h Quilesse; 7a Shirley Heights; 9 

Trail to Darby Cave; 112m Upper Pond; and 112n Wilkes’s Ghaut.
77

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Rico, Diamond (1973b) in St. Lucia, and Lack (1976: 366) in Dominica. This is 

not surprising because as a secondary cavity nester (Arendt 2004a), the thrasher 

prefers large trees which, generally, are most common in wet forests found at mid 

elevations.

The pearly-eye’s high relative abundance in wet forest may be partially 

explained in terms of its supertramp status. It is well known that the wet forests 

of West Indian islands are generally more depauperate in bird species than dry 

forests (Terborgh and Faaborg 1980). As a supertramp, Margarops should survive 

better in species-poor communities as well as on species-poor islands. As an 

example, in Puerto Rico, there are about 20 species of resident forest birds in the 

Sierra de Luquillo, which harbors wet and hygrophylous forest types, whereas 

there are some 40 resident species in dry forest within the Guánica Commonwealth Forest and Biosphere Reserve (hereafter, Guánica Biosphere Reserve) 

(Hernández-Prieto 1993, Kepler and Kepler 1970, and Faaborg et al., ongoing 

monitoring).

On a regional scale, Terborgh and Faaborg (1980: fig. 1) used mist-net censuses to show that species richness is greater in dry than in wet-forest habitat. 

They postulated that a limit to the tolerance of competitors imposes a ceiling on 

diversity, offering many examples. Their discussion also included special note of 

the pearly-eye’s distribution.

Based on mist-net captures on the 12 sampled islands in which the pearly-eye 

is resident, it reached its highest relative abundance in the wet forests of Puerto 

Rico and Montserrat (fig. 4.4). In Puerto Rico, within the Sierra de Luquillo, the 

pearly-eye is most abundant in the colorado (Cyrilla racemiflora L.) forest type at 

mid elevations, although it is found from forest edge to cloud forest (Snyder et al. 

1987, Wiley and Bauer 1985, pers. obs.). As an example, in colorado forest at elevations between 500 and 800 m, the relative abundance of the pearly-eyed thrasher is 

about 30 percent (penultimate bar—no. 4 “El Yunque”—in “Wet forest” fig. 4.4). 

Although capture rates fluctuate greatly among seasons, it is possible to capture 

as many as 25 to 30 pearly-eyes (Arendt, unpubl. data) during a standard 3-day 

netting period. This is quite a substantial total capture in comparison with the 

numbers captured in other forest types within this forest. Contrarily, within the 

tabonuco (Dacryodes excelsa Vahl.) forest type near the forest’s border at Sabana, 

located at an elevation of about 350 m, Faaborg and Terborgh (1980) failed to 

capture a single pearly-eye using the same 3-day netting regime.

On Montserrat, during banding operations in 1975, of 209 wet-forest birds 

sampled, 41 (20 percent) of the captures were pearly-eyes (Terborgh et al. 1978: 

table 4). Almost a decade later (1984), I sampled birds in dry and wet-forest 

The pearly-eye’s high 

relative abundance in 

wet forest is partially 

explained in terms of 

its supertramp status. 
78

GENERAL TECHNICAL REPORT IITF-GTR-27

habitats over most of the island. In net samples taken at Fox’s Bay and within 

each of Montserrat’s three major hill regions (Centre, Soufrière, and South Soufrière Hills), pearly-eye numbers had increased to the point that they made up 

about 50 percent of the captures at three of the six banding sites. Moreover, of 

the 389 birds captured at all 6 sites, 152 individuals (39 percent) were pearly-eyes 

(Faaborg and Arendt 1985). In March 1990, 6 months following Hurricane Hugo 

(September 18, 1989), mist netting was virtually impossible because of the volume 

of felled trees and debris left by the storm. However, I conducted standard mist-net 

sampling at three of the original six banding sites: Upper Pond, South Soufrière 

Hills (cloud forest), Jubilee Heights, Centre Hills (wet forest), and Garibaldi 

Hill (dry forest). On average, pearly-eyed thrashers constituted 43 percent of the 

captures at the three sites: 44, 65, and 21 percent, respectively (note the significant 

decrease at the species-rich dry-forest site). The pearly-eye may have reached the 

carrying capacity of the remaining habitats on Montserrat.

Of the 12 wet-forest sites sampled on 5 islands, the relative abundance of the 

pearly-eyed thrasher was low at 6 sites on 4 islands (St. Lucia, Dominica, St. Kitts, 

and Montserrat cloud forest) (fig. 4.4). Species richness is relatively high in the 

wet forests of both St. Lucia and Dominica, which may partially account for the 

thrasher’s scarcity in such habitats on those islands. The single site within Montserrat’s wet-forest habitats with a low relative abundance was in cloud forest at the 

summit of Chances Peak, the island’s highest peak (about 900 m prior to the 1990s 

volcanic eruptions). Thrashers are usually scarce at higher elevations in general, 

and in cloud forest in particular (see Beissinger et al. 2005 and Cook et al. 2003, 

2004, 2005, for a plausible explanation). The pearly-eye’s low numbers in the wet 

forests of St. Kitts is somewhat puzzling, especially as species richness is also low.

Variable-Width Line Transects

Rain-forest habitat—

Because of the inconspicuousness of many forest birds, aural-visual techniques 

often overlook the more secretive individuals. Thus, mist netting is often preferred 

when the main objective is to sample understory birds, especially the more reticent 

species. However, there are inherent problems associated with mist netting, especially in tall-statured forests and when time and personnel are extremely limited 

(see Waide and Hernández-Prieto 1981 for a comparative review of census methods 

and their tradeoffs, emphasizing Puerto Rican rain-forest vegetation). In these 

situations, mist netting should be used in conjunction with aural-visual census 

methods to obtain a reliable estimate of species richness and relative abundance 

of each species.
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

For years, many ornithologists interested in estimating densities per unit area 

favored a variable-width line-transect census technique designed by Emlen (1971) 

to census terrestrial bird populations. Working within the Luquillo Experimental 

Forest, Puerto Rico, Snyder et al. (1987) used a modified “Emlen method” in conducting 12 monthly censuses between 1970 and 1971. They found that, of 17 resident species, the pearly-eye was among the three most abundant in colorado forest 

(preferred habitat), and among the four most abundant in tabonuco forest (second 

most preferred habitat), but rather sparse in sierra palm (Prestoea acuminate var.

montana [Graham] Nicholson) and cloud (dwarf) forest (Snyder et al. 1987: table 

4.4). A decade later, Arendt conducted 45 monthly censuses (15 per year, 5 in each 

forest type) between March and July from 1980 to 1982 in the previous authors’ 

four study areas. However, only two censuses were carried out in cloud forest 

habitat (El Toro Trail) in May and June 1982. During the more than 10-year lapse 

between censuses, thrasher populations almost doubled in all four sampled areas 

(table 4.1) but, remained scarce in sierra palm and cloud forest. To make a 10-year 

comparison with the 1980, 1981, and 1982 results, and about a 20-year comparison 

overall, Emlen censuses were conducted in May and June of 1992. Results show 

that during the third decade, the thrasher population in the Sierra de Luquillo 

leveled off, and now, as a result of constant-effort monitoring, there are signs that 

it is on a slight decline (see text under “Study Area and Methods”).

Table 4.1—Twenty-two-year comparison of pearly-eyed thrasher abundancea in 

the Sierra de Luquillo, Puerto Rico

Forest type

Years Tabanuco Colorado Sierra palm Cloud Reference

1970–71 8.0 8.9 1.5 0.9 Snyder et al. (1987)

1980–82 13.5 17.5 2.2 1.5 This study

(9–26) (5–17) (1–7) —

1992 12.7 19.3 2.9 1.3 This study

(3–15) (9–28) (1–11) (0–3)

— = no data. aTabular entries are mean number of thrashers seen and heard per kilometer by using the Emlen (1971) 

line transect census method. Numbers in parentheses are minimum and maximum numbers of individuals 

detected. All minimum numbers occurred before the main breeding season (e.g., November–March), 

whereas all maximum numbers occurred during the main breeding season (May–June).

Each year, the presence of 30 to 40 thrasher nest boxes, forming part of my 

long-term thrasher research within a few kilometers of the census areas, may 

account, at least in part, for the precipitous increase during the first decade following Snyder et al.’s censuses. Also, each year beginning in 1972, thrasher boxes 

have been available for occupancy in known and potential Puerto Rican parrot 

Over the past three 

decades the thrasher 

population in the 

Sierra de Luquillo 

leveled off, and is now 

on a slight decline.
80

GENERAL TECHNICAL REPORT IITF-GTR-27

(Amazona vittata) nesting areas (James Wiley 1992, pers. Comm.; Victor Cuevas 

1992, pers. Comm.; and personal involvement). However, in light of the exceedingly 

high densities of thrashers recently found on other islands (see point-count results 

below), it is unlikely that a few artificial nest boxes have significantly increased the 

pearly-eyed thrasher population in the Sierra de Luquillo (see also discussion in the 

“Preface”).

Dry-forest habitat—

In dry forest within the Guánica Biosphere Reserve, when Kepler and Kepler 

(1970) assessed species richness and relative abundance of its forest birds, the 

relative abundance of the pearly-eye was <1 percent (10 thrashers observed out of 

1,821 individual birds). Twenty-eight years of mist-net censuses reflect an average 

relative abundance of about 7 percent (134 thrashers out of about 2,000 total new 

captures). Thus, although aural-visual and mist-net census techniques are not 

comparable, both methods in the above examples show that the Guánica dry-forest 

thrasher abundance is low—much lower than in the wetter Sierra de Luquillo.

In conclusion, as evidenced by numerous investigators working in several 

different localities, pearly-eyes are more abundant in a species-poor, highland 

rain forest than in a more species-diverse, lowland dry forest in Puerto Rico, but 

thrasher populations fluctuate widely on a temporal basis. This is a common 

characteristic of r-selected, tramp species.

Point Counts

Hutto et al. (1986) designed a fixed-radius point-count method that can be used to 

generate data for either a density estimate or a relative index of bird abundance. 

Because of its convenience (cost effective in personnel and time spent in the field), 

it gained popularity over the years and was often preferred in comparative studies 

of bird populations for almost a decade before the advent of the more recent distant 

sampling method (Buckland et al. 1993). To compare data gathered by others on 

the abundance of the pearly-eye in Puerto Rico (tabonuco forest), St. John, and St. 

Thomas (dry and mesic forest), I conducted a series of fixed-width point counts in 

additional forest types (colorado and cloud forest) in Puerto Rico (1989 to 2000) 

and in dry- and wet-forest types in Montserrat (1990, 1997).

As shown previously by mist-net censuses, with the exception of Guana Island 

(see below), results from point-count censuses also show that the pearly-eye 

reaches its highest abundance in the dry and wet forests of Montserrat (table 4.2 

and fig. 4.5). Note that in Puerto Rico’s tabonuco and cloud forest types, thrasher 

abundances are lower than all four dry-forest sites in Montserrat (table 4.2). To 

attach significance to the abundance results shown in table 4.2, a Kruskal-Wallis 
81

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

One-way ANOVA on Ranks test (hereafter K-W ANOVA) was conducted by 

using the mean number of detections per count within a 25-m radius of each point 

(H = 466.4; df = 9; P <0.001). A matrix of pair-wise comparison probabilities 

resulting from Tukey’s HSD (post-hoc) Multiple Comparisons Test is presented 

(table 4.3). Results show that, in general, the pearly-eye is significantly more 

abundant in moist and wet forest than in dry forest. But, there are exceptions (see 

below). Thrashers are significantly more abundant in Montserrat wet forest than 

in wet forest on any other island that I have sampled. Likewise, the pearly-eye is 

significantly more abundant in Montserrat’s dry forest than in either of the U.S. 

Virgin Island’s dry forests.

In 1994, the pearly-eyes inhabiting tiny (299 ha) Guana Island (xeric vegetation with a max. elev. of 245 m) in the BVIs were sampled by using a fixed-radius 

point-count census method (Arendt 1995, unpubl. rep.). The mean number of 

individuals per 25-m-radius point count was 2.12, the highest number yet recorded 

for either dry or wet forest (compare with table 4.2 means). Thus, whereas thrasher 

abundance is generally higher in wet-forest habitat, there are notable exceptions. 

For example, although he used a somewhat modified point-count census method, 

Table 4.2—Descriptive statistics of thrasher abundance derived from fixedradius point-count censuses in dry and wet forests on five West Indian islands 

between 1987 and 1994a

Number of Number of individuals per point

Forest type and island points (n) Mean ±SE Mode

Dry forest:

80.b Guana Island (BVIc) 60 2.12 0.22 2

112. Montserrat (e) 30 1.24 .10 1

154. St. Johnd (a) 107 .81 .06 1

158. St. Thomasd (a) 18 .61 .16 0

Moist and wet forest:

112. Montserrat (f, g, i, j, k, l, n) 50 1.89 0.19 2

131. Puerto Rico (b) 317 1.58 .07 1

154. St. Johnd

 (b) 181 1.4 .06 1

112. Montserrat (a, b, m) 40 1.08 .1 1

158. St. Thomasd (b) 37 .89 .12 1

131. Puerto Rico (a) 309 .24 .03 0

131. Puerto Ricoe (c) 293 .2 .03 0

aSee table 4.3 for a statistical comparison of thrasher numbers within and among forest types on

four of the five islands.

bRespectively, island and forest type numeric and alpha codes follow those in appendix 2.

cBVI (British Virgin Islands).

dData for St. John and St. Thomas, U.S. Virgin Islands (1987, 1990, 1992), courtesy of R.A. Askins, 

D.N. Ewert, and R.L. Norton.

eData for (131c) tabonuco forest in Puerto Rico (1989-1991), courtesy of R.B. Waide.
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GENERAL TECHNICAL REPORT IITF-GTR-27

by sampling four sites within the Guánica Biosphere Reserve (some very close to 

Faaborg and Arendt’s mist-netting sites), Hernández-Prieto (1993: appendices J 

through O) found large numbers of pearly-eyes at several well-forested sites. He 

also showed that as species richness increases, the number of thrashers decreases 

(Hernández-Prieto 1993: fig. 8b), thus lending additional support to the supertramp concept.

Figure 4.5—Relative abundance of pearly-eyed thrashers detected during fixed-radius (≤25 m) point-count 

censuses in dry and wet forests on five islands. With a few notable exceptions, in general, the pearly-eye 

is more abundant in moist- and wet-forest rather than in dry-forest habitats. Pearly-eyes are especially 

abundant in the dry and wet forests of Montserrat, an island on which the thrasher undergoes density 

compensation as a result of ecological release. Island and forest type numeric and alpha codes follow 

appendix 2: 158a fragments of dry forest; 154a extensive tracts in dry evergreen woodland; 112e Garibaldi 

Hill (trail to Fox’s Bay); 80 dry forest (Guana Island); 131c Guánica; 131a Cabezas de San Juan lighthouse, 

Fajardo; 158b fragments in moist forest; 112b Chances Peak, summit near fresh water pond (Soufrière 

Hills); 154b extensive tracts in moist forest; 131b Colorado forest (Cyrilla racemiflora L.) Icacos Valley, 

Luquillo Experimental Forest; 112g Jubilee Heights (Centre Hills). Capped vertical bars (⊥, ⊺)) are ±1 

standard errors. Data for St. John and St. Thomas, U.S. Virgin Islands, were supplied by R.A. Askins, 

D.N. Ewert, and R.L. Norton; for the two islands, 207 samples were taken, averaging 52 (min. = 35, max. = 

75) samples per habitat. Data for Puerto Rico’s tabonuco forest type were supplied 

by R.B. Waide (n = 293 samples).

As species richness 

increases, the number 

of thrashers decreases 

and it undergoes 

density compensation 

on Mona Island.
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Table 4.3—Comparison of pearly-eye relative abundance in dry and wet forest habitats on four West Indian 

islands.a Presented is a matrix of pair-wise comparison probabilities (α = 0.05) resulting from Tukey’s HSD 

post-hoc multiple comparisons test. Clearly, pearly-eyed thrasher abundance varies significantly among 

islands and forest types

Forest typea

MTDRY SJDRY STDRY MTCDF PRCDF PRCF PRTF MTWET SJWET STWET

MTDRY 1.00

SJDRY .00 1.00

STDRY .00 .08 1.00

MTCDF .97 .87 .23 1.00

PRCDF .00 .00 .00 .00 1.00 

PRCF .00 .77 .98 .36 .48 1.00

PRTF .00 .00 .00 .00 .99 .17 1.00

MTWET .00 .00 .00 .02 .00 .00 .00 1.00

SJWET .57 .00 .00 .89 .00 .00 .00 .03 1.00

STWET .13 .99 .41 .99 .00 .71 .00 .00 .00 1.00

aRespectively, island and forest type numeric and alpha codes follow those in appendix 2: 112(e) Montserrat dry forest (MTDRY); 

112(a, b, m) Montserrat cloud forest (MTCDF); 112(f, g, j, k, l, n) Montserrat moist and wet forest (MTWET); 131(a) Puerto Rico cloud 

forest (PRCDF); 131(b) Puerto Rico colorado forest (Cyrilla racemiflora L.) (PRCF); 131(c) Puerto Rico tabonuco forest (Dacryodes 

excelsa Vahl.) (PRTF); 154(a) St. John dry forest (SJDRY); 154(b) St. John wet forest (SJWET); 158(a) St. Thomas dry forest (STDRY), 

and 158(b) St. Thomas wet forest (STWET).

Density Compensation in Mona Island Birds

Of the nine dry-forest sites sampled, the relative abundance of the pearly-eye is 

low in all but two islands, Mona and Montserrat (fig. 4.4). Mona is a very small 

island that experiences extended droughts. Consequently, it has very few permanent resident landbirds. Only tramp species such as the thrasher, gray kingbird, 

and common ground-dove abound there. The results of a comparison of total 

captures and the relative abundance of pearly-eyed thrashers mist-netted in dry 

forest on Mona and the Guánica Biosphere Reserve on nearby Puerto Rico are 

presented in fig. 4.6. Although there are no significant differences in capture rates 

or relative abundances of other forest birds at either site, the capture rate and 

relative abundance of pearly-eyes on Mona are significantly higher than those in 

the Guánica reserve. Mist-net results clearly show that the pearly-eye undergoes 

density compensation on Mona. Encouragingly, the mist-net results under discussion (and those that follow) are in full agreement with results of a comprehensive 

avian community study using aural-visual census techniques conducted by 

Hernández-Prieto (1993). He compared avian species richness and abundance, 

among many other parameters, on a year-round basis and over several years (1986 

to 1990) on Mona Island and within the Guánica reserve.

Like the pearly-eye, the common ground-dove (a D-tramp) is abundant on 

Mona Island, and it is also exemplary of density compensation in an avian tramp 
84

GENERAL TECHNICAL REPORT IITF-GTR-27

species. Ecological release and, consequentially, density compensation among 

Mona Island birds, have been observed by other authors (e.g., Faaborg 1980a, 

Terborgh and Faaborg 1973). However, the most quantitative information to date 

confirming previous observations has become available only recently (HernándezPrieto 1993).

To illustrate the prevalence of, and density compensation in, Mona Island 

ground-doves, total numbers of individuals captured and relative abundance 

of the four most commonly netted species are compared (fig. 4.7). Whereas the 

relative abundance of the dove averages 41 percent (SE = +8.02) per sample, with 

as many as 137 individual captures per sample, that of the remaining three species averages only 5 percent (avg. SE = +1.66), with never more than 7 individual 

Figure 4.6—Capture rates and relative abundance of pearly-eyed thrashers (PETH) and other dry forest birds in mist-net samples from the 

Guánica Commonwealth Forest and Biosphere Reserve, Puerto Rico, and nearby Mona Island. Whereas captures of other birds are similar on 

both islands, thrasher numbers and relative abundance (shaded rectangles) on Mona are significantly greater than in the Guánica forest where 

species richness is much higher. Solid horizontal lines are medians, lower and upper hinges on the rectangles encompass the 25th and 75th

percentiles, respectively; lower and upper caps (⊥, ⊺) mark the 10th and 90th percentiles, respectively; open circles below and above the 

caps mark the 5th and 95th percentiles, respectively.
85

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

captures per sample. It is noteworthy that like the common ground-dove, the 

remaining three species constituting Mona’s most common resident landbirds are 

also D-tramps, and are among the eight most widely distributed species in the 

West Indies (fig. 2.5a).

As a consequence of Hernández-Prieto’s research, all four species treated 

here (fig. 4.7) were shown to undergo density compensation on Mona, a fact 

not so apparent in the results obtained by the more brief and area-limited mistnet samples. His findings are not surprising. As D-tramps, all four species are 

expected to greatly increase in numbers and to broaden their ecological niches 

in the absence of interspecific competitors for water, food, and nesting resources. 

Thus, Hernández-Prieto’s data reassuringly confirm Diamond’s argument that 

tramps can, and often do, undergo density compensation on small, species-poor 

islands (see also Baker-Gabb 1986, Cox and Ricklefs 1977).

Figure 4.7—Capture rates and relative abundance (shaded rectangles) of Mona Island’s four most common species of resident landbirds. The 

common ground-dove was the only species clearly showing density compensation as revealed by mist netting. However, more recently, by using a 

point-count census technique, Hernández-Prieto (1993) showed that all four species undergo density compensation on Mona (see text for further 

discussion; see app. 1 for species’ four-letter alpha codes; box-plot parameters are explained in fig. 4.6).
86

GENERAL TECHNICAL REPORT IITF-GTR-27

The pearly-eye’s impressive abundance in Montserrat’s dry forest may be 

explained in two ways. First, unlike the five low-relief islands on which thrashers are scarce, Montserrat, although also small, is a high-relief island with 

ample wet-forest habitat in which thrashers are abundant (source population) 

and undoubtedly continue to supply dry-forest (sink) populations. In addition, 

unlike in Puerto Rico, species richness in Montserrat’s dry forest is low, thus 

making it easier for thrashers to compete.

The extreme variability in relative abundance of thrashers among islands, 

habitat types, and even the same, but geographically separated, habitat types is 

obvious and was anticipated. Not so obvious, however, is the fact that thrasher 

abundance differs, sometimes significantly, within the same habitat type at a 

single locale. In dry forest within the Guánica Biosphere Reserve, Puerto Rico, 

native and migratory forest birds have been monitored by using mist nets for 

more than 30 years (1972 to 2005) (for a review, see Faaborg and Arendt 1990; 

Faaborg et al. 2000). Nine sites have been sampled throughout the forest since 

1989 to detect microhabitat differences among species richness and relative 

abundance of the various species. One site (fig. 4.8: Plateau 2) has been monitored for 30 of the 32 years (not sampled in 1977 or 1979). The eight remaining 

sites have been sampled from 1989 to present, with data analyses pertaining 

solely to the pearly-eyed thrasher covering a period of seven seasons (1989 to 

1995). The relative abundance of pearly-eyed thrashers differs little among six 

of the nine sites (fig. 4.8), with an average relative abundance of 3 percent (range: 

1 to 4 percent) among the six sites. However, the relative abundance of pearlyeyes is significantly higher at the remaining three sites, apparently owing to 

the more complex, closed-canopy vegetation and more humid microclimates 

at these sites. Results of vegetative analyses conducted by Hernández-Prieto 

(1993) along the Dinamita 2 trail show a closed canopy including a 10-m-high 

karst promontory, and complex vegetation. In an independent study of the 

vegetative characteristics along the remaining net lines included in figure 4.8, 

R.C. Kennedy and J.R. Faaborg (1993, in litt.) also found more stems per area 

at the Dinamita 2 banding site, once again attesting to the more complex and 

closed nature of the vegetation at this site. Although the relative abundance of 

pearly-eyes differs greatly among several banding sites within the same forest, 

as the tramp theory dictates, capture rates are inversely proportional to species 

richness (fig. 4.9a) (see also “Point Counts” below for discussion of fig. 4.9b).

Thrasher abundance 

differs, sometimes 

significantly, within the 

same habitat type at a 

single locale.
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Figure 4.8—Capture rates (•) and relative abundance (box plots) of the pearly-eyed thrasher in mist-net samples at nine sites 

along trails within the Guánica Commonwealth Forest and Biosphere Reserve, Puerto Rico (spelling of trail names follows 

that of Silander et al. 1986 and Farnsworth 1991). Susúa Forest captures are included for comparison (see “chapter 4” within 

“Study Area and Methods”). Capture rates and relative abundance differ significantly among some of the sites. Pearly-eyes 

prefer moister sites with trees of taller stature, often forming a closed canopy. Plot-wide dotted line shows the overall relative 

abundance of thrashers among all sites. Box-plot parameters are explained in figure 4.6.
88

GENERAL TECHNICAL REPORT IITF-GTR-27

Figure 4.9—Inverse correlations between numbers of pearly-eyed thrashers (PETH) and species richness as a result of mistnet samples (A) and point-count censuses (B) conducted along trails in the Guánica Commonwealth Forest and Biosphere 

Reserve, Puerto Rico (spelling of trail names follows that of Silander et al. 1986 and Farnsworth 1991). In both sampling 

techniques, as the supertramp theory dictates, as species richness increases, thrasher numbers decrease. Mist-net data are 

from a long-term study by Faaborg and Arendt (Faaborg and Arendt 1990, Faaborg et al. 2000), whereas point-count data 

were taken from Hernández-Prieto (1993).
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Extreme Temporal Fluctuations in 

Mona Island’s Thrasher Population

Even on islands where the pearly-eye is common, its populations may undergo 

significant fluctuations over time. On Mona Island for example, beginning as 

early as 1903 most visiting ornithologists reported pearly-eyes as common or 

even abundant (app. 3). Beatty (cited by Bond 1964) found thrashers common 

along the coast and on the plateau. He estimated that 1,000 thrashers inhabited 

the island during the time of his visit. Thus, for centuries, the pearly-eye has 

been a very conspicuous component of the island’s resident landbird community. 

However, some written accounts suggest periodic and, at times, extreme fluctuations in Mona’s thrasher population. For example, Rollé (1961b), although listing 

the thrasher as “common on the plateau” after a visit from November through 

December 1960, later reported that he had seen “only a few individuals” [?] during 

his first visit (Rollé 1961a). He concluded that pearly-eyes had undergone “a large 

reduction in numbers” since the last (1944) avian survey was conducted by Barnés 

(1946).

Why should the abundant pearly-eye population on Mona undergo such 

extreme population fluctuations? Mona is a small island with xerophytic vegetation adapted to very dry conditions. Water is scarce at best and is sometimes 

contaminated; more often it is completely lacking owing to extended drought 

conditions. The harsh climatic and ecological conditions on Mona apparently 

impact even highly adaptable species such as the pearly-eyed thrasher.

Seasonal Fluctuations in Mona Island’s 

Thrasher Population

Mist-net censuses of thrashers taken during three “wet-season” months (May to 

July) and three “dry-season” months (December to February) were used to determine potential seasonal population fluctuations. Banding results (fig. 4.10) are 

total captures and relative abundance (thrashers as a percentage of total captures) 

of birds caught during each 3-day banding session. Data were gathered over a 

12-year period from two dry-forest sites by Faaborg and his colleagues from 1972 

to 1976, and Faaborg, Arendt, and their associates in February 1984. Data were 

pooled after no significant difference in capture rates of thrashers (Mann-Whitney 

U-Statistic (hereafter M-W U) = 10.5; X2 = 0.15; df = 1; P = 0.69) or total birds 

captured (M-W U = 13.0; X2 = 1.06; df = 1; P = 0.69) was detected between the 

two banding sites on Mona’s extensive plateau region. The relative abundance of 

pearly-eyes is significantly higher in the “wet-month” samples (range: 42 to 86; 

Pearly-eye populations 

undergo significant 

fluctuations over time.
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GENERAL TECHNICAL REPORT IITF-GTR-27

Figure 4.10—Comparison of the total number of individuals and relative abundance (percentage of all 

captures) of pearly-eyed thrashers (PETH) and all species captured in mist-nets on Mona Island during 

June and July (wet season) and December–February (dry season) 1972–1984. Whereas median capture rates 

of other forest birds (unshaded rectangles) are similar during both seasons, the capture rate (gray shaded 

rectangles) and relative abundance (RA, blackened rectangles) of the pearly-eye are significantly lower in the 

dry season. Box-plot parameters are explained in figure 4.6.

median = 65 percent) than in the “dry-month” samples (range: 24 to 28; median = 

26 percent) (M-W U = 74.5; X2 = 9.83; df = 1; P <0.001). These banding results are 

illustrative of the highly dynamic seasonal nature of Mona’s thrasher population.

From December to February, some of Mona’s driest months outside of the 

main thrasher breeding season, populations are generally sedentary and capture 

rates are low. During June and July, both of which fall within the thrasher’s 

primary lowland breeding season, thrashers are more abundant and mobile, as 

adults are seeking food for their young. The low capture rates during January 

and February suggest that thrashers remain sedentary if there is an adequate 

supply of water. However, during severe and prolonged droughts, thrashers move 

long distances in search of food and water. For example, in figure 4.10, the maximum number of individuals (174) captured during a 3-day netting session in the 
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

normally “wet” months of May to July coincides with severe drought conditions 

occurring on Mona and nearby Guánica reserve during the summers of 1973 and 

1974 (J. Faaborg 1982a, and 1983, pers. comm.).

The pearly-eye’s contemporary and historical ranges have been reviewed, and 

three census techniques have been used to show striking differences in the pearlyeye’s abundance on different islands, among different habitat types, and even 

spatiotemporally within a single island and habitat. But, what are some of the 

factors governing the observed range and numbers of the pearly-eyed thrasher?

Environmental Factors Influencing 

Range and Abundance

Distribution and abundance of the pearly-eyed thrasher have been shown to be 

dynamic over space and time, and both geographic and demographic parameters 

continue to change even today. What are some of the principal factors influencing these spatiotemporal components of the thrasher’s range and abundance? 

The contemporary range of the pearly-eye is no doubt the culmination of many 

geological, climatological, ecological, genetic, and human-induced influences 

acting together over many millennia (see discussions in Ricklefs 1989; Vuilleumier 

and Simberloff 1980; Wiens 1989a, 1989b).

From an autecological perspective, three of the most obvious environmental 

factors influencing the thrasher’s distribution and abundance are climate and two 

human-induced factors, namely habitat destruction and hunting.

Climate

Climate, past and present, no doubt has greatly influenced the pearly-eye’s 

historical and contemporary ranges. Using the vertebrate fossil record and 

sea level changes, Pregill and Olson (1981) showed that environmental conditions 

in the West Indies during the last Pleistocene glaciation favored predominantly 

arid savanna, grassland, and xeric scrub forest. Subsequent environmental 

changes that led to more mesic habitats had a profound effect on the distribution 

of West Indian flora and fauna, including the pearly-eye (see also Olson and 

Hilgartner 1982). Although climate continues to influence the general distribution 

of West Indian birds, other, often more proximate, factors must also be considered 

when attempting to explain the pearly-eye’s contemporary range and population 

fluctuations throughout the region.

The pearly-eye’s 

contemporary range 

is the culmination 

of many geological, 

climatological, 

ecological, genetic, 

and human-induced 

influences.
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GENERAL TECHNICAL REPORT IITF-GTR-27

Habitat Destruction

Natural stochastic events, such as droughts and hurricanes, have no doubt significantly impacted populations of West Indian birds throughout the region’s long 

history (see review by Wiley and Wunderle 1993). The importance of disturbances 

in shaping forest-bird populations is clear; more than 8,000 tropical cyclones have 

been recorded within the region in the last 100 years (Evans 1990). Such frequent 

and violent habitat disturbances may even partially account for the observed present-day distributions of some species (Raffaele 1977, Wiley 1985a, Wunderle et al. 

1992). However, with few exceptions, e.g., avian specialists and small populations 

of island endemics (Westermann 1953, Wiley 1985a), those species that persist in 

the region today are well adapted to such habitat disturbances and recover well 

(Ackerman et al. 1991, Wauer and Wunderle 1992).

Unfortunately, however, not many species have acquired the physiological, 

ecological, or behavioral traits necessary to withstand the types and extent of habitat alteration produced by humans (see Ricklefs and Wikelski 2002). Before the 

arrival of European settlers in the 16th and 17th centuries, forests dominated the 

West Indian landscape. Most forests were destroyed to accommodate agricultural 

crops such as sugar cane and bananas. Loss of forest habitat was often complete 

on low-relief islands such as Antigua and Barbados, and even on the small, higher 

relief islands such as Montserrat. This undoubtedly had profound effects on the 

distribution and viability of native plant and animal populations, greatly influencing the distributional patterns observed today. Encouragingly, with the return of 

secondary forests on many once-denuded islands, e.g., Montserrat (100-percent 

deforestation) and Puerto Rico (>95-percent deforested), forest-bird populations 

are recovering. In the case of the pearly-eyed thrasher, two islands, Barbados and 

Antigua, are exemplary of two reforestation extremes. On Barbados, virtually 

no reforestation has occurred, resulting in the extirpation of both Margarops species. However, on Antigua, a once-denuded island on which substantial tracts of 

secondary forests have been established, the pearly-eye has returned to some 

of its traditional forested areas.

The pearly-eye probably occurred throughout Antigua in limestone forests 

prior to deforestation in the 18th and 19th centuries, when forests gave way to 

sugar cane (Pregill et al. 1988). For many years following deforestation, pearlyeyes were confined to the forested hills in the volcanic southwestern part of the 

island (Holland and Williams 1978). Recently, however, pearly-eyes have begun 

to repopulate secondary dry limestone forest in the eastern and south-central 

portions of the island, including English Harbour. During wildlife assessments 

in 1983 and 1984 (Faaborg and Arendt 1985), numerous sightings were made in 
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

these areas. Similarly, in January 1984 at a net line in the Shirley Heights area 

bordering English Harbour to the east, 1 of the 159 birds captured during a 

3-day sampling period was a pearly-eyed thrasher (Faaborg and Arendt 1985). 

If intensive searches were made throughout the year, doubtless more pearly-eyes 

would be found at varying elevations in many additional locales in Antigua.

Hunting

Together with habitat degradation, hunting by aboriginals has been shown to 

be a contributing factor influencing the contemporary distributions of island 

birds in the Pacific (Steadman 1989, Steadman and Kirch 1990, Steadman et al. 

1991) and the West Indies (Pregill et al. 1994, Steadman et al. 1984a). In the West 

Indies, humans did not arrive until well after the end of the Pleistocene, about 

4500 to 7000 years B.P. in the Greater Antilles and 3700 years B.P. in the Lesser 

Antilles (Pregill et al. 1994, Steadman et al. 1984a). On Antigua, Lesser Antilles, 

the first Amerindians arrived at least 3600 years B.P. They were mainly hunters, 

gatherers, and fishermen and, although thought to have contributed to the 

demise of some species of native fauna, their influence may have been less than 

that of more recent arrivals. Europeans, with their arrival commencing some 

500 years ago, began major habitat alterations that continue today throughout 

the region. Through overhunting and environmental degradation, Europeans 

and their introduced animals, e.g., rats (Rattus spp.), mongooses (Herpestes 

auropunctatus), and browsing-grazing livestock, have caused the extinction of 

at least nine native vertebrate taxa, some within recorded history (Steadman 

et al. 1984a).

The remains of many bird taxa found in archaeological and paleontological 

sites on islands throughout the West Indies attest to the fact that birds were 

widely used as food by pre-Columbian peoples (Pregill et al. 1994, Watters et 

al. 1984, Wing 1989). The first record of the pearly-eye being used for human 

consumption comes from the Trant’s archeological site, Montserrat, discussed 

above (Steadman et al. 1984b).

Numerous historical accounts (e.g., Bond 1945, 1950; Schwartz and 

Klinikowski 1965; Westermann 1953; Wiley 1985a) affirm that present-day 

populations of the pearly-eye on some West Indian islands are the results, at 

least in part, of colonial and even current hunting practices. For example, in 

his history of St. Lucia, Breen (1844) reported that “thrushes” (which no doubt 

included the pearly-eye) were favored gamebirds. Semper (1872) stated that, like 

the forest thrush, the pearly-eye was also a major gamebird on St. Lucia during 

the late 19th century. By the 1930s, Danforth (1935b) noted that the pearly-eye 
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GENERAL TECHNICAL REPORT IITF-GTR-27

had become uncommon and local on that island. He was unable to procure a 

specimen, as were most early 20th-century collectors. Diamond (1973b) found 

the pearly-eye to be “patchy,” inhabiting only the highland forests of St. Lucia, 

whereas the scaly-breast was equally abundant in highland and lowland forest 

habitats. Arendt, in 1981, and Faaborg and Arendt (1985) found the pearly-eye 

to be uncommon in mist-net samples in wet forest, and absent at dry-forest 

sites. Of almost 1,000 birds captured (1981 and 1985 totals combined), only 7 

were pearly-eyes. Pearly-eyes were captured in wet forest only, and at but three 

of seven sites sampled in dry, mesic, and wet-forest habitats along an elevational 

gradient from coastal to montane forests (n = 2, 2, and 3 individuals, respectively). Likewise, on Guadeloupe, Noble (1916) stated that by the early 1900s, 

pearly-eyes were very retiring, very arboreal, and found only in highland forests 

(see Pregill and Olson 1981, Terborgh et al. 1978). On Guadeloupe, the pearlyeyed thrasher is considered as valuable a gamebird as that island’s pigeons and 

doves. As recent as 1971, during a visit to a national park in Basse Terre, C.B. 

Kepler (cited in Snyder et al. 1987: app. 1) stated that “...hunters were bagging 

birds as small as thrashers.” Much more recently, however, and probably owing 

to the establishment and protection afforded by a national park in Basse-Terre, 

during a census of forest birds within the park, the two Margarops species (total 

individuals of both species combined) were among the most common species, 

with 8.9 individuals per hectare (Feldmann et al. 1998). Clearly, on islands 

without the establishment of national parks (or the enforcement of wildlife 

laws within them), thrasher numbers have been substantially reduced owing 

to overhunting, as have populations of the Antillean psittacines.

Ecological Correlates of Range and Abundance

Geological, climatological, and human-induced phenomena are not the only 

factors effecting the diversity, distribution, and abundance of West Indian 

avifauna. Many ecologists interpret distributional patterns from a more interactive, or synecological, perspective. They believe that contemporary avian 

communities are dynamic, continually influenced by an array of biological and 

ecological interactions taking place regionally and locally over space and time.

Evaluation of Ecological Correlates Via Categorical Modeling

There are a few obvious ecological factors that influence the present-day range 

of the pearly-eye and, at the same time, are amenable to statistical analyses. 

Categorical modeling (see Arendt 1993 for methodology) was used to analyze 

the contemporary distribution of the pearly-eyed thrasher as a function of 

Contemporary avian 

communities are 

dynamic, continually 

influenced by an array 

of biological and 

ecological interactions.
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

(1) island size (SIZE); (2) the number of landbird species (potential competitors) 

per island in which the pearly-eye is either absent or present (NUMSPEC); and 

(3) the presence or absence of a potential competitor, the red-legged thrush 

(Turdus plumbeus). The red-legged thrush was selected over other mimids and 

other thrushes because other mimids, even the pearly-eye’s congener, the scalybreasted thrasher, and several species of thrush are sympatric with the pearly-eye. 

Consequently, most apparently are not strong interspecific competitors for shared 

resources. Conversely, in comparisons of the pearly-eye and red-legged thrush on 

a regional scale, there tends to be an inverse correlation, i.e., an absence of one 

species follows the presence of the other (app. 2). In general, where both species 

are sympatric and syntopic, if one species abounds, the other is scarce. Likewise, 

research results in Puerto Rico have revealed that many aspects of the thrush’s 

ecology, such as diet and foraging niches, are similar to those of the thrasher’s 

(A.I. Arendt, unpubl. data). Information gathered from that study, together with 

results from monthly point counts, show that these thrasher and thrush populations are interspecifically territorial.

Island Size

The pearly-eye inhabits islands and cays with landmasses ranging from 0.03 km2 

(Bird Rock, Bahamas) to 9000 km2 (Puerto Rico) (table 4.4 and app. 2). However, 

it is virtually absent from islands with landmasses less than 0.1 km2 (n = 25 

islands), and does not occur on half of the islands with landmasses ranging 

between 0.1 km2 and 1.0 km2 (n = 46). In short, the pearly-eye generally does not 

colonize islands less than about 1 km2, most likely because most small islands in 

the region are treeless, low-relief cays and islets that do not offer suitable habitat 

for this predominantly arboreal thrasher (app. 2). However, at the other extreme, 

the pearly-eye does not generally inhabit large islands with landmasses greater 

than about 1000 km2. It is found in species-poor habitats on only four islands 

larger than 1000 km2 (app. 2).

The effects of island size are nonsignificant but are masked because they are 

strongly correlated with number of species (tables 4.5, 4.6, and app. 2). Therefore, 

the variable “SIZE” was dropped from the model to allow for a clear evaluation 

of the effects that the number of species has on the thrasher (tables 4.7 and 4.8).

Number of Species

The pearly-eyed thrasher does not inhabit small islands with three or fewer resident landbird species (app. 2). However, it is resident on small islands with as few 

as four landbird species (Dutch Cap and Grass Cay, U.S. Virgin Islands) and on 

The pearly-eye inhabits 

islands and cays with 

landmasses ranging 

from 0.03 km2 to

9000 km2.
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GENERAL TECHNICAL REPORT IITF-GTR-27

Table 4.4—Landmasses and number of species of breeding 

landbirds for 177 islands on which the pearly-eyed thrasher is 

absent (n = 97) or resident (n = 80)

Pearly-eyed thrasher

Statistic All islands Absent Resident

Landmass in square kilometers

Mean 1,312.01 2,214.88 241.93

SD 10,150.46 13,718.34 1,026.47

Median 3.6 .9 12

Mode .1 .1 .2

Minimum .002 .002 .03

Maximum 114,524 114,524 9,000

Number of breeding landbirds

Mean 14.42 12.78 16.37

SD 12.73 14.96 9.16

Median 12 7 15

Mode 1 1 14

Minimum 1 1 4

Maximum 73 73 55

Table 4.5—Maximum likelihood analysis of variance table resulting 

from an analysis of the geographical distribution (presence or 

absence) of the pearly-eyed thrasher as a function of island size 

(SIZE), number of landbird species per island (NUMSPEC), and the 

presence or absence of the red-legged thrush (RLTH)

Source df Chi-square Probability

INTERCEPT 1 26.73 <0.001

SIZE 1 .41 .52

NUMSPEC 1 28.1 <.001

N2 a 1 12.67 <.001

RLTH 1 15 <.001

LIKELIHOOD RATIO 149 174.86 .07

aThis model includes a quadratic term (N2

) for “number of species” to allow for the 

fact that the pearly-eye does not inhabit especially small or large avian communities 

(NUMSPEC). Note that both terms are significant. Note also that the likelihood ratio 

(residual variation) is not significant, implying that the model fits.

Table 4.6—Maximum likelihood estimates (MLE) obtained in the 

analysis of variance procedure from table 4.5

Effect Estimate Standard error Chi-square Probability

INTERCEPT 2.22 0.42 26.73 <0.001

SIZE 0 .41 .52

NUMSPEC .25 .04 28.1 <.001

N2 0 0 12.67 <.001

RLTHa 2.87 .74 15 <.001

a In this model, “RLTH” (red-legged thrush) was treated as a quantitative variable. 

Thus, the maximum likelihood estimate of 2.87 represents the gain in expected value 

of the logit when the red-legged thrush is present in the avian community.
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

large islands with as many as 60 or more landbird species (Puerto Rico) (table 4.4). 

The number of landbird species per island is highly correlated with the presence 

or absence of the pearly-eye (tables 4.4 through 4.8). As the supertramp theory 

predicts, the pearly-eyed thrasher inhabits species-poor islands. On average, the 

pearly-eye is present on islands with about 16 landbird species (table 4.4).

The overall number of breeding landbirds per island does not necessarily 

translate directly into the number of resident landbirds in particular forest communities on any given island. Mist netting at 25 locations at varying elevations 

and in two general (dry and wet) forest types on the Caribbean’s 10 most speciesrich (>20 species) islands in which the pearly-eye occurs, has shown that the actual 

size of forest bird communities is much smaller than the total reported for number 

of resident landbirds inhabiting any given island (Faaborg and Arendt 1985, 

Terborgh and Faaborg 1980, and table 4.9). Whereas the mean number of breeding landbirds per island reported for the 10 most species-rich islands in which 

the thrasher is resident was 35 (SD = 12.29; range: 21 to 61), the mean number of 

species per mist-net sample was 12 (SD = 2.65; range: 5 to 23). The difference is 

significant (t = -4.45; P <0.001).

Table 4.7—Maximum likelihood analysis of variance table resulting 

from an analysis of the geographical distribution (presence or 

absence) of the pearly-eyed thrasher as a function of the number 

of landbird species per island (NUMSPEC) and the presence or 

absence of the red-legged thrush (RLTH)a

Source df Chi-square Probability

INTERCEPT 1 26.89 <0.001

NUMSPEC 1 29.44 <.001

N2 1 14.49 <.001

RLTH 1 15.16 <.001

LIKELIHOOD RATIOb 44 67.37 .01

aThe variable “SIZE” has been dropped from the model.

bNote that now the likelihood ratio (residual variation) is significant (0.01), 

but this is owing to a large decrease (105) in degrees of freedom.

Table 4.8—Maximum likelihood estimates obtained in the analysis 

of variance procedure from table 4.7

Effect Estimate Standard error Chi-square Probability

INTERCEPT 2.18 0.42 26.89 <.001

NUMSPEC .24 .04 29.44 <.001

N2 0 0 14.49 <.001

RLTHa 2.87 .73 15.16 <.001

aRLTH = red-legged thrush.

The number of 

landbird species 

per island is highly 

correlated with the 

presence or absence 

of the pearly-eye.
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GENERAL TECHNICAL REPORT IITF-GTR-27

Geographical Distribution of the Red-Legged Thrush

Field studies of the red-legged thrush in Puerto Rico suggest that it is a competitor of the pearly-eye, especially for food (A. I. Arendt, unpubl. data). In the Sierra 

de Luquillo, where there are dense populations of both species, they are parapatric (contiguous, but not overlapping) for the most part. In habitats in which the 

species are syntopic, e.g., the Guánica Biosphere Reserve, neither species is common. The thrush prefers clearings, edge, and open canopy with little understory. 

In the Sierra de Luquillo, it is found primarily at lower elevations in tabonuco 

forest and forest edge. However, seasonal migration occurs to upper elevations in 

colorado forest, where it follows gaps and corridors such as roads. The thrasher 

prefers mid-elevation colorado forest where its density is greatest, but is found at 

all elevations, even at the summit, in open and closed-canopy forest (Snyder et al. 

1987: table 3.1).

There are a few historical accounts documenting that the thrasher and 

thrush have been geographically separated in the Greater Antilles for many 

decades, and possibly hundreds of years. On Hispaniola, Cory (1881) found the 

thrush abundant in the vicinity of Fort Jacques, Haiti, at an elevation of 762 m. 

In the Dominican Republic, Cherrie (1896) reported the thrush as “...not at all 

uncommon...” and “Found everywhere on the coast and high up in the interior.” 

Also reporting on the birds of the Dominican Republic, Christy (1897) likewise 

Table 4.9—Comparison of the number of resident landbird species 

reported for the 10 most species-rich islands (≥20 species) on which 

the pearly-eye occurs (app. 2), with the number of resident landbird 

species found in 25 mist-net samples (1 to 7 per island)

Number of species

Species Mist-net sample

Island per island Mean Range

131.a Puerto Rico (a through h) 61 19.0 15—23

156. St. Lucia (a through h) 45 13.6 12—15

48. Dominica (a, b) 41 11.0 9—13

159. St. Vincent (a, b) 39 12.0 10—14

79. Guadeloupeb (a, b) 37 12.0 10—14

155. St. Kitts (ab, b) 23 11.0 9—13

112. Montserrat (b, c, e, g, h, m, n) 27 9.9 5—12

7. Antigua (a, b) 25 13.0 5—11

11. Beatab 30 11.0 —

105. Marie Galanteb 21 10.0 —

— = single banding session.

aRespectively, island and forest type numeric and alpha codes follow those in appendix 2.

bMist-net data provided by John Faaborg.

The red-legged thrush 

is a competitor of 

the pearly-eye; in the 

Sierra de Luquillo 

they are parapatric 

(contiguous, but 

not overlapping). 
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

found the thrush “pretty common” at La Vega (North-Central region), although 

shy and observed more often “...in the thicker parts of the forest.” On the Isle of 

Pines (Isla de la Juventud), Cuba, Read (1911) reported the thrush, but not the 

thrasher, on two consecutive Christmas bird counts. Red-legged thrushes, together 

with northern and Bahama mockingbirds, are resident on Cayo Coco (Garrido 

1976), whereas the thrasher is absent. Likewise, Garrido and Schwartz (1969) 

reported the thrush resident on Cayo Cantiles, whereas the thrasher is absent. 

The thrush’s presence on the small islands off Cuba’s coasts may partially explain 

the thrasher’s paradoxical absence. On Cuba, the thrasher is absent, whereas 

the thrush is a well-established and widespread resident. Garrido and Schwartz 

(1968) reported the thrush uncommon but widespread in Cuba’s Guanahacabibes 

Peninsula in the southwestern extreme of the island. More recently (1991), in the 

Zapata Peninsula near the Zapata Swamp (also in the SW), Martin McNicholl 

found the thrush to be common (J. Faaborg 1992, in litt.). At 6 banding sites, of 

560 birds captured (mean = 93.3; SE = +9.12; range: 60 to 118 per site), 124 were 

red-legged thrushes (mean = 20.6; SE = +2.44; range: 12 to 27 per site), resulting 

in a relative abundance (percentage of thrushes per total individual captures) of 

22 percent (mean = 24; SE = +4.77; range: 14 to 45 percent).

Likewise, the red-legged thrush may be a contributing factor behind the 

pearly-eye’s absence on the larger, northern Bahama Islands. Cory (1890a) stated 

that the thrush is “especially common” on New Providence and Abaco. Northrop 

(1891) found the thrush to be “common” on Andros, whereas the thrasher was 

absent. Similarly, Bangs (1900) reported the thrush as resident on New Providence, 

whereas the thrasher was not listed.

Following Diamond’s (1974) competitive classification scheme, the red-legged 

thrush is a C-tramp (app. 1). As a tramp, it should be a good disperser and colonizer. However, there are virtually no published accounts documenting the thrush 

expanding its range or even dispersing to other islands. However, Johnston (1969) 

reported a single, resident female red-legged thrush on Grand Cayman (first record 

for that island) from 1963 to 1966. He noted that it could have come from Cuba, 

the Isle of Pines, or Cayman Brac (the only island of that group inhabited by 

Turdus plumbeus). It is possible that the disappearance of the Grand Cayman 

thrush has allowed the commencement of a range expansion by the red-legged 

thrush onto all the Cayman Islands. The elevated species richness of Grand 

Cayman Island (25 resident species) may account for the absence of the pearlyeye on that island, but its absence on Little Cayman, with only 17 species of 

resident landbirds, is more puzzling.
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GENERAL TECHNICAL REPORT IITF-GTR-27

Because of the distinct geographical and habitat separation of the two species, the distribution of the red-legged thrush was included as a variable in the 

categorical models to better quantify its apparent influence on the distribution of 

the pearly-eyed thrasher.

Island Size

Its disjunct distribution (Greater Antilles and Dominica) suggests that the thrush 

also may once have had a more extensive range. Nevertheless, presently, the redlegged thrush is found on only 19 of the 177 islands under consideration (app. 2). 

Unlike the pearly-eye, the red-legged thrush does not inhabit islands with landmasses smaller than about 20 km2

 but rather is found more often on large islands 

(table 4.10). Of the 19 islands inhabited by the thrush, 16 (84 percent) have landmasses larger than 100 km2. Moreover, half of them have landmasses larger than 

1000 km2 (app. 2). That island size is not as important as the number of landbird 

species in governing the distribution of the red-legged thrush, is further illustrated 

by the fact that of the 177 islands under consideration, the thrush is absent on 52 

islands with landmasses larger than 20 km2 (app. 2).

Table 4.10—Descriptive statistics of landmasses and 

number of landbird species for 177 islands on which the 

red-legged thrush is absent (n = 158) or resident (n = 19)

Red-legged thrush

Statistic All islands Absent Resident

Landmass in square kilometers

Mean 1,312.01 132.25 11,122.62

SD 10,150.46 886.89 29,778.35

Median 3.6 1.45 660

Mode .1 .1 None

Minimum .002 .002 26.8

Maximum 114,524 10,989 114,524

Number of breeding landbirds

Mean 14.42 11.85 35.78

SD 12.73 9.44 16.36

Median 12 10.5 31

Mode 1 1 20

Minimum 1 1 20

Maximum 73 67 73
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Number of Species

As with the thrasher, the effects of island size in the categorical modeling 

analysis of the thrush’s geographical distribution was masked by the more 

important “number of species” variable. The number of landbird species per 

island is significantly correlated with the geographical distribution of the redlegged thrush (tables 4.11 and 4.12). Turdus plumbeus does not inhabit islands 

with fewer than 20 resident landbird species but rather is found more often on 

the most species-rich islands (app. 2). Of the 158 islands from which the thrush 

is absent, 132 islands (84 percent) have fewer than 20 landbird species, whereas 

only 26 islands (16 percent) have >20 species. Elevated species richness increases 

the probability that the thrush will be resident on an island.

Table 4.11—Maximum likelihood analysis of variance table resulting 

from an analysis of the geographical distribution (presence or absence) 

of the red-legged thrush as a function of island size (SIZE) and number 

of landbird species per island (NUMSPEC)

Source df Chi-square Probability

INTERCEPT 1 45.96 <0.001

SIZE 1 .03 .86

NUMSPEC 1 21.37 <.001

LIKELIHOOD RATIO 151 82.01 1

Table 4.12—Maximum likelihood estimates resulting from the analysis 

of variance procedure from Table 4.13

Effect Estimate Standard error Chi-square Probability

INTERCEPT 4.59 0.67 45.96 <0.001

SIZE 8.94*e-6 0 .03 .86

NUMSPEC .12 .02 21.37 <.001

Thrasher-Thrush Distribution Analysis

To emphasize the inverse correlation between the presence or absence of the 

pearly-eye as a function of the presence or absence of the thrush and island size, 

a restricted analysis was done. It included only islands in the size range in which 

both the thrasher and the thrush occur (although they are not necessarily on the 

same island) (tables 4.9 and 4.10). The presence or absence of the thrush becomes 

the only significant factor in predicting the presence or absence of the thrasher 

(table 4.13). The coefficient was large and negative, indicating that the presence 

of the thrush implies an absence of the thrasher (table 4.14). However, that the 

thrasher and thrush are negatively associated over this range of the data, does 

The presence or 

absence of the thrush 

becomes the only 

significant factor 

in predicting the 

presence or absence 

of the thrasher.
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GENERAL TECHNICAL REPORT IITF-GTR-27

not imply a cause-and-effect relationship. As Wiens (1989a) pointed out, inferences drawn from correlative data are suggestive at best and remain equivocal in 

the absence of quantitative manipulation of a series of ecological variables and 

testing of alternative process hypotheses.

No doubt, besides the red-legged thrush, there are many other potential competitors of the pearly-eye that have gone undetected in these analyses, especially 

owing to diffuse competition. To further explore a cause-and-effect relationship of 

the thrasher’s contemporary range, what must be undertaken is extensive multivariate analyses by using ordination and serial inclusions-exclusions of an array of 

physical and ecological variables. This would be, of course, a major study in itself.

Table 4.13—Maximum likelihood analysis of variance table resulting 

from a restricted analysisa of the geographical distribution of the 

pearly-eyed thrasher as a function of island size (SIZE) (where 20 ≤SIZE 

≤11 000 km2) and number of landbird species per island (NUMSPEC) 

(where 20 ≤NUMSPEC ≤60) and the presence of the red-legged thrush 

(RLTH)

Source df Chi-square Probability

INTERCEPT 1 0.45 0.5

SIZE 1 .6 .43

NUMSPEC 1 .2 .65

RLTH 1 9.48 <.001

LIKELIHOOD RATIOb 36 42.92 .19

aThis analysis includes only the data space in which both the pearly-eyed thrasher and the 

red-legged thrush are present, but not often on the same island (see app. 2). By restricting 

island size, and thus number of species, these two variables become insignificant (see the 

following tables for the model with a full range of island sizes and number of species).

bThe likelihood ratio is nonsignificant (0.19) for the range of the data in which both the 

pearly-eye and the red-legged thrush are present. Thus, the model fits, i.e., there is no 

significant unexplained variation.

Table 4.14—Maximum likelihood estimates resulting from the restricted 

analysis of variance procedure from table 4.13

Effect Estimate Standard error Chi-square Probability

INTERCEPT 1.02 1.53 0.45 0.5

SIZE 0 0 .6 .43

NUMSPEC .02 .05 .2 .65

RLTHa 1.37 .44 9.48 <.001

a In this model, “RLTH” (red-legged thrush) was treated as a categorical (not quantitative) variable. 

Thus, the maximum likelihood estimate of 1.37 represents the effect of RLTH. Therefore, adding 

it to the intercept for cases where RLTH = 0 (absent), and subtracting it when RLTH = 1 (present) 

implies that:

P(PETH|no RLTH) = 1/(1+exp*(1.0278+(-1.3728))) = 0.584

and

P(PETH|RLTH) = 1/(1+exp*(1.0278-(-1.3728))) = 0.083

Thus, the thrasher is about seven times more likely to be found where the thrush is absent.
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

In summary, categorical modeling has shown that, as Diamond’s supertramp 

theory predicts, the pearly-eyed thrasher is virtually restricted to small, species-poor islands. Moreover, the pearly-eye’s distribution is inversely correlated 

with a potential passerine competitor, the red-legged thrush, although additional 

research is needed to shed light on the cause-and-effect relationship of this 

observed pattern.

Competition Within West Indian Bird Communities

Inter- and intraspecific competition for resources such as food and nest sites may 

affect species richness, composition, population size, and even an individual’s 

form and function within avian communities in some environments. This fact led 

to the early conceptualization and establishment of the competition paradigm 

in the study of community ecology. Interspecific competition occurs on at least 

three levels: individual, species, and community. As defined by Lincoln et al. 

(1983), competition at the individual and species levels (e.g., interference and 

resource competition, respectively) can be included in the competitive exclusion 

principle; i.e., two individuals or species having identical ecological requirements 

cannot coexist indefinitely (this is similar to the classic Gaussian hypothesis and 

Grinnell’s axiom). Competition at the community level is indirect or “diffuse” 

(MacArthur et al. 1972); or more precisely, it is the “....simultaneous interspecific 

competition between numerous species each having a small degree of niche 

overlap with other species;” (Lincoln et al. 1983).

Supertramps face interspecific competition at all three levels with other 

members of the community. In this study, interspecific competition involving the 

pearly-eye has been inferred, but not confirmed, at the individual and species levels (presence or absence of the thrasher correlated to that of a potential competitor and predator, the red-legged thrush) and at the community level (absence of 

the thrasher in large avian communities) (see also chapter 8 for more compelling 

evidence). But, so far, little mention has been made of the ecological constraints 

influencing pearly-eye distribution and abundance.

In his comprehensive review, Wiens (1989b: chap. 3) outlined all of the major 

ecological factors influencing the structure of biological communities, including 

competition. He noted that support for intra- and interspecific competition is 

“convincing” in some instances (e.g., nectarivorous species in local assemblages), 

suggestive or circumstantial in others (e.g., comparative, correlative observations), 

or lacking in some (e.g., unstable, highly variable ecosystems such as grasslands). 

Supertramps 

face interspecific 

competition at 

three levels with 

other members of the 

avian community.
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GENERAL TECHNICAL REPORT IITF-GTR-27

How important, then, might competition be in West Indian bird communities? 

Wiens stated that “Despite the weakness of a good deal of the observations (and 

some of the experimental) evidence, there are clear, strong indications that competition does occur as a proximate process among some groups of birds.” He further 

noted that “In some groups, such as the West Indies birds examined by Case et al. 

(1983)...the species that occur together do appear to differ morphologically more 

than would be expected by chance.”

Many of the data and ideas presented by Case et al. (1983) stemmed primarily 

from the investigations of Faaborg (1985; and more recently, 1988), who has been 

studying West Indian bird communities for more than 30 years. Although his 

data are of a correlative nature and thus open to alternative interpretations as 

to the cause-and-effect aspects of resulting patterns, Faaborg has shown that 

many complex ecological isolating mechanisms play a decisive role in influencing 

species richness and distributions of West Indian birds. By separating the 

region’s landbird fauna (excluding Falconiformes) into four basic foraging guilds 

(sets of species with similar diets and/or foraging habits), he found (1) a direct 

relationship between the total number of species and coexisting number of species 

in each guild and the size of the resident landbird community; (2) saturation 

curves for total number of species (about 30 in dry forest, 20 in wet) and guild 

membership within habitats on large islands; i.e., the number of species found 

in a habitat is constant on islands with more than about 30 species; and (3) size 

differences among coexisting guild members that closely adhered to the so-called 

“Hutchinsonian ratio” (Hutchinson 1959), i.e., in smaller guild members, one 

species is twice as large as the other, whereas in larger members, one species is 

1.3 times larger than the other. Considering the last of these important and 

insightful observations, is it possible that the pearly-eye’s morphology also 

could be influenced by intra- and interspecific competition? This question 

will be addressed in the following chapter.

Summary: Distribution and Abundance

A review of the contemporary and historical distribution and abundance of the 

pearly-eyed thrasher shows that it fulfills all of the area and population-size 

prerequisites of a superior colonizer. It has a wide geographic and habitat range, 

exhibits local and regional abundance, and undergoes density compensation on 

small islands and in species-poor habitats. Today, the pearly-eye inhabits some 

80 islands and cays throughout the Greater Caribbean basin, from Rum Cay and 

San Salvador in the Southern Bahamas south to Bonaire, Netherlands Antilles, 

over a north-south geographical range of almost 3000 km. With the exception of 
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Puerto Rico, on which it is common only in species-poor habitats, the pearly-eyed 

thrasher is noticeably absent from the larger islands, such as Cuba, Hispaniola, 

and Jamaica, inhabiting only their species-poor satellites. It generally does not 

colonize islands less than about 1 km2 or with three or fewer resident landbird 

species. Nor does it inhabit large islands greater than 1000 km2 unless they are 

species-poor and contain extensive, disturbed habitats. Although the pearly-eyed 

thrasher is resident on small islands with as few as four landbird species, it is 

found most often on islands and within habitats with about 16 resident species.

From an historical perspective, fossilized bones from an archaeological site 

near Trant’s Bay, Montserrat, represent the first prehistoric record for both Margarops species anywhere in the Caribbean. Fossilized bones from Antigua suggest 

that the pearly-eye might have inhabited nearby Puerto Rico much earlier than 

has been reported. Written accounts show that the pearly-eye has experienced 

severe population fluctuations over time, resulting in a southern range contraction and northern expansion. It has undergone significant intra-island population 

increases within the past 30 to 60 years in Puerto Rico, Montserrat, and Bonaire. 

As a supertramp, excluding density-compensation islands, e.g., Mona (Puerto 

Rico), and samples taken during periods of ecological stress (e.g., droughts and 

hurricanes), the pearly-eye reaches its highest relative abundance in wet-forest 

habitats because wet forests are generally more depauperate in bird species than 

dry forests of the West Indian islands.

The pearly-eye 

reaches its highest 

relative abundance 

in wet-forest habitats 

because wet forests 

are generally more 

depauperate in bird 

species than dry 

forests.
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GENERAL TECHNICAL REPORT IITF-GTR-27
107

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Morphological characteristics of insular birds involving body dimensions and 

plumage have been interpreted as adaptive responses to the environmental 

conditions on islands (Grant 1965a, 1965b; Grant and Grant 1999). An important condition is the relative paucity of species. In the absence of close relatives, resident birds often extend their foraging activities, with concomitant 

modification of the feeding and locomotor apparatus and resulting in enhanced 

morphological variation (Grant 1965a). This phenomenon is known as ecological 

(or character) release. It is basically a broadening of the niche in the absence of a 

competitor or restrictive species. As both supertramp and competition theories 

predict, the pearly-eye shows substantial variation in body size among islands and 

habitats throughout its range. Does the pearly-eye undergo ecological release in 

the absence of close competitors? Conversely, could the evolution of ecological 

character displacement1 have been a prerequisite to the observed syntopy (cooccurrence in a habitat) on some islands by the pearly-eyed thrasher and other 

interspecifics, e.g., the congeneric scaly-breasted thrasher, competing for the 

same resources via diffuse competition? (see discussions by Brown and Wilson 

1956, Schluter and Smith 1986). Various analyses were performed to address this 

question. However, before investigating the potential effects of ecological release 

in the absence of competitors (or possible character displacement in their presence), one has to first consider potential intraspecific geographical, biological, and 

ecological sources of variation that might account for the pearly-eye’s variable 

body size among sympatric and allopatric populations (for a general discussion 

of individual variation in birds, see Curio 1977). Before offering and discussing 

the results from numerous intra- and interspecific analyses that I conducted on 

museum specimens and live, free-flying thrashers, I will present and explain the 

statistical procedures used and the rationale behind their selection.

Statistical Analyses

There are two main approaches to the most effective means of calculating overall 

body size in birds and other vertebrates. The more traditional approach advocates 

the use of uni- or bivariate analytical approaches using a variety of parametric 

and nonparametric statistical tests (see, for example, Johnson and Marten 1992). 

More recently, there has been a trend toward using multivariate procedures. 

The multivariate techniques most commonly used to determine and compare 

Chapter 5: Morphology

1Ecological character displacement may be defined as the effect of competition in causing 

two initially allopatric species to diverge from each other in some character upon attaining 

sympatry...” (Diamond et al. 1989).

 In the absence of 

close relatives, 

resident birds 

exhibit enhanced 

morphological 

variation.
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GENERAL TECHNICAL REPORT IITF-GTR-27

vertebrate body size and shape are canonical variate analysis (Abbott and Abbott 

1978, Albrecht 1980); principal components analysis or “PCA” (e.g., Freeman and 

Jackson 1990, Lande and Arnold 1983, Lougheed et al. 1991, Rising and Somers 

1989); discriminant function analysis (DFA) (Sites and Willig 1994b, Storer 1989); 

and multivariate analysis of variance and covariance (MANOVA and MANCOVA) (Lande and Arnold 1983, Sites and Willig 1994a, Willig and Owen 1987, 

Willig et al. 1986). Some investigators use a combination of uni- and multivariate 

analyses (e.g., Abbott 1978).

For comparative purposes, confirmation of analytic results, and to appeal to 

a wider audience (namely, proponents of univariate methods and those advocating 

multivariate techniques), I chose both uni- and multivariate techniques to analyze 

the pearly-eye’s morphometric data. I used external morphological characters 

(and various ratios—see Amadon 1943) easily obtained in the field and from 

museum specimens, namely (1) body mass (mainly field) and various longitudinal measurements (appendicular lengths), (2) wing chord, (3) ninth primary, 

(4) culmen from the feathered base (exposed culmen), (5) culmen from the anterior 

portion of the nares (bill length from nares), (6) center rectrix, and (7) tarsus. For 

the multivariate analyses, I chose multivariate multiple regression, MANOVA, 

MANCOVA, and PCA (see “Study Area and Methods” for descriptions).

Before discussing several endogenous and exogenous factors affecting pearlyeye morphology, I will present the outcome of numerous statistical and comparative analyses to facilitate a more rapid and precise interpretation and assimilation 

of the information contained in this chapter (fig. 5.1). In summary, biological, e.g., 

genetic factors, followed by geographical and climatic factors, tend to have more 

effect on pearly-eye morphology than ecological factors, although the latter are 

significant contributors in many instances.

Sources of Intraspecific Variation in Avian Body Size

Geographic Effects

Among allopatric populations of birds, body size is greatly influenced by a 

variety of site-specific and isolation factors, although such geographical isolation 

factors do not always precede other kinds of isolation (Thorpe 1945). In general, 

size variation is ordered clinally and is significantly related linearly to latitude, 

longitude, elevation (or isophane, a single function of the latter three), and 

landmass, or area (Abbott 1974b, Baker 1980, Diamond 1973b, Hamilton 1961, 

Murphy 1983, Power 1970, Rand 1936, Zink and Remsen 1986; but see Grant et 

al. 1985). Dispersal is also a factor, albeit indirect (Grant et al. 2001). Although 

there are exceptions (see Abbott 1974b), including the pearly-eye (see below), 

Biological factors, 

followed by geographical and climatic 

factors, have more 

effect on pearly-eye 

morphology than 

ecological factors.
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Biology Genetics

Age

Gender

Ontogenesis

(and allomorphosis)

Ontogenetic migration

Physiology

Metabolism

*

*

Pearly-Eye Morphology

Phenotype

Heritability

Polygenetic inheritance

Centripetal selection

Centrifugal selection

Sexual dimorphism

Sexual selection

Genotype

Homozygosity

Heterozygosity

Multiple allelism

Pleiotropy

Genetic recombination

Mutation

Immigration

Gene flow

Genetic drift

Season

Insolation

Temperature

(Allen’s rule)

Rainfall

Humidity

Populations and Isolation

Dispersal

Parapatry

Allopatry

Sympatry

Elevation

Landmass

Latitude

(Bergmann’s rule)

Geography Climate Ecology

* *

Competition

Intra- and intergeneric

interference (individuals)

Diffuse (community)

Exclusion (individual

and/or community)

* Character displacement

Niche shifts

Adaptive radiation

Ecological release

Figure 5.1—Biological, geographical, climatic, and ecological influences governing body size of the pearly-eyed thrasher throughout its range. 

The greater the influence, the more pronounced is its respective arrow (interior arrows only; exterior arrows not differentially sized). Asterisks 

signify the most influential biotic and abiotic factors (the larger the asterisk, the more important the factor). The most important factors are, in 

descending order, (1) age, (2) gender, (3) elevation, (4) season, and (5) competition.
110

GENERAL TECHNICAL REPORT IITF-GTR-27

birds as a taxon generally follow Bergmann’s ecogeographic rule, i.e., body size 

in intraspecific and interspecific homeotherms tends to be positively correlated 

with landmass, latitude, and colder climates (see Graves 1991, James 1970, 

and references therein). Graves’ (1991) and James’ (1983, 2001) results support 

the hypothesis that climate, especially temperature, seasonality, and habitat 

type, are important environmental determinants of geographic size variation 

in homeotherms (see also Murphy 1985, Snow 1954). Likewise, a phenomenon 

similar to Bergmann’s rule occurs in regard to elevation. In general, avian body 

size increases with elevation (see Diamond 1973a, Grant 1979, Graves 1991, and 

their respective references), which in turn is confounded with landmass as larger 

islands tend to be of greater topographical relief.

As a case in point, McLaughlin and Roughgarden (1989) noted a significant 

difference in the pearly-eyed thrasher’s body size and diet among individuals 

from Anguilla and St. Eustatius. They attributed this apparent niche shift in 

the pearly-eye’s body size to the abundance of its congener, the scaly-breasted 

thrasher. However, regardless of potential competitors or food resources, it is 

intuitive that Anguilla pearly-eyes would be smaller because the island’s highest 

point is 65 m. Similarly, the pearly-eye is expected to be much larger on the island 

of St. Eustatius because all data were collected “...at various points along the rim 

of the Quill (highest elevation: 600 m)....” (McLaughlin and Roughgarden 1989). 

Although these authors’ conclusions may be valid (and thus supportive of the 

competition and supertramp theories advocated in the present study), this may 

in fact simply be a good example of the widely observed thrasher body size shifts 

with elevation (see below).

To view the effects of latitude, landmass, and elevation on thrasher body 

size on a regional scale, these variables were included in the analyses of museum 

specimens from different island populations.

Latitudinal Effects (Museum Specimens)

To investigate the anticipated direct correlation between latitude and pearly-eye 

body size, I measured 584 specimens from five North American museums (see 

“Study Area and Methods” for details). Of the five characters measured, three 

were correlated with latitude. However, contrary to Bergmann’s Rule, all three 

were negatively correlated with increasing latitude, i.e., external appendages were 

shorter at higher latitudes: tarsus (R = -0.24, P <0.001, n = 499); wing chord (R =

-0.18, P <0.001, n = 584); and culmen from nares (R = -0.14, P = 0.001, n = 492). 

Length of the exposed culmen also tended to decrease with increasing latitude, 

but not significantly (R = -0.03, P = 0.38, n = 549). Only tail length tended to 
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

increase with increasing latitude, but not significantly (R = 0.07, P = 0.08, n = 

485). To reemphasize the importance of elevation in influencing the pearly-eye’s 

morphology, most Lesser Antillean islands from which many specimens were 

collected are high-relief islands (e.g., Montserrat, Guadeloupe, Dominica, St. 

Lucia), whereas the northern islands such as those in the Bahamian Archipelago 

are all low-relief islands (max. elev. 67 m). Therefore, the inverse correlation 

between pearly-eye body size and increasing latitude may well be caused, or at 

least confounded, by the inclusion of large individuals from montane rain forests 

on several Lesser Antillean islands and small individuals from low-lying islands in 

the northern latitudes (app. 4.1). The narrow span of latitude (about 12 to 27°N) no 

doubt was also a contributing factor. It became evident from the onset that many 

more factors, besides latitude, affect the pearly-eye’s body size.

Landmass and Elevational Effects (Field Measurements)

When the pearly-eye’s external morphological characters were correlated with 

landmass, only tail length significantly increased as island size increased (R = 0.15, 

P <0.001, n = 485). As with latitude, although none of the other four characters 

was strongly correlated with island size, there was a tendency for each to diminish 

in length with increasing landmass: tarsus (R = -0.08, P = 0.06, n = 499); wing 

chord (R = -0.008, P = 0.84, n = 584); culmen from nares (R = -0.03, P = 0.41, 

n = 492); and exposed culmen (R = -0.02, P = 0.63, n = 549). These simple 

correlation analyses notwithstanding, when included in more comprehensive 

multivariate analyses (see below), latitude and, to a lesser extent, landmass were 

shown to be significant factors in governing pearly-eye body size in four of five 

and two of five characters, respectively. After considering the effects of latitude 

and landmass, I then evaluated the more significant influences of elevation and 

climate on the pearly-eye’s body size.

Elevational information is usually missing from museum tags, especially from 

older specimens. Therefore, to determine the effect of elevation on the pearly-eye’s 

body size, I chose individuals from among populations within a single island. 

Three allopatric populations were sampled in Puerto Rico. Thrashers were captured and measured along an elevational gradient: (a) low (0 to 150 m, including 

coastal areas, the Guánica Biosphere Reserve, and other, mostly dry-forest habitats); (b) mid (250 to 400 m, mesic and wet forest, with several samples from the 

mid elevations of the Luquillo Experimental Forest and surrounding slopes); and 

(c) upper (650 to 800 m, the transition zone between colorado (Cyrilla racemiflora

L.) and cloud forest, and the Icacos Valley on the southern slopes of the Luquillo 

Experimental Forest). For comparison with Puerto Rico’s coastal and lowland 
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GENERAL TECHNICAL REPORT IITF-GTR-27

thrasher populations, the nearby, low-relief island of Mona (0 to 80 m) was 

included in the analyses. To reduce the influences of reproduction (potentially 

heavy females) and season (feather wear), samples were taken between November 

and February. During these months, plumage is “fresh” and few thrasher pairs 

are breeding. To investigate the potential role of seasonal effects, March through 

October samples were then compared to November through February samples 

(fig. 5.2).

In general, the pearly-eye’s body size, especially body mass, increases with 

elevation (see fig. 5.2; and discussion below). Coastal and lowland pearly-eyes 

average between 90 and 100 g, whereas highland individuals can average 

between 110 and 123 g. To date, the heaviest El Yunque (highland rain forest) 

pearly-eye measured was a nongravid female weighing 138 g, almost 50 g more 

than the lower average for coastal and lowland individuals. However, this is 

not the heaviest thrasher on record. In a montane rain forest on St. Lucia, 

J.L. Gulledge (pers. comm.) collected a nongravid (small ovaries) female that 

weighed 153 g. To my knowledge, this is the heaviest individual reported thus far. 

The extreme size and striking plumage differences in St. Lucia’s pearly-eyes led 

Garrido and Remsen (1996) to propose a fourth subspecies (M. f. klinikowskii) 

for that island’s chromatically and morphologically distinct geographic race.

Climatic Effects

It is well known that the environment, especially climate (together with 

underlying genetic components), is an important factor influencing avian 

body size and appendicular characters (Boag 1984). Insular allomorphosis, 

i.e., heterogeneity in allometric (differential) growth rates, has been shown to 

be a result of selection for heat exchange (Harrison et al. 1964). Still, it seems 

unlikely that the cornified surface of the tarsus and bill would be of much value 

in heat exchange (but see Power 1970). Instead, the larger dimensions of the bills 

and tarsi of insular birds are generally thought to be adaptations primarily to 

ecological conditions (but see Soulé and Rindge Stewart 1970, Willson 1969, 

Willson et al. 1975) concomitant with sexual dimorphism, ecological release, and 

niche shifts (Baker-Gabb 1986; Diamond 1975b; Diamond and Marshall 1977a, 

1977b; Faaborg 1980a; Grant 1965a; Johnson 1966; Selander 1966; Terborgh 

and Faaborg 1973; Winkler and Leisler 1985). Island birds use a wider variety of 

perch types, tend to be more terrestrial, and are euryphagous (eat a wide variety 

of food items), resulting in a greater range of body sizes (Grant 1983b). For 

general discussions on ecomorphology and underlying hypotheses, see Lederer 

(1984) and James (2001).
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Figure 5.2—Descriptive statistics (mean, ±1 standard error, [S.E.] and sample size [n]) of groups used in 

statistical analyses and defined by combinations of season (March–October vs. November–February) and 

elevation (Mona vs. Guánica vs. El Yunque [mid-elevation] vs. El Yunque [high elevation]) on each of six 

morphometric characters of the pearly-eyed thrasher. Treatment levels, e.g., combinations of season and 

island such as Mona in March–October, which share a common letter (a, b, c...h) are statistically indistinguishable at the specified alpha level of the test. Levels of significance are as follows: [*] 0.05 ≥ P > 0.01, 

[**] 0.01 ≥ P > 0.001, and [***] 0.001 ≥ P >0.0001. Because few measurements of length of tail and bill length 

from the nares were available for specimens from Mona, seasonal data for that island could not be included 

in analyses for those characters, and those characters could not be included in multivariate analyses. All 

measurements are in millimeters.
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GENERAL TECHNICAL REPORT IITF-GTR-27

Three of the six characters sampled from Mona Island thrashers were quite 

similar (exposed culmen and wing), or even larger (tarsus), in their linear dimensions when compared to those sampled from highland individuals (fig. 5.3). This 

was not surprising because it has been known for decades (e.g., Grant 1965a, 1980; 

Murphy 1938; Van Valen 1965) that island birds exhibit longer bills and tarsi than 

those of the mainland (or in this case a nearby high-relief island with a greater 

range of habitats and elevations as well as many more potential competitors). Is it 

possible that Allen’s ecogeographic rule may explain the attenuated appendicular 

characters of Mona’s pearly-eyes? This rule states that in polytypic species of 

endotherms, extensions of the body tend to be larger (and have a higher surface/

volume ratio) in the warmer parts of the range and shorter (with a lower surface/

volume ratio) in cooler parts (see also Baker 1980, Power 1970). Mona Island is 

desert-like in many respects, e.g., xerophytic vegetation, numerous species of cacti, 

and prolonged and severe droughts. Moreover, thrashers in general have long 

beaks, wings, tails, and tarsi, and most species are thought to have originated in 

xeric habitats (e.g., Toxostoma spp. and Mimus spp.).

Effects of Dispersal on Local Body Size

Because the pearly-eye is known to undergo inter-island dispersal, e.g., Barbuda 

to Guadeloupe (chapter 3), the possibility that Mona may be an ecological sink for 

dispersing thrashers from neighboring, high-relief islands, many with wide elevational gradients (see below), must be taken into consideration. As a case in point, 

I was informed (M. Frost 1999, in litt.) in June of that year, Dr. Phil Hansbro of 

Cambridge University, United Kingdom, examined a pearly-eye specimen now 

housed at Cambridge that was collected at Bagatelle, Barbados, on March 2, 1889, 

by C.J. Manning (see also app. 3). After considering all the external plumage and 

appendicular measures, Dr. Hansbro concluded that the specimen is of the newly 

designated (Garrido and Remsen 1996) St. Lucian subspecies (M. f. klinikowskii). 

Confirmation through genetic testing is desirable. Still, this may be yet another 

example of inter-island dispersal by a large-bodied pearly-eyed thrasher from a 

high-relief island that, when collected, was assumed to be a permanent resident 

on a low-relief island.

Effects of Investigator Bias, Genes, Age, Gender, and Season

Investigators themselves influence avian morphometric results by making simple, 

individually biased measurement errors. However, if care and standard measurements are taken, accuracy (repeatability) is usually over 95 percent, and measurement error among investigators is minimal and will not significantly affect the 

Transmarine dispersal 

affects local body size.
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Figure 5.3—Variation in the pearly-eye’s body size with increasing elevation. Individuals were captured and measured at low 

(0–150 m), mid (250–400 m), and upper elevations (650–800 m) in Puerto Rico. To investigate the possibility of small-island effects, 

a sample from nearby Mona Island was included for comparison. Although overall body size increases with ascending elevation, 

there were some unexpected exceptions, e.g., significantly longer bills and tarsi in the Mona Island population. Body mass is in 

grams and longitudinal measures are in millimeters.
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GENERAL TECHNICAL REPORT IITF-GTR-27

results (Arendt and Faaborg 1989, Boag 1984, Gannon et al. 1992, Price and Grant 

1984). More important are the biological and ecological effects of age, gender, and 

season on the body size of individuals within a population (Arendt and Faaborg 

1989, Rising and Somers 1989).

Among birds, real size differences in internal characters (e.g., skull and 

dimensions of long bones) as well as external characters, e.g., body mass and 

the lengths of such characters as culmen, tail, wing, and tarsus, result from 

the influence of many biological and ecological factors. Three relatively easily 

measured and highly germane biological and ecological factors are age, gender, 

and season. Several ecological factors will be discussed below. But first, from a 

genetic perspective, it is noteworthy that phenotypic variation reflects underlying 

genetic (heritability) and ontogenetic (dynamic allometry) variation (Grant 1984, 

Smith and Dhondt 1980, Smith and Zach 1979). Heritability often accounts for 

80 to 90 percent or more of the observed variation in passerine morphological 

characters (Boag 1983, Grant 1983a, Grant et al. 2001). Furthermore, morphological character variation increases with allozymic (multiform enzymes) homozygosity (identical alleles at a given locus) and decreases with heterozygosity (two 

alleles at a given locus) (Eanes 1978, Fleischer et al. 1983, Lande 1980, Lougheed 

and Handford 1993) (but see Handford 1980 as an exception). In general, island 

populations have less allozyme variation than mainland populations, and insular 

endemic populations show reduced genetic variation (Frankham 1997). Genetic 

studies have shown that the basic features of complex morphologies result from 

polygenic (multifactorial) inheritance with multiple allelism (multiple forms on the 

same locus), pleiotropy (more than one phenotypic effect), and genetic recombination, mutation, and immigration. Genetic models show population variation is in 

a state of balance between the two opposing forces of mutation and immigration, 

which tend to elevate variation, whereas selection (including sexual selection) and 

genetic drift, tend to decrease it (Grant and Grant 1983, Grant and Price 1981, 

Huxley 1955, Price et al. 1984a). A similar phenomenon occurs at the level of the 

phenotype (Gerhart et al. 1997, Rothstein 1973). Elimination of extreme individuals, i.e., selection for an optimum phenotype (directional, or centripetal selection) 

results in a directional shift in gene frequencies of the character concerned and 

leads to a state of adaptation (see Grant 1972b, 1985; Schluter and Smith 1986) in a 

progressively changing environment (Lincoln et al. 1983). Conversely, selection for 

phenotypic extremes (disruptive, or centrifugal selection) in a polymorphic population preserves and accentuates discontinuity (Lincoln et al. 1983). Moreover, 

environmental heterogeneity is correlated with large continuous, morphological 

variation (see, for example, Grant and Grant 1979, Grant et al. 1976).

Phenotypic variation 

reflects underlying 

genetic and 

ontogenetic 

variation.
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Ontogenesis (growth and development), which often continues after fledging, 

is also responsible for the morphological variation observed in avian external 

characters (Smith et al. 1986). For example, wing chord and tail length vary with 

age. Wing length and body mass often increase from the first to second year in 

passerines (Freeman and Jackson 1990, Stewart 1963). This ontogenetic effect 

is treated separately below from that of seasonal influences (climatic effect) of 

feather wear (Arendt and Faaborg 1989, Boag 1984, Grant 1971, Rising and 

Somers 1989).

Internal organs (see Davis 1961), body mass, and especially bill length, vary 

through time (e.g., seasonally). Variation in bill length is a result of rhamphothecal 

growth, frictional wear, and change in diet commensurate with seasonal variation 

in food types and quantities (Clancey 1948, Davis 1954, Freeman and Jackson 

1990, Grant 1983b, Marshall 1948, Miller 1941, Packard 1967, Price et al. 1984b, 

Rising 1972, Rothstein 1973, Schluter 1982, Schluter and Grant 1984). Even with 

these factors taken into account, however, a pronounced size difference exists 

between the sexes in adult pearly-eyes. Therefore, the influences of age, gender 

(sexual dimorphism), and season will be addressed.

Age and gender of live pearly-eyed thrashers are difficult to ascertain (more so 

in free-flying individuals, and only slightly better in the hand) because the species 

is basically monochromatic and obtains its definitive “adult” plumage within its 

first year. Even with years of practice at detecting subtle differences in plumage 

coloration (including hue, contrast, and brightness, especially in the scaling effect 

of the breast feathers) in combination with morphological measures and “skulling” (the use of cranial bone ossification to determine age), it is difficult to determine age and sex of pearly-eyes with complete accuracy. Even trying to attribute 

the masculine gender to a singing pearly-eye is risky because, as in many other 

species, estrogen production in older females diminishes, causing them to exhibit 

“male” characteristics such as “male” plumage and song (Baptista et al. 1993; see 

also Baumgartner 1986 for an example in a 4-year-old northern cardinal Cardinalis cardinalis; and Arendt 1987 for an example and explanation of sex reversal in a 

7-year-old minimum-aged American redstart, Setophaga ruticilla, that had made 

at least 15 flights between North America and Puerto Rico). During the course of 

the present study, three female thrashers older than 4 years of age (5, 7, 13) often 

were observed singing from within 2 m of their nest boxes. One female habitually 

sang from a song perch several meters high in a bamboo stand about 4 m from 

her nest box. Female song in mimids was reported as early as 1899 (Cohen 1899). 

Cohen shot a female California thrasher (Toxostoma redivivum) that “...was singing, perched on top of a bush.” Interestingly, Cohen added that it “was covered 
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GENERAL TECHNICAL REPORT IITF-GTR-27

with vermin, especially the neck and breast.” Although not specified, the vermin 

might very well have been a species of the common chewing lice (Mallophaga), 

an ectoparasite that has been found in much higher intensities in very old female 

pearly-eyes (see discussion under lifetime reproductive success in chapter 7). This 

heavy infestation of ectoparasites, especially in the head and upper breast regions, 

suggests that Cohen’s singing female most likely was a very old individual that had 

acquired male characteristics as she aged.

Female song in general, although once thought to be rare in birds, especially 

in female oscines in north temperate latitudes (Kling and Stevenson-Hinde 1977, 

Nice 1943, Ringleben 1982), is now known to occur more frequently in nature than 

originally thought. Moreover, besides aging, female song has been attributed to 

several other physiological, biological, and ecological functions such as fluctuating estrogen levels in females of all ages throughout the year, and guarding of 

nest sites, territories, food supplies, and even male breeders (Arcese et al. 1988, 

Baptista et al. 1993, Kern and King 1972, Thorpe 1961). As more and more longterm studies are undertaken, undoubtedly not only several more examples but, 

more importantly, many more functions of female song within much broader 

contexts will be discovered.

Owing to the difficulty in determining age and sex of free-flying thrashers, 

the following analyses are based on banded adults of known gender, including 

known- and minimum-aged individuals, that breed in thrasher boxes as part of 

my ongoing life-history study within the Luquillo Experimental Forest. For purposes of comparisons on a regional scale, additional analyses are then conducted 

based on museum specimens of pearly-eyes of known sex (age data were scant, 

and thus excluded). Males often prevail in museum collections simply because 

singing males are more readily detected and, thus, are more frequently collected 

than are females.

Early in this study, age was shown to be a better predictor of pearly-eye body 

size than was season (Arendt 1993: app. 4.2), although seasonal feather wear 

is appreciable in the pearly-eye, and thus can significantly affect wing and tail 

measurements (Arendt and Faaborg 1989). Tail length in recently fledged pearlyeyes falls far short of its asymptotic (definitive) length and thus increases with 

age. Therefore, it is not surprising that age was the best predictor of tail length, 

followed by gender. A regression of the three effects (age, gender, and season) on 

tarsus length showed that any one (or combination) of them does not significantly 

affect it. Furthermore, tarsus length reaches more than 90 percent of its asymptotic length before nestlings fledge (Arendt 1985a). Thus, the tarsus is a good 

predictor of the pearly-eye’s body size.

Age is a better 

predictor of pearlyeye body size than is 

season: pearly-eyes 

are sexually dimorphic; 

each sex is smaller in 

some characters, but 

larger in others.
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

In another study of avian body size by using a combination of statistical 

methods (bivariate correlations, stepwise regression, and principal-components 

analysis), Freeman and Jackson (1990) also found tarsus (and mass) to be the best 

predictor of overall body size. In an earlier study using a multivariate approach, 

Miles and Ricklefs (1984) determined that only tarsus and mid-toe lengths contributed strongly to foraging behavioral correlations. For most field biologists who 

rely solely on measurements of live birds, tarsus may be the best single predictor of 

a bird’s overall size and foraging behavior. Although intuitively body mass would 

seem to be a good indicator of body size, it is well known that body mass varies in 

response to many geographical, genetic, physiological, and environmental factors. 

Body mass, although a good candidate for comparing body size, is too variable 

owing to these many and diverse influences. That is, unless linear (appendicular) 

measurements are compared as ratios to the cube root of body mass (Amadon 

1943, Winkler and Leisler 1985). Comparisons using body mass alone would be 

misleading in the case of the pearly-eye because body mass increases throughout 

the day and with the ingestion of fruits with large seeds. Conversely, body mass 

generally decreases in both sexes as the reproductive season progresses, except 

in fecund females during the egg-laying months, at which time it significantly 

increases (Arendt 1993, Arendt and Faaborg 1989).

Also in the infancy of this study, to test for sexual dimorphism in the pearlyeye, both uni- and multivariate statistics were performed on the El Yunque 

nest-box adults (Arendt 1993: app. 4.4). Results revealed that each sex is smaller in 

some characters, but larger in others. For example, whereas the wing chord, ninth 

primary, and tail are longer in males, females measure as large (tarsus) or larger 

(e.g., bill) in other external longitudinal characters (fig. 5.4). As early as the late 

1800s, Newton and Newton (1859: 142), referring to pearly-eyes collected in St. 

Croix, Virgin Islands, stated that “The sexes do not differ materially,” [but] “...the 

bills of individuals vary greatly in size.” Furthermore, as in the pearly-eye, females 

of the closely related tremblers also have longer bills than do males (Storer 1989), 

which may be an example of character displacement to allow both sexes to exploit 

different-size plant and animal foods within the same territory.

A multivariate multiple regression model showed that gender, and interaction 

terms including it, had the most influence on six of the seven measured characters, 

tarsus being the only exception (Arendt 1993: app. 4.1). Storer (1989) also found 

tarsus to be the least variable between sexes of the closely related tremblers.

Additional univariate and multivariate analyses were conducted to further 

evaluate the spatiotemporal effects of elevation and season on the pearly-eye’s 

morphology. Descriptive statistics (mean, standard error, and sample size) for 
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GENERAL TECHNICAL REPORT IITF-GTR-27

Figure 5.4—Sexual dimorphism in the pearly-eyed thrasher. Typical of most mimids, males (M) tend to have longer wings and tails 

than females (F). Pearly-eye females are generally heavier, with longer bills. Data were taken between 1979 and 1992 from 95 female 

and 88 male thrashers inhabiting the Sierra de Luquillo Mountains, Puerto Rico. To demonstrate the extent of sexual dimorphism on 

a regional scale, 546 museum specimens collected from 40 different islands are included for comparison (box plots). “Primary 9” is 

the ninth primary flight feather. Body mass is in grams, all other measurements are in millimeters.
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

treatment groups are summarized in figure 5.2, along with results of a priori and 

a posteriori analyses. Loadings of characters on size and shape, respectively, are 

base, -0.103, 0.949; mass, -0.990, -0.095; wing, -0.097, -0.080; and tarsus, -0.014, 

-0.289.

Both multivariate and univariate approaches suggest that elevational 

differences in morphology depend on season (table 5.1). There was a highly 

significant two-way interaction in the MANOVA, and significant two-way 

interaction for five of the six characters separately. Moreover, all six characters 

contributed to this response. Alternatively, this may be conceptualized as if 

Table 5.1—Results of statistical analyses assessing the effects of season 

(March–October vs. November–February) and elevationa (Mona vs. Guánica 

vs. El Yunque [midelevation] vs. El Yunque [high elevation]) on each of six 

morphometric characters (two-way ANOVAb), as well as based on all characters 

together (two-way MANOVAc)

P-values

Explained Elevationd Season Elevation × season

Character df = 7 df = 3 df = 1 df = 3

Exposed culmen <0.001* <0.001 <0.001 <0.001

Bill from naresd <.001* <.001 .035 .369

Tarsus <.001* <.001 .154 <.001

Wing <.001* .015 .637 <.001

Taild <.001* .039 .147 <.001

Body mass <.001* <.001 <.001 .021

MANOVA <.001 .04 <.001

*Denotes significance after Bonferroni’s sequential adjustment.

aAnalyses are based on the ELEVCOMP data set (see “Study Area and Methods”).

bANOVA results for each should be interpreted with df = 5, 2, 1, 2 for each of the sources of variation in 

columns 2 through 5, respectively.

cBonferroni’s sequential adjustment corroborates the overall significance of the MANOVA and suggests 

that all six characters contribute to the differences in multivariable space.

d “Bill length from nares” and “Tail” have few data for elevation = 1 (Mona Island); therefore, they were 

not included in the MANOVA.

seasonal differences in morphology are contingent on elevation. Furthermore, for 

each elevation category (fig. 5.5), seasonal differences are related to size (PC-1) 

rather than shape (PC-2). At various elevations, diverse climatic and ecological 

factors such as insolation, temperature, humidity, rainfall, and consequential 

edaphic effects all govern the life cycles of the residing flora and fauna, and thus 

the availability and abundance of the pearly-eye’s food resources. This in turn 

influences the timing of molt in individuals of all ages and, in immatures, the rate 

and pattern of growth (increase in body mass, long bones, and rhamphotheca). 

Note that in figure 5.5 the variation (+1 SE) in size and shape is greatest in the 

March through October samples, the period of heaviest breeding, peaks in 
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GENERAL TECHNICAL REPORT IITF-GTR-27

flowering and fruiting food trees, and inclusion of most postfledged young in 

the net samples. In addition, variation is greatest in the coastal lowland thrasher 

populations (Mona and Guánica), the geographic locations and habitats most 

likely to first receive dispersing immatures from other islands or more saturated 

and highly competitive upland forest habitats (explained further in Ricklefs and 

Cox 1972, 1978, in their discussions of the taxon cycle). Conversely, the November 

through February samples at all elevations show less variation among sampled 

individuals (fig. 5.5). This is the period of less primary and secondary production 

in the forest, and the time by which the early-fledged (March through August) 

immatures have attained asymptotic proportions in most external morphological 

characters and are preparing to breed for the first time.

Controlling for temporal effects, elevational trends within seasons (fig. 5.5) 

mostly involved size, although shape is important in distinguishing groups in 

November through February. More specifically, the elevational sequence “Mona 

Figure 5.5—Graphical portrayal of results from a principle components analysis (PCA) of log-transformed 

morphometric characters (table 5.1) of the pearly-eyed thrasher. Groups were defined by combinations of 

season and elevation as in table 5.1. Vertical and horizontal lines that intersect centroids represent one standard error for PC-2 (shape) and PC-1 (size), respectively. Analyses are based on the ELEVCOMP data set.
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Island–Guánica–high-elevation El Yunque–mid elevation El Yunque” forms a 

gradient of size from small to large in March through October samples. Intuitively, one would predict the correct sequence to be “Mona Island–<Guánica–

<El Yunque mid–<El Yunque high” and thus might consider these test results 

erroneous, possibly owing to other unknown physical or ecological influences. 

However, subsequent analyses of thrasher populations on other high-relief islands 

(e.g., Montserrat) showed the same trend; that is, an increase in body size from 

the coast to montane forest, but then a decrease in body size from samples on the 

highest peaks, especially near summits in cloud forest. Plants and their fruits are 

known to decrease in size on the higher slopes owing to physical, climatic, and 

edaphic conditions. I offer two possible explanations that are by no means the 

only ones, nor are they mutually exclusive. Highland thrasher populations feeding 

near mountain summits may have adapted to the reduction in food size, resulting 

in smaller phenotypes. Or, it is possible that more of the individuals foraging at 

the highest elevations are dispersers or “floaters” (mostly young, and thus smaller 

individuals) searching for food in less competitive habitats (avian species richness 

decreases at higher elevations in the Sierra de Luquillo as well as on other islands). 

Although as many as 30 nest boxes have been available for nesting in cloud forest 

on the higher peaks for the past 10 years, nesting has occurred only at the lowest 

extremes in the transition zone from colorado to cloud forest (see Beissinger et al. 

2005 and Cook et al. 2003, 2004, 2005 for plausible explanation).

Based on size (PC-1) in the November through February categories (fig. 5.5), 

as anticipated, groups from Mona Island and Guánica are indistinguishable, and 

elevational groups from El Yunque are larger than those from Mona Island or 

Guánica. Elevational groups from El Yunque are sometimes indistinguishable 

from each other probably because the elevational range is minimal.

In the November to February samples, large shape differences characterize 

the contrast between Mona and Guánica samples. A plausible explanation 

would be that climatic and environmentally induced microhabitat differences, 

e.g., differences in rainfall and vegetation (especially plant species richness and 

composition), may be causing the divergence in shape among the sampled external 

characters. The fact that Mona Island pearly-eyes have longer tarsi corroborates 

the assumption that, owing to the island’s drought-resistant vegetation with its 

more diminutive stature, thrashers there are more terrestrial than are those on the 

main island. Coastal and dry-forest populations on the main island often inhabit 

forests of greater stature with a wider variety of plant and animal food resources 

(but see discussion below on adaptation to terrestrial living).
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GENERAL TECHNICAL REPORT IITF-GTR-27

Extent of Regional Sexual Dimorphism

From a regional perspective, and to obtain a more composite picture of the effects 

of gender and season on pearly-eye morphology, more comprehensive univariate 

and multivariate tests were performed. Descriptive statistics (mean, standard 

error, and sample size) for all morphometric characters differed considerably with 

season and gender (table 5.2). After controlling for the effects of age as a covariate, 

season and gender each had a consistent effect on morphometric variation based 

on MANOVA and Bonferroni’s sequential adjustment of univariate results (table 

5.3). In general, males are distinguished clearly from females based on size and 

shape, and this is consistent in all seasons (fig. 5.6). Loadings of characters on size 

and shape, respectively, are wing, 0.004, 0.133; mass, -0.883, 0.385; base, -0.297, 

-0.612; nares, -0.306, -0.639; primary 9, -0.168, 0.114; tarsus, -0.101, 0.164; and tail, 

-0.020, 0.112. Results from these more indepth analyses uphold those previously 

obtained from simpler univariate tests and multiple regression models.

To further investigate the extent of sexual dimorphism within sympatric 

populations and among subspecies throughout the region, following techniques 

used by Storer (1989), I compared the variation in external morphological characters by calculating a dimorphism index. Age was excluded in this analysis because 

it is often unattainable from museum specimens. Within a single population (El 

Yunque), as shown earlier (Arendt 1993: app. 4.4), sexual dimorphism was most 

prevalent in body mass and bill measurements (table 5.4). A comparison of the El 

Yunque population to an average of several region-wide populations (table 5.5) 

supports the ecological release concept. Recall, with the exception of the redlegged thrush in some “edge” habitats, there are virtually no keen competitors 

of the pearly-eye in the Sierra de Luquillo. Thus, as predicted, both sexes in this 

single population show greater divergence in bill size because they are exploiting 

a greater variety and size of food items than if they had to compete with several 

other species. On the other hand, wing and tail lengths (indicators of overall body 

size) differ much less than in multiple populations in the absence of keen competitors and several other island and geographic effects.

In contrast to the El Yunque thrasher population considered separately, on 

a regional scale, i.e., including sympatric and allopatric populations of pearlyeyes, sexual dimorphism was most prevalent in wing and tail lengths, with males 

measuring longer in both (table 5.6). Conversely, females generally had the longest 

bills. Once again, tarsus length was least variable between the sexes. As anticipated, sexual dimorphic trends among the four subspecies were less definite, most 

probably owing to several confounding effects, especially elevation. The most 

isolated subspecies, M. f. bonariensis, presented prominent sexual dimorphism 

Region-wide 

comparison of 

pearly-eye sexual 

dimorphism supports 

the ecological release 

concept.

(text continues on page 129)
125

Table 5.2—Descriptive statistics (mean, standard error [SE], and sample size [n]) of groups used in statistical analysesa (tadefined by combinations of season and gender for each of seven morphometric characteristics of the pearly-eyed thrashe

Season = 1 (March–June) Season = 2 (July–October) Season = 3 (Novem

Males Females Males Females Males 

Character (mm) Mean SE n Mean SE n Mean SE n Mean SE n Mean SE n 

Millimeters

Exposed culmen 27.32 0.097 152 27.72 0.106 176 27.16 0.343 15 28.82 0.262 23 26.90 0.194 32 

Bill from nares 18.09 .069 152 18.70 .064 176 18.12 .161 15 19.10 .141 23 18.00 .129 32 

Tarsus 37.82 .088 152 37.59 .076 176 37.73 .228 15 36.99 .399 23 38.04 .159 32 Wing 138.46 .261 152 135.01 .276 176 137.37 1.370 15 134.49 .886 23 139.44 .463 32 1

Tail 115.13 .261 152 113.18 .273 176 115.58 .834 15 113.20 .800 23 116.91 .494 32 1

Body mass 106.70 .197 152 112.17 .183 176 103.41 .628 15 112.23 .507 23 106.36 .430 32 1

Primary 9 90.79 .342 152 89.18 .281 176 89.84 .630 15 89.17 .558 23 89.56 .466 32 

aAnalyses are based on the ELYUNQUE data set (see “Study Area and Methods”).
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GENERAL TECHNICAL REPORT IITF-GTR-27

Table 5.3—Results of statistical analysesa assessing the effects of 

season, gender, and age on each of seven morphometric characters 

after log-transformation (two-way ANOVA), as well as based on all 

characters together (two-way MANOVAb)

P-values

Explained Gender Season Gender × season Age

Character df = 6 df = 1 df = 2 df = 2 df = 1

log (exposed culmen) <0.001* <0.001 0.017 0.015 0.218

log (bill from nares) <.001* <.001 .191 .290 .743

log (tarsus) .004* .025 .011 .305 .133

log (wing) <.001* <.001 .002 .511 <.001

log (tail) <.001* <.001 .022 .581 .006

log (body mass) <.001* <.001 .514 .218 .057

log (primary 9) .001* .002 .705 .331 .001

MANCOVA <.001 .002 .069 .003

*Denotes significance after Bonferroni’s Sequential Adjustment.

aAnalyses are based on the ELYUNQUE data set (see “Study Area and Methods”).

bBonferroni’s Sequential Adjustment corroborates the overall significance of the MANOVA and suggests 

that all seven characters contribute to the differences in multivariate space.

Figure 5.6—Graphical portrayal of results from a Principle Components Analysis (PCA) of logtransformed morphometric characters (tables 5.2 and 5.3) of the pearly-eyed thrasher. Groups were 

defined by combinations of season and sex as in tables 5.2 and 5.3. Vertical and horizontal lines that 

intersect centroids represent one standard error for PC-2 (shape) and PC-1 (size), respectively. Males 

exhibit consistently smaller size (PC-1) than females and differ in shape (PC-2) as well, with no overlap 

between the sexes in any season. Analyses are based on the ELYUNQUE data set.
127

Table 5.4—Extent of sexual dimorphism in a pearly-eyed thrasher population at 600 m elevation in the Luquillo ExperimenPuerto Ricoa

Male Female

Morphological No. of mimid and 

character N Min.–Max. Mean SD N Min.–Max. Mean SD thrush competitor

- - - - - - - - - Millimeters - - - - - - - - - - - - - - - - - - Millimeters - - - - - - - - -

Wing 159 127.8–148 138.52 3.44 211 116.9–145.1 135.21 3.96 0 

Body mass 164 92.3–128 106.39 6.5 212 86–138 112.18 8.91 0 

Exposed culmen 149 22.71–30.59 27.22 1.19 192 20.4–32.74 27.82 1.39 0 

Bill from the nares 158 12.9–19.95 18.03 .82 209 12.63–20.69 18.79 .85 0 

Culmen depth 146 5.64–9.55 8.11 .49 189 5.34–9.71 8.47 .44 0 

Culmen width 147 5.35–8.17 6.58 .7 188 5.44–8.72 6.81 .71 0 

Primary 9 152 82.7–141.2 90.96 5.85 205 77.8–105 89.3 3.67 0 

Tarsus 159 33.98–40.6 37.89 1.04 213 27.72–40.14 37.57 1.18 0 

Tail 153 106.5–126 115.61 3.16 204 103.7–123 113.59 3.61 0 

aThere are no other mimid or thrush competitors except for the red-legged thrush, which is usually parapatric in this forest. However, immediately following twmajor hurricanes (Hugo in 1989 and Georges in 1998), thrashers and thrushes (together with other species) formed loose, meandering flocks in search of food.bDimorphism index obtained by dividing the difference between the means for the sexes (male minus female) by the overall mean and multiplying by 100; negativalues denote females are larger in the particular character.
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Table 5.5—Comparison of dimorphism indices to evaluate the extent of 

sexual dimorphism in a single (“EL Yunque”) pearly-eye population within the 

Luquillo Experimental Forest (LEF), Puerto Rico, compared to several allopatric 

populations throughout the Caribbean

Morphometric Sexual dimorphism indexa Extent of sexual dimorphism

character (mm) LEF Many populations LEF vs. many populations

Bill from nares -4.15b -1.13b Almost quadruple

Exposed culmen -2.19b -1.77b Pronounced

Tarsus .84 -.11b Greater

Wing chord 2.41 3.45 Less than

Tail 1.76 3.39 About half

aDimorphism index was derived by dividing the difference between the means for the sexes 

(male minus female) by the overall mean and multiplying by 100.

bNegative values denote females are larger in a particular character.

Table 5.6—Use of a dimorphism indexa to compare body size differences between the sexes among subspecies of 

the pearly-eyed thrasher

Male Female

 Dimorphism

Subspecies N Min.–Max. Mean SD N Min.–Max. Mean SD index

- - - - - - - - - Millimeters - - - - - - - - - - - - - - - - - - Millimeters - - - - - - - - -

Wing length

M. f. fuscatus 214 134.1–142.4 138.07 3.00 169 130.4–139.6 135.48 3.47 1.86

M. f. densirostris 51 130.4–142.1 137.74 3.88 46 128.6–138.3 133.14 3.62 3.39

M. f. klinikowskii 10 141.1–148.2 145.02 2.26 11 124.0–144.9 139.73 5.94 3.72

M. f. bonariensis 18 125.2–137.2 132.68 3.47 14 120.4–133.5 126.41 4.18 4.84

Tail length

M. f. fuscatus 208 109.6–118.9 114.12 3.43 176 107.0–116.6 111.82 3.75 2.15

M. f. densirostris 52 106.6–116.8 111.96 3.55 44 101.1–112.1 106.25 4.05 5.25

M. f. klinikowskii 10 112.2–122.6 116.52 2.96 11 105.5–120.1 115.12 4.48 1.21

M. f. bonariensis 18 100.1– 119.0 110.95 4.49 14 100.4–114.8 105.61 4.24 4.93

Tarsus length

M. f. fuscatus 215 36.3–38.8 37.46 0.95 167 36.0–38.9 37.71 1.09 -0.41

M. f. densirostris 54 35.2– 38.9 37.1 1.34 44 35.4–38.0 36.86 0.94 0.62

M. f. klinikowskii 10 38.5–40.6 39.61 0.67 11 38.1–41.7 40.03 1.09 -1.07

M. f. bonariensis 18 33.9– 38.3 37.05 1.1 14 34.8–39.5 36.89 1.12 0.43

Exposed culmen length

M. f. fuscatus 214 25.0–28.3 26.67 1.23 172 25.6–28.7 27.09 1.2 -1.4

M. f. densirostris 51 24.5– 27.5 26.05 1.04 43 24.7–27.5 26.3 0.93 -0.92

M. f. klinikowskii 10 25.5–28.3 26.85 0.98 11 25.4–29.5 27.14 1.14 -1.08

M. f. bonariensis 18 26.4– 29.6 27.57 0.93 14 26.4–30.5 28.6 1.22 -3.67

Bill length from nares

M. f. fuscatus 233 17.5– 19.4 18.38 0.69 169 18.0–19.7 18.87 0.65 -2.11

M. f. densirostris 47 17.1– 19.0 18.11 0.64 31 17.1–18.8 18.18 0.59 -0.33

M. f. klinikowskii 10 18.1– 20.2 19.06 0.64 11 18.1–19.8 19.1 0.61 -0.23

M. f. bonariensis 18 17.9– 20.5 19.26 0.82 14 18.0–21.2 19.62 0.83 -1.85

a

 Dimorphism Index was derived by dividing the difference between the means for the sexes (male minus female) by the overall mean 

and multiplying by 100; negative values denote females are larger in the particular character.
129

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Figure 5.7—Discriminant Function Analysis results from using five morphological characters (lengths of exposed culmen, bill from the 

nares, tarsus, wing chord, and tail) to compare body size among three subspecies of the pearly-eyed thrasher (Margarops fuscatus fuscatus, 

M. f. densirostris, and M. f. bonariensis). The most predictive variables showing bonariensis’ dissimilarity to the other two subspecies were, 

in descending order of importance, bill length from the nares, tarsus, and wing chord (similar to tail in importance). To illustrate gene flow 

among the M. f. fuscatus and M. f. densirostris populations, the classification matrix, as well as a jackknifed classification matrix, showed 

that 80 percent of the M. f. bonariensis characters was correctly classified, whereas only 50 percent of each of the other two was.

in four of the five characters (table 5.6 and fig. 5.7). In contrast to the El Yunque 

population that has few competitors, the thrashers on species-rich (30+) Bonaire 

show a much greater divergence in overall body size, not just bill size. Bonaire 

is a dry, low-relief island on which water is scarce and patchy. As a result, both 

interference and diffuse competition are keen (see discussion in chapter 4 and 

below). As in the tremblers (Storer 1989), character displacement in the pearly-eye 

may be an adaptation conducive to its continued survival on Bonaire.

Character displacement in the pearly-eye 

may be an adaptation 

conducive to its 

continued survival 

on Bonaire.
130

GENERAL TECHNICAL REPORT IITF-GTR-27

Tremblers are now thought to be one of the closest relatives of the pearly-eye 

(Hunt et al. (2001). Thus, it is not surprising that the trends and extent of sexual 

dimorphism are strikingly similar among at least three external morphological 

characters (wing, tail, and tarsus) amid subspecies of tremblers and pearly-eyes 

(table 5.7 and Storer 1989: table 1). Even culmen and bill lengths follow the same 

trend (longer in females), but the extent of the sexual dimorphism is greatly exaggerated in the tremblers (Storer 1989: table 1). With the exception of the tarsus, the 

same sex of each species measures longer or shorter in the remaining characters. 

Although sexual dimorphism in the trembler does not increase with an increase in 

competitors, it tends to in some characters of the pearly-eye.

As anticipated, the more ubiquitous subspecies M. f. fuscatus (Bahamas to 

Antigua—and thus presumed greater gene flow and increased homozygosity) 

showed the least dimorphism in wing and tarsus lengths. Both of the more allopatric subspecies M. f. densirostris and M. f. klinikowskii presented the least variation among one or more of the remaining three characters (table 5.6). Little sexual 

dimorphism in bill measurements for the subspecies M. f. densirostris and M. f. 

klinikowskii was unexpected because of their more limited and isolated ranges and 

smaller populations. Furthermore, samples were taken from high-relief islands in 

the Lesser Antilles, which should increase variation owing to elevational effects 

(app. 4.1). Finally, the small sample size for the subspecies M. f. klinikowskii must 

be considered as a potential factor contributing to the observed results.

The pearly-eye population inhabiting la Désirade, a small satellite island off 

of Guadeloupe in the Lesser Antilles, is currently assigned to the subspecies M. f. 

densirostris. Yet, with the exception of the tarsus, it shows the least sexual dimorphism among all the populations of the five islands on which the race occurs (app. 

4.1). In fact, its dimorphism indices results are most similar to those of the much 

more northern populations of M. f. fuscatus. Why should this be? Recall that 

DNA testing (chapter 3) showed very little mitochondrial differentiation between 

pearly-eye populations on Puerto Rico, Barbuda, Montserrat, Guadeloupe, and 

Dominica. And, inter-island natal dispersal (Barbuda to Guadeloupe) was documented (chapter 4). It is likely that the thrasher propagules colonizing la Désirade 

were of the subspecies M. f. fuscatus, originating from islands much farther north. 

A statement by Garrido and Remsen (1996) corroborates this view. These authors 

noted plumage differences in La Désirade specimens from those collected on 

nearby islands. This led them to “...agree with Hellmayr’s (1934) assignment of 

specimens from La Désirade to more distant nominate M. fuscatus rather than to 

M. densirostris of nearby Guadeloupe and Montserrat.”

It is likely that the 

thrasher propagules 

colonizing la Désirade 

were of the subspecies 

M. f. fuscatus, 

originating from 

islands much 

farther north. 
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Table 5.7—Comparison of sexual dimorphism indices between pearly-eyes (PETH) and their close relativies, 

the tremblers (TREM)

TREM Dimorphism PETH

competitors indexacompetitors

N (TREM) TREM subspecies x (min.–max.) TREM PETH x (min.–max.) PETH subspecies N (PETH)

Cinclocerthia Margarops

Wing length

35 C. r. pavida 2.3 (2–3) 2.40 1.89 1.2 (0–3) M. f. fuscatus 383

35 C. r. tremula 4 3.57 3.39 4.2 (2–6) M. f. densirostris 97

88 C. g. ruficauda 6 3.39 3.72 7.0 M. f. klinikowskii 21

12 C. g. gutturalis 5 3.50 4.84 10.0 M. f. bonariensis 32

47 C. g. macrorhyncha 8 3.42

41 C. r. tenebrosa 4 4.75

 Average 3.50 3.46

Tail length

28 C. r. pavida 2.3 (2–3) 3.22 2.15 1.2 (0–3) M. f. fuscatus 384

27 C. r. tremula 4 2.06 5.25 4.2 (2–6) M. f. densirostris 96

61 C. g. ruficauda 6 3.01 1.21 7.0 M. f. klinikowskii 21

9 C. g. gutturalis 5 — 4.93 10.0 M. f. bonariensis 32

30 C. g. macrorhyncha 8 2.93

27 C. r. tenebrosa 4 5.02

 Average 3.25 3.39

Tarsus length

43 C. r. pavida 2.3 (2–3) 1.48 -.41 1.2 (0–3) M. f. fuscatus 382

41 C. r. tremula 4 .38 .62 4.2 (2–6) M. f. densirostris 98

89 C. g. ruficauda 6 .65 -1.07 7.0 M. f. klinikowskii 21

10 C. g. gutturalis 5 — .43 10.0 M. f. bonariensis 32

49 C. g. macrorhyncha 8 -.13

40 C. r. tenebrosa 4 3.30

Average 1.14 -.11

Exposed culmen length

25 C. r. pavida 2.3 (2–3) -14.27 -1.40 1.2 (0–3) M. f. fuscatus 386

17 C. r. tremula 4 -21.85 .92 4.2 (2–6) M. f. densirostris 94

46 C. g. ruficauda 6 -17.11 -1.08 7.0 M. f. klinikowskii 21

4 C. g. gutturalis 5 — -3.67 10.0 M. f. bonariensis 32

21 C. g. macrorhyncha 8 -10.21

18 C. r. tenebrosa 4 -12.69

 Average -15.23 -1.77

Bill from nares

43 C. r. pavida 2.3 (2—3) -18.75 -2.11 1.2 (0–3) M. f. fuscatus 402

35 C. r. tremula 4 -19.61 -.33 4.2 (2–6) M. f. densirostris 78

88 C. g. ruficauda 6 -19.55 -.23 7.0 M. f. klinikowskii 21

12 C. g. gutturalis 5 -16.56 -1.85 10.0 M. f. bonariensis 32

45 C. g. macrorhyncha 8 -12.75

37 C. r. tenebrosa 4 -16.21

 Average -16.93 -.80

— = no data.

aDimorphism index was derived by dividing the difference between the means for the sexes (males minus females) by the overall mean and 

multiplying by 100; negative values denote females are larger in the particular character.
132

GENERAL TECHNICAL REPORT IITF-GTR-27

To visually portray and compare sexual dimorphism within and among island 

populations, morphometric data taken from the free-flying thrashers and museum 

specimens included in previous analyses are presented in figure 5.4. Note that 

mean lengths of the five morphological characters included in both analyses are 

remarkably similar, although as would be expected, the ranges (data extremes) in 

the museum specimens are generally much wider because they represent samples 

from numerous islands and include geographic and island effects.

Terrestrial Vs. Arboreal Adaptations (Museum Specimens)

Before moving on to interspecific determinants of avian body size, I must treat 

one final intraspecific factor influencing the observed pronounced variation in 

the pearly-eye’s morphology, i.e., the potential for a predisposition to a more 

terrestrial existence on dry, heat-stressed low-relief islands and cays. The wingto-tarsus ratio in birds has been used in the past (e.g., Grant 1966, Storer 1989) 

to evaluate the adaptation to a more terrestrial life (smaller ratio) than the more 

common arboreal habit (larger ratio). Recall that on Mona Island (exemplary of 

the above-mentioned physical and climatic conditions), pearly-eyes unexpectedly 

rivaled, or even surpassed, highland individuals in wing and tarsus lengths. To 

evaluate the possibility of a transition to a more terrestrial mode, I compared 

wing-to-tarsus ratios not only from Mona Island pearly-eyes with those of Puerto 

Rico’s highlands, but also among thrasher populations from several islands and 

habitats. An analysis of the available museum specimens did not reveal a consistently smaller wing-to-tarsus ratio in coastal and lowland individuals, often 

found in vegetation associations of short and diminutive stature, or vice versa 

(larger ratios in upland thrashers from tall-stature forests). Tarsus and wing length 

among the 546 specimens of pearly-eyes from some 40 islands (with elevations 

from sea level to more than a thousand meters) ranged from, respectively, 35.70 

to 40.11 and 129.9 to 142.1). Although lowland pearly-eyes were somewhat longer 

legged than upland individuals (wing-to-tarsus ratio 3.5 vs. 3.6), the overall wingto-tarsus ratio revealed very little variation (range: 3.36 to 4.03; avg. = 3.65) among 

islands (table 5.8). Furthermore, there was no significant correlation between the 

wing-to-tarsus ratio and landmass. As an example, the wing-to-tarsus ratio of 

Mona Island pearly-eyes was 3.62, whereas that of St. Lucian thrashers (most of 

which were collected in montane rain forest) was 3.52, or just the opposite of what 

was expected. In any case, the mean wing-to-tarsus ratios were not significantly 

different between the two groups (Mann-Whitney Rank Sum Test (hereafter 

M-W R S): T = 91; P = 0.86). In support of these results, Hernández-Prieto (1993), 

after conducting intensive comparative foraging studies on Mona and in adjacent 

Is the pearly-eye 

predisposed to a more 

terrestrial life on dry, 

heat-stressed islands?
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Table 5.8—Wing-to-tarsus ratios derived from museum specimens of the pearlyeyed thrasher from several low- and high-relief islands as a measure of potential 

adaptation to a terrestrial existence

No. of L (low reliefa) Average wing

Island study skins Elevation (m) H (high relief) tarsus ratio

Barbuda 12 44.5 L 3.71

Caicos 27 <48 L 3.68

Beata 7 50 L 3.56

Sombrero 1 <65 L 3.68

Anguilla 2 65 L 3.58

Crooked Island 2 <67 L 3.76

San Salvador 80 <67 L 3.75

Great Inagua 32 <67 L 3.7

Mayagüana 1 <67 L 3.6

Rum Cay 12 <67 L 3.57 

Desecheo 10 79.1 L 3.72

Mona 17 80 L 3.62

Culebra 13 198.1 L 3.65

Isla Piñeros 3 <200 L 3.77

Luís Peña 1 <200 L 3.72

Culebrita 4 <200 L 3.72 

Caja de Muertos 8 <200 L 3.66

Bonaire 32 239 L 3.51

La Désirade 18 270 L 3.64

Vieques 17 300 L 3.59

Jost Van Dyke 1 335.3 L 3.36

St. Croix 9 355 L 3.63

St. John 4 392 L 3.65

Antigua 18 402 L 3.58

Tortola 5 <450 L 3.67

Virgin Gorda 2 <450 L 3.47

St. Thomas 26 477 L 3.66

St. Martin 4 <880 H 3.62

St. Eustatius 14 <880 H 3.6

Saba 7 887 H 3.68

Montserrat 13 900 H 3.67

St. Lucia 26 950 H 3.54

Nevis 4 985.1 H 3.56

St. Kitts 12 1156 H 3.56

Puerto Rico 41 1338 H 3.66

Martinique 4 1397 H 3.62 

Dominica 34 1441 H 3.75

Guadeloupe 22 1484 H 3.68

Jamaica 1 2256 H 4.03

Total or (count): 546 — (39)

Minimum 1 44.5 3.35

Maximum 80 2256 4.03

Mean 14.3 512.5 3.64

SD 15.4 535 0.11

— = incalculable. aLow relief = <600 m; high relief = >600 m, or about midway up the moisture and vegetational gradient 

on most Caribbean islands; ratios should be minimal for small, low-relief islands, but no clear trend is 

evident.
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GENERAL TECHNICAL REPORT IITF-GTR-27

coastal dry forest within the Guánica Biosphere Reserve, Puerto Rico, showed 

that Mona’s pearly-eyes were not more terrestrial than Guánica’s, but rather 

both populations are mainly arboreal. It is obvious that factors other than perch 

substrates and an adaptation to terrestrial life are having a greater impact on the 

longitude of appendicular characters in the pearly-eyed thrasher.

What else might be behind the evolution of disparate body size and sexual 

dimorphism in the pearly-eye? From an ecological perspective, morphological 

disparity between the sexes may be the result of interference competition and 

consequential character displacement (see Schluter and Smith 1986, Schoener 

1965). Both sexes compete for the same food resources, often in highly disturbed 

or “edge” habitats in which food may be limited. From a reproductive perspective, 

whereas females tend to guard the nest and a small area around it (reflected in 

their more pugnacious behavior and larger beaks), males guard a much greater 

area from intra- and interspecifics, especially around the perimeter of the territory. Over time, this behavior could result in an increase in wing and tail length 

to enhance the male’s aerial maneuverability. Not only is wing chord longer in 

males than in females, so is the ninth primary, a very important feather in forward 

propulsion, speed, and agility: M-W R S: P <0.001, including 209 females (median 

= 88.8; 25 to 75 percentiles = 87.4 to 91.4, respectively) and 155 males: (median = 

90.3; 25 to 75 percentiles = 88.2 to 91.9, respectively) from the El Yunque sampled 

population.

Some of the intraspecific factors governing pearly-eye body size have been 

reviewed. Next, interspecific effects will be treated, namely the possibility of 

competition from closely related species, such as the scaly-breasted thrasher, 

among others.

Sources of Interspecific Variation in Avian Body Size

Effects of Competition on Body Size

In habitats where avian communities are not often subjected to resource limitation, e.g., grassland and shrubsteppe, intra- and interspecific competition play 

at best a minor role in influencing community structure or phenotypic variation 

(Wiens 1977, Wiens and Rotenberry 1981). In general, however, although occasionally disputed (Connor and Simberloff 1978, Simberloff 1978, Simberloff and 

Boecklen 1981, Strong and Simberloff 1981, Strong et al. 1979), competition and 

its consequences (e.g., ecological release, character displacement, and adaptive 

radiation) have been well documented (Alatalo 1982; Amadon 1966; Boag and 

Competition plays a 

role in avian body size.
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Grant 1981; Brown and Wilson 1956; Case and Sidell 1983; Case et al. 1983; Cox 

and Ricklefs 1977; Diamond 1978; Faaborg 1982b, 1985; Grant 1968; Grant and 

Abbott 1980; Grant and Grant 1982; Graves and Gotelli 1993; Hendrickson 

1981; Hernández-Prieto 1993; Lundberg et al. 1981; Schluter and Grant 1984; 

Schoener 1965). Therefore, competition was included as a variable in several 

statistical analyses.

Potential for Intrageneric Competition

Competition theory predicts that the pearly-eye’s keenest competitor for 

resources should be a similar species, i.e., the most closely related phylogenetically, and with the most similar ecology (e.g., feeding, nesting, and behavioral). 

The pearly-eye and its current congener (pending possible replacement in the 

genus Allenia), the scaly-breasted thrasher, will be treated first because both 

species are (1) similar in plumage, (2) mostly arboreal, (3) mainly frugivorous, 

and (4) habitat generalists. These similarities led early taxonomists to conclude 

that the scaly-breast must be the pearly-eye’s closest living relative. Indeed, even 

ecologists have used the two Margarops species as evidence for character displacement in sympatric congeneric species (see McLaughlin and Roughgarden 

1989). As a further example, Schoener (1965: table 1.34) reported that Margarops

spp. show the most pronounced “ratio of large to small culmen” (1.56) among 

14 species of mimids (min. = 1.02, avg. = 1.2).

Recently, however, with the aid of DNA testing, Ricklefs and Bermingham 

(1997) and Hunt et al. (2001) have shown that the two species of tremblers 

(Cinclocerthia gutturalis and C. ruficauda) are the pearly-eye’s closest relatives. 

To many field ornithologists, the apparent close affinity of the pearly-eye and 

scaly-breast was always questioned because of several striking dissimilarities, 

including morphology (body size), behavior, aspects of the diet (preferences, 

shifts, seasonal trends), and foraging techniques. The scaly-breast’s morphology, behavior, and disposition are more similar to that of the sage thrasher 

(Oreoscoptes montanus) of North America, which, incidentally, is more closely 

related to mockingbirds (Mimus) than to other continental (Toxostoma spp.) 

thrashers (Sibley and Ahlquist 1990: fig. 379; Zink et al. 1999). Nonetheless, 

because of the similarities between the two species’ geographic and habitat 

ranges, and especially their nesting ecologies and frugivory, the scaly-breast 

is still considered a good candidate species to explore the possibility of a shift 

in the pearly-eye’s body size in the absence (or presence) of a potentially keen 

competitor.
136

GENERAL TECHNICAL REPORT IITF-GTR-27

Combination of Uni- and Multivariate Analyses 

(Museum Specimens)

To identify and evaluate potential competitive factors responsible for observed 

differences in the pearly-eye’s body size, analysis of variance and covariance, 

both univariate (ANOVA and ANCOVA) and multivariate (MANOVA and 

MANCOVA) tests, were performed on museum specimens (tables 5.9 and 5.10, 

respectively). Dependent variables included the same set of five external morphological characters used in the preliminary univariate analyses. These more comprehensive and inclusive analyses also were chosen to compare results with those 

from the preliminary analyses and to summarize simultaneously all of the main 

treatment (and covariate) effects of gender, season, and the presence or absence of 

the scaly-breast, on the pearly-eye’s body size. It is noteworthy that regardless of 

either a univariate or multivariate approach, three-way or two-way interactions 

generally were not significant before (table 5.9) or after (table 5.10) controlling 

for the effect of covariates. Thus, the effects of the main treatment factors are 

consistent when significant. Season, gender, and presence or absence of the scalybreasted thrasher (SBTH) each had a significant effect in the MANOVA (table 

5.9), although only season and gender had a significant effect after controlling for 

the influence of the covariates in the MANCOVA (table 5.10).

The disparity in significance (before versus after controlling for covariates) for 

the effect of the scaly-breast on morphometric variation in the pearly-eye has at 

least two possible explanations. First, the scaly-breast has an effect on the pearlyeye, but the covariates: SSPNET (species present per netting session), SPPISLA 

(landbird species per island), elevation, latitude, and area, represent environmental variables that determine the distribution of the scaly-breast, so that once the 

effects of those variables are removed in ANCOVAs or MANCOVA, the effect of 

the scaly-breast is removed as well. Alternatively, the covariates may have a direct 

effect on morphometric variation in the pearly-eye, and the scaly-breast only 

appears to have an effect in ANOVAs and MANOVA because the distribution of 

the scaly-breast is highly correlated with the underlying environmental variables 

to which the morphology of the pearly-eye responds. These two interpretations 

are not mutually exclusive, and are equally plausible. The covariates, coupled 

with the other two main treatment effects (gender and season), no doubt greatly 

influence the distribution of both the pearly-eye and the scaly-breast and, at the 

same time, are influencing both the pearly-eye’s and the scaly-breast’s morphologies. Consequently, although interspecific competition may be present, and thus 

influencing body size of both species, a multiplicity of environmental and ecological factors acting in concert may play a more major role in governing both species’ 
137

Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

Table 5.9—Results of statistical analysesa assessing the effects of season (March–June vs. July–October 

vs. November–February), gender (male vs. female), and presence or absence of the scaly-breasted thrasher 

(SBTH) on each of a set of five morphometric characters (three-way ANOVA), as well as on all characters 

together (three-way MANOVAb), of the pearly-eyed thrasher

ANOVA

SOV df Exposed culmen Culmen from nares Tarsus Wing Tail MANOVA

Explained 11 <0.001* <0.001* <0.001* <0.001* 0.008*

SBTH 1 .014 .105 .005 <.001 .721 <0.001

Gender 1 <.001 <.001 .759 <.001 .001 <.001

Season 2 <.001 .635 <.001 .537 .218 <.001

SBTH × gender 1 .645 .786 .585 .046 .041 .406

SBTH × season 2 .465 .708 .067 .514 .198 .430

Gender × season 2 .745 .687 .929 .492 .61 .902

SBTH × gender × season 2 .110 .758 .7 .732 .213 .337

*Denotes significance after Bonferroni’s Sequential Adjustment.

aAnalyses are based on the MUSEUMPET data set (see “Study Area and Methods”) and do not control for the effects of a set of 

geographical and ecological covariates (SOV), i.e., elevation, latitude, area, species per net, and species per island, on morphometric 

variation in the pearly-eyed thrasher.

bBonferroni’s Sequential Adjustment corroborates the overall significance of the MANOVA and suggests that all five characters 

contribute to the differences in multivariate space.

Table 5.10—Results of statistical analysesa controlling for the set of geographical and ecological covariates 

(SOV), i.e., elevation, latitude, area, species per net (SPPNET), and species per island (SPPISLA), and 

assessing the effects of season (March–June vs. July–October vs. November–February), gender (male vs. 

female), and presence or absence of the scaly-breasted thrasher (SBTH) on each of a set of five morphometric 

characters (three-way ANCOVA), as well as on all characters together (three-way MANCOVAb), of the pearlyeyed thrasher

ANOVA

SOV df Exposed culmen Culmen from nares Tarsus Wing Tail MANOVA

Explained 16 <0.001* <0.001* <0.001* <0.001* 0.008*

Season 2 <.001 .047 <.001 .511 0.32 <0 .001

Gender 1 <.001 <.001 .751 <.001 <.001 <.001

SBTH 1 .189 .067 .671 .147 .927 .234

Gender × season 2 .733 .415 .827 .653 .639 .954

SBTH × season 2 .554 .329 .017 .237 .133 .148

SBTH × gender 1 .613 .855 .603 .062 .065 .586

SBTH × gender × season 2 .128 .692 .749 .71 .221 .389 

All covariates 5 <.001 <.001 <.001 <.001 .263 <.001

SPPNET 1 .36 <.001 .37 .033 .32 .239

SPPISLA 1 .005 .001 .063 .86 .518 .811

Elevation 1 .001 .028 .198 .159 .78 .084

Latitude 1 <.001 <.001 <.001 <.001 .714 <.001

Area 1 .069 .622 .773 .151 .03 .326

*Denotes significance after Bonferroni’s Sequential Adjustment.

aAnalyses are based on the MUSEUMPET data set (see “Study Area and Methods”).

bBonferroni’s Sequential Adjustment corroborates the overall significance of the MANCOVA and suggests that all five characters contribute to 

the differences in multivariate space. 
138

GENERAL TECHNICAL REPORT IITF-GTR-27

body sizes (review fig. 5.1). For extreme examples of how, through natural selection, the environment and a species’ foraging ecology can direct the size and shape 

of the resultant phenotypes, see Boag and Grant (1981) and Price et al. (1984b).

Besides the ANOVA tests, a PCA was conducted, not only to help identify and 

prioritize predictor variables but also to visually depict their effects on thrasher 

morphology. Based on the PCA, a detailed exposition of group differences among 

museum specimens before controlling for covariates is presented in fig. 5.8. Loadings of characters on size and shape, respectively, are base, -0.710, -0.145; nares, 

-0.658, 0.055; tarsus, -0.247, 0.331; wing, -0.025, 0.503; and tail, 0.035, 0.783.

Figure 5.8—Graphical portrayal of results from a Principle Components Analysis (PCA) of log-transformed morphometric characters (tables 5.9 

and 5.10) of the pearly-eyed thrasher. Groups were defined by combinations of season, presence (SBTH-P) and absence (SBTH-A) of the scalybreasted thrasher (SBTH) and sex as in tables 5.9 and 5.10. Vertical and horizontal lines that intersect centroids represent one standard error 

for PC-2 (shape) and PC-1 (size), respectively. Within each season, groups defined by combinations of sex and the scaly-breasted thrasher do not 

overlap, with males smaller than females and the presence of SBTH affecting size and shape. Analyses are based on the MUSEUMPET data set.
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As competition theory predicts, body size of both male and female pearlyeyes is more variable within and among the three seasons when the scaly-breast 

is absent (fig. 5.8). Neither male nor female pearly-eye body size is highly variable 

in either the postbreeding period (July to October) or early in the breeding 

season (November to February) in habitats in which the scaly-breast is present. 

The presence of more juveniles in the samples may contribute to the greater 

variation in body size during the main breeding season (March to June). Also, 

as would be anticipated if competition were influencing morphology, shape of 

the pearly-eye’s external characters is highly variable during the breeding season 

when the scaly-breast is present (fig. 5.8).

Combination of Uni- and Multivariate Analyses 

(Field Measurements)

To corroborate the results obtained from museum specimens, i.e., that season 

and the presence or absence of the scaly-breast affect pearly-eye morphology, 

both univariate and multivariate approaches were undertaken on data collected 

from free-flying pearly-eyes and scaly-breasts captured during mist-net sessions 

on several Caribbean islands. Descriptive statistics for morphometric characters 

(mean, standard error, and sample size) are summarized in table 5.11. Loadings of characters on size and shape, respectively, are base, -0.627, 0.093; nares, 

-0.766, -0.211; tarsus, -0.117, 0.356; wing, -0.010, 0.510; tail, -0.076, 0.748. External 

appendicular characters vary considerably with the influences of season and the 

presence or absence of the scaly-breast. In general, the effect of the scaly-breast 

on the morphology of the pearly-eye depends on season (significance of two-way 

interactions) based on univariate and multivariate approaches (table 5.12). As 

Table 5.11—Descriptive statistics (mean, standard error [SE], and sample size [n]) of groups used in statistical 

analysesa and defined by combinations of season and presence vs. absence of the scaly-breasted thrasher 

(SBTH) for each of five morphometric characters of the pearly-eyed thrasher

SBTH present SBTH absent

March–October November–February March–October November–February

Character Mean SE N Mean SE N Mean SE N Mean SE N

Millimeters

Exposed culmen 26.06 0.109 501 26.89 0.103 197 27.45 0.09 167 23.2 0.279 60

Culmen from nares 18.42 .053 335 18.69 .078 136 18.64 .06 167 18.45 .229 13

Tarsus 37.77 .065 358 38.54 .179 199 37.7 .094 166 38.05 1.152 11

Wing 136.41 .181 571 135.53 .29 290 135.83 .322 166 137.13 .464 91

Tail 113.99 .206 348 112.85 .386 146 110.3 .384 163 112.57 1.052 11

aAnalyses are based on the FIELDPET data set (see “Study Area and Methods”).

Seasons and 

competitors affect 

pearly-eye body size.
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expected, from March to October, the presence of scaly-breast has no effect on 

the pearly-eye’s body size but does have a significant effect on its shape (fig. 5.9). 

Corroborative of the season and scaly-breast effects obtained from museum specimens (fig. 5.8), and supportive of competition theory, results from the same set of 

statistical tests conducted on free-flying thrashers once again reveal that season 

is a major factor governing pearly-eye body size. Still, pearly-eyes definitely show 

much more morphological variation in the absence of the scaly-breast (fig. 5.9, 

November to February), which suggests that competition does in fact influence 

its morphology.

Intergeneric Competition

On most islands, because the pearly-eye is a habitat generalist, it competes with 

several species of thrushes and other mimids besides the scaly-breasted thrasher 

(see, for example, Zusi 1969), all of which are fairly similar in size, diet, and 

reproductive ecologies. Therefore, I performed a simple correlation analysis 

of five of the pearly-eye’s external morphological characters taking into account 

the total number of potential competitors on each island. That is, those species 

most likely to compete with the pearly-eye through diffuse, interference, and 

exclusion competition.

Twelve mimid and thrush species are potential competitors of the pearly-eye 

(app. 4). Despite the pearly-eye’s wide geographic distribution, it is absent from 

most large islands. As a result, it generally inhabits islands with an average of 

three competitors, ranging from zero to eight (app. 4). As shown earlier for the 

Table 5.12—Results of statistical analysesa assessing the effects of 

season (March–October vs. November–February) and presence or 

absence of the scaly-breasted thrasher (SBTH) (see table 5.11) on each of 

five morphometric characters (two-way ANOVA), as well as based on all 

characters together (two-way MANOVAb)

P-values

Explained SBTH Season SBTH × season

Character df = 3 df = 1 df = 1 df = 1

Exposed culmen <0.001* 0.881 0.011 <0.001

Bill from nares .008* .033 .011 .094

Tarsus <.001* .489 <.001 .461

Wing .005* .563 .194 .001

Tail <.001* <.001 .042 .016

MANOVA <.001 .010 .037

*Denotes significance after Bonferroni’s sequential adjustment.

aAnalyses are based on the FIELDPET data set (see “Study Area and Methods”).

bBonferroni’s sequential adjustment corroborates the overall significance of the MANOVA and 

suggests that all five characters contribute to the differences in multivariable space.
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El Yunque thrasher population, and as competition theory predicts, in the 

absence of keen competitors all vying for limited resources (e.g., food), body 

size in the pearly-eye (particularly bill dimensions) will increase, whereas sexual 

dimorphism will decrease as its niche widens, especially the foraging niche. 

Conversely, under keen competition for food and other resources and, as a consequence of the aforementioned individual and community-based competition, 

body size and appendicular characters should diminish and sexual dimorphism 

should increase as the foraging niche narrows. To test this hypothesis, I compared 

pearly-eye sexual dimorphism indices (SDI) to the number of mimid and thrush 

competitors either absent or present on respective islands. I did this assuming 

there would be a direct correlation between the SDI of the five sampled external 

morphological characters with an increase in the number of interspecific competitors. Among the results, there were two significant correlations, wing chord 

Figure 5.9—Graphical portrayal of results from a Principle Components Analysis (PCA) of log-transformed morphometric characters (tables 5.11 and 5.12) of the pearly-eyed thrasher. Groups were defined by combinations of season and 

presence (SBTH-P) and absence (SBTH-A) of the scaly-breasted thrasher (SBTH). Vertical and horizontal lines that 

intersect centroids represent one standard error for PC-2 (shape) and PC-1 (size), respectively. The effect of the scalybreast on morphology depends on season. From March to October, the effect of the scaly-breast is reflected mostly 

in shape differences. From November to February there is substantial variation in morphology in the absence of the 

scaly-breast (black circles). Analyses are based on the FIELDPET data set.
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GENERAL TECHNICAL REPORT IITF-GTR-27

Figure 5.10—Correlation between the extent of sexual dimorphism and the number of potential mimid and 

thrush competitors. Only wing and tail lengths showed a significant direct correlation with an increase in 

the number of competitors.
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Adaptations of An Avian Supertramp: Distribution, Ecology, and Life History of the Pearly-Eyed Thrasher (Margarops fuscatus)

and tail length, indicating an overall shift in body size (fig. 5.10). There was no 

correlation between tarsus and, in terms of food acquisition and handling, the 

more important bill dimensions (fig. 5.10). These results suggest that interference 

competition (one on one with individual competitors, and thus selection for larger 

body size) plays a more important role than that of diffuse competition, namely 

selection for smaller phenotypes with greater disparity in bill measurements 

between the sexes resulting from the influence of the “competitor community” 

as a whole (see chapter 8 for further discussion).

Conclusion

As depicted in figure 5.1, there are many geographic, climatic, biological, and 

ecological factors affecting the pearly-eye’s body size and morphology throughout 

its extensive geographical and elevational range. Although all of these factors (and 

surely many more) are intertwined and thus cannot be treated separately, both 

uni- and multivariate statistical analyses showed that biological factors, coupled 

with geography (especially elevation) and climate (season), play a more major 

role in governing the pearly-eye’s overall size and shape than do ecological and 

environmental factors, although the latter are often important contributors. Thus, 

a hierarchy of the key factors influencing pearly-eye morphology would be Age > 

Gender > Elevation > Season > Competition.

Summary: Morphology

Character displacement resulting from ecological release has been observed 

in various vertebrate taxa, including island birds. Univariate and multivariate 

statistical procedures were used in assessing and interpreting shifts in body size 

within and among thrasher populations in species-poor (and rich) communities 

and in the absence or presence of potential, interspecific competitors. More 

than 1,200 museum specimens and live, free-flying individuals of the pearlyeyed thrasher were measured on a local and regional scale. Various external 

morphological characters were then compared to quantify and explain several 

observed and anticipated patterns in the often-pronounced morphological 

disparity found within sympatric, and among allopatric, populations.

Five main effects (in descending order of importance): age, gender, elevation, 

season, and the presence or absence of potential competitors, greatly influenced 

the analytic results. For example, the pearly-eye’s body size (especially mass) generally increases with elevation. Morphological variation in allopatric populations 

at similar elevations is, as predicted by the supertramp and competition theories, 

characterized by shape, but not size, differences. The effects of interspecific 

Hierarchy of 

the key factors 

influencing pearly-eye 

morphology: age > 

gender > elevation > 

season > competition.
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competition were diminished when several environmental and ecological variables 

were controlled because competition itself is a product of all these factors, as well 

as many others. Furthermore, climatic effects, although previously shown to be 

an important factor, play an even more integral role in influencing the pearly-eye’s 

body size than was first recognized. From these results, it is evident that there 

is no single, major causative mechanism dictating the observed variation in the 

pearly-eye’s body size within and among populations throughout the Caribbean, 

but rather a profusion of intrinsic and extrinsic determinants that influence its 

morphology throughout each year, and throughout its life. One must use caution 

when attempting to attribute avian body size differences to various environmental 

or ecological factors. Prudence is especially paramount when attempting to evoke 

competition as a major causative factor.