\ .‘. _

United States

Department of

Agriculture

Forest Service

Southern Forest

Experiment Station

New Orleans,

Louisiana

General Technical Report

SO-98

April 1994

Census Methods for

Caribbean Land Birds

Joseph M. Wunderle, Jr.
SUMMARY

Different census methods can be used to survey the distribution

of a terrestrial bird species or to monitor population changes. The

appropriate method depends on whether the objective is simply to

document the presence of a species or to quantify its relative abundance, population density, population trends over time, habitat use,

survivorship, or the physical condition of individuals. Once the

objective is defined, consideration should be given to selecting a

study site, the number of sampling units, the time of day, the time

of year, and the experience of field personnel. The various census

techniques, and their advantages and disadvantages in the Caribbean Islands, discussed in this paper are: point counts without

distance estimation, variable-radius point counts, fixed-radius

point counts, point counts for parrots, line transects without distance estimates, variable-distance line transects, strip transects,

spot mapping, territory mapping of color-banded birds, mist petting, and use of tape-recorded playback to enhance detectability.

For most studies of land birds in the Caribbean, the fmed-radius

point count method is recommended.

ACKNOWLEDGMENTS

Wylie C. Barrow, Jr.; Robert W. Colburn; Francisco J. Vilella;

Robert B. Waide; and participants in workshops on wildlife census

techniques held on St. Lucia and Jamaica provided constructive

comments on an early draft of this work. In addition, this work

benefitted from the constructive comments of Robert A. Askins,

Peter G. H. Evans, John R. Faaborg, Peter Pyle, C. John Ralph,

Nigel Varty, and James W. Wiley. Finally, I thank Peter Pyle for his

permission to use his previously published figures. Illustrations in

figures 4 through 11 were adapted from drawings by S.N.G. Howell

published in Pyle and others (1987).
Census Methods for Caribbean Land Birds

Joseph M. Wunderle, Jr.

INTRODUCTION

In the Caribbean Islands, 37 species of resident birds

are currently threatened with extinction; 34 of these

are terrestrial species (Collar and Andrew 1988). In

addition, the Caribbean is visited by thousands of

North American migrants each winter, and many of

these may be threatened by habitat disturbance on the

breeding grounds or on their wintering areas in the

Caribbean. In most cases, little or nothing is known of

the basic population biology necessary to prevent

extinction of these island residents or visitors. Our

ignorance frequently includes a lack of essential information on habitat distribution, natural population fluctuations, and population response to a variety of

natural and human-caused disturbances. This ignorance could prove tragic to some of the region’s threatened species, particularly on islands where human

disturbances and frequent natural events, such as

droughts and hurricanes, could drastically reduce bird

populations. Thus, there is a need to encourage and

assist Caribbean biologists in surveying and monitoring bird populations on their islands.

Ornithologists have used a variety of techniques to

survey the abundance and distribution of Caribbean

bird populations. For example, some surveys have

involved observers who remained stationary for a furedtime interval and counted all birds detected at a point

(Cox and Ricklefs 1977, Rivera-Milan 1992, Wunderle

1985) or counted only those birds within a given distance from the observer (Wunderle and others 1992)

before proceeding to the next point. In other cases, an

observer counted and tallied all birds while walking

slowly along a transect line (Lack 1976, Lack and Lack

1973, Lack and others 1973). In some cases, in addition

to counting the birds, the observer noted the distance

between the birds and the transect (Cruz and Delannoy

1984, Emlen 1977a, Faaborg and Arendt 1985) or

counted only the birds within a given distance from the

transect line (Vilella and Zwank 1987). Some surveys

have involved playing back tape-recordings of songs or

other vocalizations of specific secretive species to elicit

a response from them in order to document habitat

distribution and abundance (Blockstein 1988, Varty

1991, Wunderle 1992). Birds captured in mist nets have

also provided an indication of abundance (Diamond

1974, Faaborg and Arendt 1985, Terborgh and Faaborg

1973, lbrborgh and others 1978, Wunderle and others

1987). Finally, density estimates (birds per unit area>

have been obtained by mapping the positions of

unmarked territorial birds (Recher 1970) or by more

intensive methods requiring that birds be captured

with mist nets and be given unique combinations of

color-bands. Aft&r banding, the birds are released; then

the observer maps their territories (Holmes and others

1989). .:

In only a few cases have bird populations been

monitored for long time periods in the Caribbean.

Long-term monitoring with mist nets in the dry forest

of Puerto Rico has documented population changes in

resident species in relation to rainfall over 9 years

(Faaborg and others 1984) and declines in wintering

North American migrants over 16 years (Faaborg and

Arendt 1991). Christmas bird counts, sponsored by the

National Audubon Society headquartered in New York,

have been consistently held (for over 15 years in some

areas) throughout the Caribbean. Although Christmas

counts have numerous weaknesses, they have enabled

investigators to detect some general patterns in distribution and abundance (Pashley and Martin 1988). In

some instances, monitoring efforts have focused on

endangered species such as the Puerto Rican parrot

(Amazona vittatu). With this species, censuses involving numerous observers situated in widespread lookouts in trees, on ridge tops, or in towers are made at

sunrise or sunset as the birds move from or to roost

sites (Snyder and others 1987). These systematic parrot counts have been made since 1968 and continue

today.

To the uninitiated it appears that a bewildering variety of methods have been used to sample terrestrial

bird populations in the Caribbean. Yet, in the examples

cited previously, the method used was generally appropriate for the question or questions posed by the investigators. Thus, the selection of a particular sampling

method is dependent upon the question of interest to

the investigator and the time and resources available to

conduct the sampling.

Joseph M. Wunderle, Jr., is a research wildlife biologist at the International Institute of Tropical Forestry, U. S. Department of Agriculture,

Forest Service, Rio Piedras, PR 00928-2500. In cooperation with: University of Puerto Rico, Rio Piedras, PR 00936-4984.

1
In this review, some of the most appropriate bird

A variety of publications have described sampling

census methods suitable for sampling Caribbean land

methods and techniques for songbirds, and some have

birds, particularly songbirds (Passeriformes), on the

Caribbean Islands are summarized. The census

provided the basis for much of this review (Pyle and

methods described here can be used for surveys of

abundance and habitat distribution. For example, cenothers 1987; Ralph and others, 1993; Verner 1985).

suses might be conducted in a variety of different habitats to determine in which habitat the species of

Because these previously published census methods

concern is most abundant. Locating the threatened

species and its appropriate habitat is the first stage of

focus on temperate zone species, methods most approany conservation effort. Once the appropriate habitat is

identified, regular systematic censuses should be conpriate to the Caribbean based on 15 years of sampling

ducted to monitor the populations over a long time

period. Consistent long-term population monitoring

bird populations in both the Greater and Lesser

enables the observer to examine how bird populations

change in response to natural or human-induced

Antilles are emphasized in this review. The purpose of

events. The benefit of long-term monitoring was illustrated by recent hurricane studies in which census data

this review is to encourage Caribbean bird watchers to

collected before a hurricane were available and could be

used to provide a baseline for comparison with data

adopt widely accepted census methods to routinely surcollected after a hurricane (Askins and Ewert 1991,

Waide 1991, Wauer and Wunderle 1992, Wunderle and

vey and monitor bird populations on their islands.

others 1992).

CENSUS REQUIREMENTS

General Requirements

All census methods must satisfy the following basic

requirements as listed in Manuwal and Carey (1991):

i.

2.

3.

4.

5.

6.

7.

2

Birds must be accurately identified.

Sampling efforts must be adequate to detect the

presence of species.

Sampling efforts must be adequate to obtain estimates with desired precision.

Differences among observers should be slight or

accounted for.

Differences in detectability among species should

be slight or accounted for.

Differences in detectability among habitats

should be slight or accounted for.

Differences in detectability among years should be

slight or accounted for.

Selecting Sample Sites

The selection of a site or sites for counting depends

on the purpose of the counts. If the counts are intended

to describe the distribution and abundance of specific

threatened or endangered species, then initial surveys

should be made in a variety of different habitats and

possibly along habitat “edges.” However, for long-term

monitoring purposes, it is generally best to sample

uniform habitat types. If the study is designed to sample uniform habitats, then counts should be made at

least 25 m, and preferably 50 m (Hutto and others

1986), from a habitat border, at least where conditions

permit.

Factors Affecting Census Results

Time of Year. -Both counting and mist netting can

be done in any season of the year. However, if the intent

is to monitor population changes, counting or netting

should be done every year at roughly the same time.

Comparisons of counts in the breeding period with

those in the nonbreeding period are difficult because of

changes in the behavior of birds. For example, birds are

Time of Day. -The best time for conducting a

census is in the morning from sunrise until about 10

a.m. in the lowlands and until 11 a.m. at high altitudes

easier to detect during censuses in the breeding season

(> 400 m). It is best If. censuses are begun within 15 min

of dawn. Mist netting can be done throughout the day,

when they are most vocal, but they are often easier to

although captures tend to decrease at midday in hot,

sunny, lowland habitats. -.

capture in the nonbreeding period when they wander

more. Thus, the time and season of sampling must be

standardized.

Periodic droughts can be a serious problem for censuses standardized to fixed dates during the breeding

season in many lowland areas, particularly in the subtropical dry forest life zone. Droughts involving delays

in the onset of early wet season rains (normally in

April or early May) delay breeding and thus reduce

census detections. For example, counts scheduled for

early May might normally show an abundance of breeding birds, but breeding may be delayed until early June

in a drought year, thereby causing fewer detections

than normal. One solution to this problem is to time

the censuses with the arrival of the early wet season

rains in dry forests. However, in moist and wet forests

most breeding occurs at the end of the wet season.

Therefore, the best solution is to count birds only after

breeding activity has been detected. Censuses normally

scheduled for nonbreeding periods (i.e., November

through January) are less likely to encounter this

problem.

Weather. -Conducting a census or capturing with a

net should never be done when it’s windy, rainy, foggy,

or too hot or when there is direct sun on the net.
Record Keeping

Maintaining a field journal is an essential requirement for all field studies. It is valuable to routinely

record date, time, location, weather conditions, personnel involved, the number and type of censuses made, all

birds seen or heard, and plant phenology (which plants

are flowering or fruiting). Additional observations on

behavior, nesting, and predation can be valuable. Standardized data sheets should be used for all censuses;

examples are shown in the appendix.

CENSUS METHODS

Equipment Care

Binoculars of 7 x 35 or higher magnification are

most suitable for census work. The greatest problem

faced by Caribbean bird watchers is keeping fungal

growth off binocular lenses, which occurs under warm,

humid conditions. Thus, it is essential to store binoculars in a dry location, such as a wooden box with small

holes in the top and a small light bulb set inside the

bottom for heating and drying the binoculars. The bulb

should be illuminated when the binoculars are in the

box.

Binoculars can also be stored in an airtight plastic or

metal box (some cracker or cookie containers are ideal)

with a cloth bag or an old sock containing silica gel,

which will absorb moisture from the container and the

binoculars. The silica will turn pink when saturated

with moisture and can be dried in an oven to return it

to its normal blue coloration. Silica gel can be purchased commercially from a variety of photography

stores.

Selecting a Census Method

Once the objectives of a study are defined, an appropriate method can be selected to take a census of bird

populations (table 1). Any of a variety of methods can

be used to detect the presence of a species in a habitat.

If the species of concern is particularly rare or secretive, use of a tape-recorded playback of vocalizations

may enhance the likelihood of detection. If the objective

of the study is to obtain a measurement of relative

abundance (e.g., number of individuals per point, percentage of points with one or more individuals, number

of individuals per kilometer of transect, etc.), then a

variety of methods are also available. Measurements of

relative abundance are useful for comparative purposes, such as comparing habitats or comparing years.

Population trends can be monitored using the same

methods for sampling relative abundance. Only a few

methods are adequate to measure population density

(e.g., number of individuals per hectare), but fortunately most measurements of relative abundance are

adequate for most other purposes. Finally, if the objective requires measurements of the physical condition of

the birds (fat or lean) or measurements of survival,

mist netting will be required.

Point Counts

Point counts are one of the most popular methods for

surveying bird distributions and monitoring population

changes in Caribbean land birds. This method can be

Table 1. -Summary of census methods and their appropriateness for specific objectives in the Caribbean (adapted from

Manuwal and Carey 1991)

Objective*

Method

Species Relative Population Habitat Physical

presence abundance Trends Densitv use condition Survival

Point counts

Without distance estimate

Variable-radius

Fixed-raduis

Transects

Without distance estimate

Variable-distance estimate

Strip transects

Spot mapping

Mapping color-banded birds

Mist netting

X

X

R

X

X

X

E

E

X

X

X

X

X

R X

X X R

R X

*X= adequate to achieve objective.

E = excessive, provides more detail than necessary to achieve objective.

R= recommended for the Caribbean.
used to study yearly changes in bird populations at

fixed points, differences in species composition between

habitats, and abundance of different species at a specific location. This method is particularly appropriate

for Caribbean habitats in which rugged terrain or

dense, scrubby vegetation makes walking difficult.

Point counts involve an observer standing in one location for a fixed time period and recording all birds

detected, whether by sight or by sound. Each point

count site should be at least 100 m apart, and preferably 150 to 200 m apart. A single observer can complete

12 to 15 point counts per morning, depending on the

terrain. At least 30 counts should be conducted,

depending on the abundance of the species and the

purpose of the counts. Point counts can be conducted

once or many times at a given site.

The time spent at each point should represent the

minimum time needed to sample at least 80 percent of

the species present at a point. For example, preliminary

studies by Waide and Wunderlel in a Puerto Rican rain

forest and a dry forest indicated that, in a point count,

35 to 46 percent of the species are sampled in the first 5

min, 79 to 83 percent of the species are detected in 10

min, and 85 to 88 percent of the species are detected in

15 min, assuming that 100 percent of the species are

detected in 20 min. These results, and those of Askins

and Ewert (19911, suggest that a point count of 10 min

duration may be adequate for most Caribbean surveys.

However, observers might wish to conduct some initial

experiments to establish the appropriate time period

needed to sample a point.

The point count method, because of its simplicity

and practicality in rough terrain and thick vegetation,

is well suited for most songbird survey and monitoring

work in the Caribbean. Unfortunately, point counts are

not very accurate for estimating population densities,

particularly for species with low densities. For instance, Burnham and others (1980) have shown that a

minimum of 40 point counts must contain the species

of concern to accurately estimate population density, so

that density estimates for a rare species might require

100 or more point counts. However, for most conservation purposes, a density estimate may not be required,

and the results from point counts can provide a reliable

index of abundance. In fact, Verner (1985) argues that

point counts are the preferred method for monitoring

long-term population trends because “. . . the time

spent counting can be absolutely controlled, and more

sites can be sampled, permitting more representative

sampling.” However, several factors must be considered

to obtain accurate point counts. Because estimates of

IWide, Robert B.; Wunderle, Joseph M., Jr. 1987. Changes in

habitats available to migrant land birds in the Caribbean.

Unpublished interim report submitted to the World Wildlife FundU.S., Washington, DC 20037 U.S.A.

distance must be made in both the variable-radius and

fmed-radius point count methods, it is essential that all

participants practice estimating distances with known

references in the appropriate habitat. An optical range

finder can be very useful for practicing distance estimates. Practice should continue until all observers

obtain consistent levels of accuracy for distances within

25 m.

The accuracy of point counts (as well as the accuracy

of most other methods) in different habitats is likely to

vary. Visual census methods (point counts, transects,

spot mapping, etc.) detect most species in open areas

and areas with low vegetation, but not in areas with

thick vegetation. Furthermore, as the vegetation

heights increase, the ability to detect certain species in

the canopy decreases. This deficiency was illustrated in

a point count study by Waide and Narins (1988) using a

22-m canopy tower in the El Verde rain forest in Puerto

Rico. Here they discovered that two of three canopysinging species were underestimated by 33 to 46 percent in ground point counts. Species that sang softly or

at high frequencies were most likely to be missed by

observers at ground level. To counter this problem, they

suggested using a small, 26-m, fixed-radius point count

(or small, fKed band for transects), because this tended

to minimize the bias against certain canopy-singing

species. Care also should be used in estimating the

distance to singing or calling individuals (those not

observed) because this method is most vulnerable to

error.

Point counts can be classified into three major types

depending on how the observer treats information on

distance from the birds (fig. 1) and classified into a

fourth type when modified for counting parrots:

Point Counts Without Distance Estimation. -

Detected birds are counted without regard for distance

from the observer. These counts cannot be used for

estimating density, but they are useful for measuring

species richness. Examples of the use of this method

can be found in Cox and Ricklefs (1977) and Wunderle

(1985).

Variable-Radius Point Counts. -The observer

estimates the distance to the detected birds. Analysis,

by species, can include data either grouped into concentric rings of similar radii about the point or data in

an ungrouped form (Reynolds and others 1980).

Fixed-Radius Point Counts. -Detections are

recorded for birds within a fmed-radius circle around

the observer separately from all detections beyond the

radius. The size of the radius is dependent on the

thickness of the vegetation and the observer’s ability to

detect all birds. Therefore, the observer should select

the largest radius in which all birds can be detected.

For most habitats, with the exception of dense vegetation, 25 m is commonly used as the standard

radius.

This method can be used to calculate three indices of

bird abundance, any of which can be used to test for

4
b

C OF FIXED RADIUS

Figure 1.-A stationary observer making bird observations: (a) a point count without distance estimation; (6) a

variable-radius point count where the observer estimates the distance to all birds; and (c) a fixed-radius

point count in which birds are tallied both within and outside a predefined distance (radius) from the

observer.

5
differences in community composition among sites or

for differences in the abundance of a given bird species

among sites or among years. These indices are (1) the

mean number of detections within 25 m of the

observer, (2) the percentage or proportion of points

with one or more detections within 25 m of the

observer, and (3) the percentage or proportion of points

with one or more detections, regardless of the distance

from the observer. These indices allow comparisons

with standard statistical tests (Hutto and others 1986).

Because species differ in their detectability, it is sometimes valuable to calculate a detection ratio for each

species, particularly when comparing different species,

but also when comparing the same species at different

times of the year (such as breeding vs. nonbreeding

periods). For example, vocal species, such as scalynaped pigeons (ramiers, Columba squamosa), have

high detection ratios, whereas quiet species, such as

hummingbirds (doctorbirds), have low ratios. The

detection ratio represents the number of point counts

at which a given species was recorded only beyond the

25 m radius divided by the total number of counts at

which the species was recorded, whether within or

beyond 25 m.

Examples of the calculation of the different indices

derived from point counts are shown in table 2.

Hutto and others (1986) recommend fixed-radius

point counts because they have fewer assumptions than

most other methods of estimating population density

and can be used in both the breeding and nonbreeding

periods. For examples of different ways in which the

method has been used in the Caribbean, see Askins

and Ewert (1991) and Wunderle and others (1992).

Point Counts for Parrots. -Because of their elusive behavior, parrots require special census techniques

designed to obtain counts as they move to their roost

sites before sunset or as they depart from their roost

sites after sunrise (Snyder and others 1987). Parrots

Table 2. -Example of calculations of different indices derived from 25-m fixed-radius point counts of 10 minutes duration each (Calculations are

based on only 5 point counts for illulstrative purposes, but normally would involve 30 or more independent points)

A. From original data sheet (see appendix):

B. Summary of indices calculated from field data above:

Species

Mean Mean

detections detections

within 25 m unlimited radius

2.L 3.L

Percentage of

points with

detections

within 25 m

(00

Percentage of

points with

detections

unlimited radius

I 06

Detection

ratio

0. d

-_a. f?!?” f 0.2 2.2 26 lob I* 6

IQ?. u&+.&/§ s!y 6.0 loo /OO 1. 0

*For bananaquk (Coereba flaveola), the mean detection within 25 m would be the most appropriate index of abundance.

‘For Puerto Rican emerald hummingbirds (Chlorostilbon maugaeus), both the mean detections within 25 m and the percentage of points with

detections within 25 m provide equivalent measures of abundance because the mean detection within 25 m is equivalent to one. Note the low

detection ratio, indicating the absence of detection beyond 25 m.

*For scaly-naped pigeons (Columba squamosa), the most appropriate index would be either the mean detections per unlimited radius or the

percentage of points with detection at points of unlimited radius because this species is rarely detected within 25 m of the observer. (This species

is easily disturbed by human presence.) Because of the very high detection ratio of this species, point counts should be widely separated (at least

200 m).

SFor Puerto Rican tanagers (Neospingus speculiferus), the appropriate measure would be either the mean detections within 25 m or mean

detections at points of unlimited radius. The percentage of point measurements would be inappropriate for this species because of its tendency to

occur in flocks.

6
are best counted simultaneously by several observers

situated at different vantage points (ridge tops, tree

tops, cleared hillsides, etc.). Each observer counts the

passing parrots and records the time of their passage

and their compass bearing as they fly out of sight.

Because observers at different locations are likely to see

and count some of the same birds, information on

group size, time, and direction of flight can be used

after the census to eliminate duplication.

Snyder and others (1987) have found that counts

made during the parrot breeding period need not

involve any lookout points other than those covering

the nesting areas. During the breeding period (February through May), most parrots, including nonbreeders, roost in the nesting areas where they may be

consistently counted. Counts are best conducted from

sunrise to about 8 a.m. or from 3:30 p.m. until dark.

Lookout sites should have a wide view of the landscape

and be spaced so that all flying birds are visible from at

least one site. Counts made within forests under the

canopy are likely to miss many birds. For this reason,

treetop lookouts are particularly valuable for counting

parrots in forested regions. Information on construction of treetop platforms for parrot counts can be

obtained by writing the Wildlife Program, Caribbean

National Forest, USDA Forest Service, P. 0. Box B,

Palmer, PR 00721 U.S.A.

lhnsects

Transects involve slowly walking through a habitat

and, therefore, should be conducted only in areas where

the observer can concentrate on birds instead of worrying about footing or dodging spiney or poisonous plants.

It is important that the observer traverse the transect at a fured speed (e.g., 100 m in 10 min). Transect

censuses can take many forms; three are shown in

figure 2.

Line lkansects Without Distance Estimates.-

This is the simplest form of transect census and can

enable the observer to generate a list of species present

in a habitat. By walking slowly for a given distance or

time period, the observer is able to obtain a list of

species that can be compared between habitats. Lack

(1976) effectively used this method in Jamaica. It cannot be used for estimating densities, but it does provide

information on the presence or absence of species in a

habitat.

Variable-Distance Line lkansects. -Here the

observer must estimate the perpendicular distance to

the bird from the transect line. This can be done

directly or indirectly by recording the distance from the

observer to the bird and the sighting angle between the

transect line and the bird. Using this method, transect

counts are made in which all detections, visual and

aural, out to the distance limit of detectability are

recorded. The count for each species is then multiplied

by a conversion factor (coefficient of detectability) representing the percentage of the population that is normally detected. Conversion values for each species are

derived directly from distribution curves of detection

points located laterally from the observer’s transect

line. Obviously, this technique is difficult to conduct

and is recommended only if density estimates are

required and only for habitats or terrains where transects can be easily run. For a detailed explanation and

examples of the method see Burnham and others

(1980) and Emlen (1971, 1977a, 1977b).

Strip Zkansects. -In this type of transect, fmed

boundaries are set on both sides of the transect, and all

detected birds within the boundaries (the strip) are

tallied. Boundaries are usually set at 25 to 50 m on

each side of the transect line, depending on the density

of the vegetation. These transects are simpler to run

than variable-&stance line transects because the

observer estima%es only one distance (to the outer

boundary) rather than distance estimates for each bird.

In addition, density estimates are considerably simpler,

requiring only that the total for each species be divided

by the area of the strip. However, it is often difficult to

be certain whether an individual bird is just inside or

outside the boundary.

Although a strip transect is simpler to conduct than

the variable-distance line transect, it does not allow the

observer to correct the count for differences in species

detectability. Because species differ in the ease with

which and distance at which they are detected, it is

inadvisable to make comparisons among different species. Therefore, a strip transect is best used for making

comparisons of one species in different habitats or sites,

but with the realization that detectability can vary

with habitat and that the derived densities are crude

estimates. Vilella and Zwank (1987) provide an example

of the proper use of this method for counting calling

nightjars at night.

Spot Mapping

This method involves mapping the positions of territorial birds. Detailed field maps and repeated visits

(at least 10) to the site are required before the observer

is able to determine the territories on the study site

and the population density. It is used primarily during

the breeding season because it is based on observations

of territorial males who are actively displaying on their

territories. It is often assumed that all males are paired

with a female, which can sometimes be verified during

the field visits. Some workers report only the number of

territorial males per unit area. Obviously, the method

works best with territorial species and is not effective

for colonial, highly social, or nonterritorial species.

Because spot mapping is done in the main period of

breeding, it is most effective for those species with a

7
b

LIMIT OF STRIP

C

f

LIMIT OF STRIP

Figure Z.-A slowly moving observer making bird observations while walking a transect line; the observer may: (a)

simply record all birds regardless of distance (line transect without distance estimate); (b) record all birds

and estimate theirperpendicular distance to the transect line (variable-distance line transect); or(c) record

all birds observed within a jixed distance of either side of the transect (strip transect).

8
short, well-defined breading season. For an example of

the use of this method in a Puerto Rican rain forest see

Recher (1970).

The method is intensive and is not appropriate for

broad-scale monitoring of land birds, but it is appropriate when reasonably precise pair numbers and density

estimates are required from small habitat patches or

study sites. One or two observers in a single morning

can sample 10 to 30 ha in a wooded area or 50 to 100 ha

in an open area. The minimum size of the study site

depends on the bird density, with the requirement that

at least 50 and preferably over 100 pairs be present to

obtain sufficient data-at least 10 ha in forests and 50

ha in open habitats. The study site should be as round

or square as possible to limit border length, which can

influence bird densities. Before the census begins, a

detailed map (scale at 1:2,000) is needed, showing landmarks such as trails, streams, forest edges, ridges,

roads, etc. If few landmarks are available, it is advisable

to place a grid with flagging or metal tags at 25-m

intervals throughout the site. It is important that the

map show sufficient detail to enable precise plotting of

the positions of all birds.

A clean map is used for each census, and different

symbols are used for different species and individuals.

Each visit should cover the area as evenly as possible,

and no part of the site should remain farther than 25

m off the route. The most difficult task is to separate

different individuals and not count the same individual

two or more times on a visit. This requires walking

with moderate speed through the study area and

recording birds on the map at the same time. The

observer should stop frequently to search for simultaneous observations of different individuals of the

same species. If uncertain as to whether one or two

individuals are present, the observer should return to

the area later the same morning to verify the actual

number.

This method is time consuming, particularly if all

species on the site are to have their territories mapped.

For example, in a forest it usually takes 10 mornings to

census 30 ha by the accepted lo-visit version of the

method (about 50 to 60 hours of field work). The

observer should allow 40 hours (4 hours per morning)

to prepare the species maps and approximately 5 to 10

hours to analyze them. Thus one can expect to spend

100 hours to census 30 ha of forest during the breeding

season.

The method described here is the standard method

used by the Cornell Laboratory of Ornithology in its

resident bird counts (known as Breeding Bird Censuses). The counts have been conducted throughout

North America, and the organizers welcome participation and publish the results of each plot annually in the

“Journal of Field Ornithology.” Participation by Caribbean residents is encouraged; contact the Cornell

Laboratory of Ornithology, Resident Bird Counts, 159

Sapsucker Woods Road, Ithaca, NY 14850, U.S.A. (telephone 607-254-2441).

Territory Mapping of ColorBanded Birds

By capturing birds in mist nets, leg banding them

with unique color combinations, and then‘ mapping

their locations after release, an accurate estimate of

density can be obtained. In fact, this method provides

the best measure of density, even though nonterritorial

individuals (floaters) are not accurately counted. If the

study is continued for a sufficient time period (4 to 10

years), the method has the added benefit of allowing a

measurement of survival of known individuals on their

territories. Unfortunately, the method is highly laborintensive because it requires both the amount of time

and effort of spot mapping (e.g., mapping the site followed by numerous visits for mapping individual birds),

plus an additional amount of time for capturing and

banding the bird& Because the method is so intensive,

it is usually used for studying only one or two species at

a time (fig. 3).

Mist Netting

Standardized mist netting enables biologists to

monitor population changes and provides information

about the health of a population. Captured birds can

provide detailed information on sex, age, weight, and

fat condition, all of which can assist in evaluating the

condition of a population. For example, the sex ratio of

a population can be used to assess differential survival

in the previous year and the potential for population

increases. The proportion of juveniles captured can

provide a measure of a population’s productivity in the

previous months (Baillie and others 1986). Weight and

fat condition, when related to anatomical measurements (such as wing length), can provide an estimate of

the health of individuals in the population. Careful

inspection of captured individuals can enable workers

to determine the reproductive condition of individuals

and the pattern of molt-all basic information needed

to determine the annual cycle of reproduction and molt

as illustrated by Diamond’s (1974) studies in Jamaica.

Finally, marking individuals (particularly with colorbands) can aid in studying dispersal and survivorship

among years. Examples of Caribbean mist net studies

on distribution and long-term population trends can be

found in the works of Faaborg and Arendt (1985,1991),

Faaborg and others (19841, Terborgh and Faaborg

(1973), and Terborgh and others (1978).

Caribbean land birds have been captured with 6-, 9-,

12-, and 18-m nets; net size depends upon the goals of

the project and the source of the nets. Because the

capture efficiency for different sized birds varies with

the net mesh size (Pardieck and Waide 1992), it is

important to consistently use the same mesh size for

9
iL-_

Figure 3.-A field sketch map of territories of color-banded bananaquits (Coereba flaveola), in

Cayey, PR, on 11 March 1989. Letter code refers to color-band combinations (R. red; BZL.

blue; Ba. black; W white; Y yellow; X. metal). Band code is read from top to bottom, left

to right, with bird facing the observer. Birds with the code -X are individuals with no

color-bands and only a metal band on their left leg.

long-term studies. Generally, nets with a mesh of 30 or

36 mm are used for small- to medium-sized songbirds.

Nets can be set between two upright 3.0- to 3.7-m poles

braced with two nylon cords at each end. Metal electrical conduit tubes (1.3 cm in diameter) or polyvinyl

chloride tubes of the same diameter are readily available in most building supply stores in the Caribbean

and can serve as net poles in areas where bamboo poles

are unavailable. Supporting cords can be tied to the

base of available shrubs or to stakes placed in the

ground. Before net placement, a 1.5 to 2.0-m wide strip

of vegetation should be cleared along the length of the

net to prevent entanglement in the vegetation.

Nets are most effective in areas with no wind and low

light levels and where the maximum vegetation height

is equal to or only slightly above the top of the net. Nets

10
are less effective in forests with tall trees where canopy

dwellers are poorly represented in net samples. This

problem can sometimes be alleviated by use of nets

strung in the canopy, involving a complex of cords,

pulleys, and poles to raise the nets high off the ground.

However, in most instances, capture rates with canopy

nets are so low that it is rarely worth the effort

required to set and operate them.

The most efficient means of operating nets is to place

a line of nets end-to-end, so that each net shares a pole

with the preceding net. These net lines can be placed on

unused trails or abandoned roads that pass through

the appropriate habitat. Nets placed on ridges, on habitat edges, near streams, or in mesic sites in dry habitats can have very high capture rates, but samples may

not be representative of uniform habitats. Nets should

be set up the afternoon before sampling and furled for

the evening. On the following morning, nets should be

opened at the first sign of light and left for at least 6

hours-and preferably until sundown-for 2 or 3 complete days to obtain an adequate sample. Nets placed in

the shade should be checked about every 30 minutes

and at least once each hour. Nets should be checked

more frequently during light showers and closed during heavy downpours. Nets in the sun in lowland areas

should be routinely checked and cleared of birds every

10 minutes . The need for frequent net inspections and

the capture rate sets the limit on how many nets can be

safely operated at a site. Generally, 2 experienced people can operate 10 to 12 full-length nets, assuming that

capture rates are moderate and that only a small

amount of information is obtained from each captured

bird. However, nets require almost constant vigilance

in areas with mongooses or feral cats.

The number of birds captured in mist nets decreases

with time, primarily because birds learn where nets are

located. Forest bird captures did not decrease when

netting intervals were spaced with nonnetting intervals of 2 to 3 weeks (Wunderle and others 1987). However, this procedure may not work in other habitats and

with other species; biologists must first experiment to

determine the appropriate interval between netting

efforts.

Handling and Freeing Captured Birds. - Removing birds from mist nets is a fine art that requires

hours of practice and is best learned from an experienced person. In fact, a beginner should not operate

nets without supervision and assistance until he or she

has considerable experience in removing entangled

birds. The principal concern in netting should always

be the life and health of the captured birds, and a netter

should never hesitate to close down nets and immediately release birds when overwhelmed with numerous

captures or severe weather conditions. The methods

used by most netters to remove birds have been

described by Bleitz (19701, Ralph (1967, 19881, and

Shreve (1965). Although several different removal

techniques have been recommended, the most widely

used method is that of Ralph and others (1993):

1. The first and most important step is to determine

from which direction the bird entered the net. This

requires finding the opening of the pocket caused by

the weight of the bird.

2. Once the opening of the pocket is determined,

move to the side in which the bird entered the net, and

grasp both tibiae- the tibia is the part of the leg

above the bare tarsus. If the person removing the bird

is right handed, both tibiae should be grasped with the

left hand, so that the fingers point towards the bird’s

head. Both thumb and fingers should hold the tibiae as

close to the bird’s body as possible, leaving the right

hand free to remove net strands from the entangled

bird.

3. The process of disentanglement should begin by

making certainthat all threads are pulled down and off

the tibiae and thighs below the heel joint (the prominent joint betw&n the tibia and tarsus). Note that

threads can often be high up on the thigh at the

flank.

4. Toes should be untangled by pulling strands

gently. A blunt probe, crochet hook, forceps, or pencil

can often be useful. Sometimes, if the heel joint is

straightened out, the bird’s toes will relax and release

the netting. Toes can frequently be extracted by first

freeing the opposable toe (the “thumb”) by sliding

threads over it and lifting it away from the other toes.

Next, the other three toes should be straightened and

the netting slid over these toes, thereby freeing the

feet.

5. Once the feet have been freed, the bird should be

pulled up and away from the net, still holding the legs

on the uppermost part of the tibiae. Pull the bird away

from the net creating a small amount of tension, so

that when wings and other body parts are freed, the

net pulls away from the bird. Next, the netting should

be removed from the bend of the wings by working

from the underside. Often a badly entangled wing can

be freed by carefully opening the wing and sliding the

threads off the wing. At this stage, it is useful to pull

gently on any exposed threads to free them or to see

where they are caught.

6. When the wings are freed, the head should be

disentangled by gently pulling threads from the neck,

working from the back of the head forward. The bill

should be secured by placing the thumb against the tip

while pulling the net over the head.

7. In some species, particularly thrushes, thrashers,

and woodpeckers, the tongue can be badly entangled in

the net. In this situation, the bird’s head should be held

between the index and second finger, while the third

and fourth fingers and thumb can hold the net near the

side of the mouth and relieve pressure on the tongue. A

probe, such as a pencil, pointed twig, or crochet hook

can be manipulated with the free hand to remove the

11
thread from behind the cleft of the tongue. This

method requires considerable skill, and until it is mastered, a small pair of scissors is useful. Often, just

clipping a single strand of netting will free the tongue.

8. Birds that are badly entangled and in distress may

be extracted by use of a fine knife or scissors. If threads

are to be cut, first find the area with one layer of

netting, or the area with the fewest layers. As few

threads as possible should be cut, and the bird should

be pulled through the net. The bird should then be

removed from the net in the normal manner. Finally,

check to see if any threads remain on the bird, particularly in its wings or bill.

9. Once removed from the net, birds should be placed

in small cloth bags for transportation and temporary

holding at the processing site. Ideally, birds should each

be placed in a separate holding bag, but small nonaggressive species (e.g., warblers or grassquits) can be

placed together for short periods of time. It is better to

temporarily “store” captured birds in suspended holding bags in the shade rather than hanging in the nets,

particularly when processing is delayed. To properly

hold a songbird in the hand requires that the bird’s

head be held between the knuckles and base of the

fingers so that its back rests firmly against the palm of

the hand. Held properly in this manner the bird should

be unable to flap its wings, thereby reducing the risk of

injury.

Banding. -If mist netting is used for survey work

(e.g., to locate the habitat in which a particular species

occurs or is most abundant), then it may not be necessary to band the birds. In this case, simply cutting a

small portion of the end of a tail feather will suffice to

discriminate between new and recaptured birds during

a short netting session (1 to 4 days). It will also save the

time and effort required for banding, which would be

wasted if a long-term study is not contemplated.

Banding is useful only if it will answer a specific question and is not recommended as a routine practice.

Banding is essential for studying long-term survival

or for mapping territories at a later date. Banding and

banding supplies are tightly regulated by the U.S. Fish

and Wildlife Service, and prospective banders are

required to work with an established bander before

obtaining a banding permit. Numbered aluminum

bands and information on methods, equipment, and

record-keeping can be obtained from the Bird Banding

Laboratory, Office of Migratory Bird Management,

U.S. Fish and Wildlife Service, Patuxent Wildlife

Research Center, Laurel, MD 20708, U.S.A. Information regarding banding supplies and equipment can

also be obtained from AVINET, Inc., P.O. Box 1103,

Dryden, NY 13053-1103, U.S.A.

For most studies involving color-banded individuals,

it is useful to use four bands (two on each leg), which

allows a variety of unique color combinations-three

plastic color-bands and a U.S. Fish and Wildlife Service

aluminum band. A simple code, which involves reading

the band combinations from top to bottom and left to

right (with the bird facing the observer, starting with

the observer’s -not the bird’s -left) can be used. For

example, the code RY-XW can be used to indicate an

individual with a red above a yellow band on the bird’s

right leg and an aluminum band (X) above a white

band on the bird’s left leg. Therefore, color type and

position, above or below as well as left or right, provide

many unique band combinations. The colors red,

yellow, blue, and white are easiest to distinguish in the

field.

Colored plastic coil bands are attached to the leg by

placing the outer end of the band next to the leg and

uncoiling the band so that the inner end finally resides

on the outside and helps hold the band in place.

Although these bands rarely come off the bird, they do

require practice to properly place them on the bird’s

leg. A coat hanger Can be used to practice placing the

coil bands. High-quality coil bands of several different

sizes and colors that resist fading in the tropical sun

can be obtained from AVINET, Inc. or the National

Band & Tag Co., 721 York St., Newport, KY 41072,

U.S.A. Split-ring plastic color-bands for land birds can

be obtained from A. C. Hughes, Ltd., 1 High Street,

Hampton Hill, Middlesex TW12 1 NA, UK.

Record Keeping and Data Collection. -Records

should be kept of the location, size, type, and number

of nets and the hours and weather conditions during

which nets are open.

Capture rate per net hour has commonly been used

as a unit for standardizing netting effort. The net hour

unit simply represents the product of the number of

standard-sized nets that are open multiplied by the

number of hours the nets are open. For instance, 1 net

open for 1 hour represents 1 net hour, whereas 10 nets

open for 2 hours represents 20 net hours of netting

effort (assuming all nets are of equivalent size, usually

12 m long). The total bird captures are divided by the

total net hours, providing a capture rate per net hour

that enables comparison of captures obtained from different netting efforts. It is often convenient to convert

the capture rate to 100 net hours, as opposed to having

to deal with decimal points of a bird! Unfortunately, for

most resident species in the Caribbean, captures

decrease with netting effort; therefore, capture per net

hour is not a useful measurement of effort. For a discussion of the appropriate measurement see Terborgh

and Faaborg (1973). The best way to solve this problem,

at least when using mist netting for long-term monitoring purposes, is to standardize the number of nets, net

placement, hours of operation, and time when nets are

opened and closed. For example, Faaborg and others

(1984) have always used 3-day samples, usually with a

16-net line, once a year for their long-term studies.

The date, location, species, and (if possible) sex and

age should be recorded for each captured individual.

12
Sex and age are important data, but unfortunately are

difficult to determine in many species due to the highly

variable nature of size, plumage, and molt patterns in

each species. It is important to realize that it is not

possible to reliably sex, age, and/or identify all live

individuals. In the Caribbean, in particular, reliable

methods for aging and sexing have not been determined

for many species, and this subject represents an area in

which Caribbean ornithologists could make major contributions. Thus, it is better to be cautious than inaccurate when determining age and sex, and there will be

many birds for which these determinations will remain

unknown.

Aging Birds by Skull Ossification. -The best

method for aging songbirds is by determining the

extent of ossification (defined below) in the postbreeding period (April through December in most Caribbean

habitats). Unfortunately, the method allows the determination of only two age classes -juvenile (hatching

year) and adult (after hatching year).

e.

Figure 4. - llvo major patterns of skull ossif=ation proceeding from

a very young bird with an urwssified skull (a) to a mature

adult with a completely ossified skull (e) (adapted from

Pyle and others 1987).

This method is based on the fact that, when a fledgling leaves the nest, the upper section of the skull

(parietals and frontals) consists of only a single layer of

bone. However, as the fledgling matures (until 4 to 12

months old, depending on the species) a second layer of

bone develops below the first layer. As this second layer

grows, a series of tiny bone supports is produced (very

much like the struts in an airplane wing), which

extends through the air space between the two layers.

This process is called “ossification” or “pneumatization.” When these tiny bone supports are viewed from

above, they appear as tiny white dots (ossified) on the

skull surface in contrast to the unspotted (unossified)

skull of a very young bird.

Although the progression pattern of ossification

varies among species, two major patterns are evident

(fig. 4). The peripheral ossification pattern is more

common in small species, whereas the median line pattern is more common in large species. However, considerable individualsrariation exists, and in some species

either pattern may occur. The speed at which this

process occurs varies among species. Unfortunately,

the rate of ossification has not been studied in Caribbean birds; therefore, it must be assumed (correctly or

incorrectly) that patterns found in North American

species are also typical of some Caribbean species. In

most North American songbirds, the skulls of the earliest hatching-year birds (hatched in May through early

June) may be completely ossified by October or November, but later hatching-year birds (hatched late June

through July) may not be completely ossified until

November, December, or January. This pattern appears

to apply generally to Caribbean birds, at least in Grenada and Puerto Rico. However, it is possible to find

birds with incompletely ossified skulls at any time of

the year in the Caribbean.

To age birds by the extent of skull ossification

requires that the observer first wet the feathers on top

of the skull just behind the eyes. Then the feathers

must be carefully parted to allow an area of the skull to

be viewed through a small area of bare skin. Fortunately, the skin on the skull is very thin and almost

transparent. Thus, with sufficient light, one should be

able to see the extent of ossification by moving the area

of bare skin over the skull. If the skin is thick on the

crown, it is often helpful to make the opening in the

feathers on the side of the skull or neck where the skin

is more transparent. Unossified areas of the skull are

pinkish and have no white dots (i.e., have no bone

supports), in contrast to the ossified areas with white

dots that may have an overall white, pinkish-white, or

gray appearance.

The observer’s main goal is to locate areas without

white dots (unossified), requiring that the bare wet

skin be moved over much of the skull in search of

unossified regions. By moving the skin over the skull

surface, the observer should be able to differentiate

13
between white dots on the skin (which move with the

skin) and ossified white dots on the skull (which are

stationary when the skin is moved). Except for the

youngest fledglings, at least some white dots should be

present along the crown midline or the posterior base

of the skull (fig. 4). With “advanced” juveniles, the

observer must be careful to look for small unossified

“windows” on the crown behind the eyes. Care should

always be taken to look for these small windows

because they are easy to miss. It is often useful to

sketch the position and relative size of the unossified

windows on the data sheet or in the field notebook.

These sketches can contribute to badly needed information on the development of aging criteria for Caribbean species. Some Caribbean species, such as

thrashers, may be difficult to age by skull condition

because fat on the skull makes it almost impossible to

see the skull.

Sex Determination. -Birds without sexual differences in size or plumage can usually be sexed by the

presence of either a brood patch (mostly females) or

cloacal protuberance (males only) during the breeding

period. The presence of a brood patch or cloacal protuberance can generally be used to reliably identify the

appropriate sex, whereas the absence of either structure cannot be used to determine sex, particularly in

the Caribbean where breeding and nonbreeding individuals of many species can be found at any time of the

year.

Brood Patch. -To transfer body heat effectively to

the eggs, females develop a highly vascularized, featherless area on the lower breast and abdomen known as a

brood or incubation patch. This enables her to place her

bare skin directly on the eggs or young, thereby facilitating heat transfer. Because females do most, if not

all, of the incubation and brooding, they have the most

highly developed brood patches. Thus, the presence of a

substantial brood patch indicates that an individual is a

female in most songbirds.

Brood patches begin to form with the loss of abdominal down feathers, 3 to 5 days before the first eggs are

laid (Blake 1963). Afterwards, the blood vessels of the

area begin to expand, and fluid collects under the skin

giving a puffy or swollen appearance, often described

as edematous (for the presence of an edema or fluid

buildup). Females with this edematous condition are

actively incubating eggs or brooding newly hatched

young (fig. 5). However, once the nestlings leave the

nest, a decrease in ‘vascularization and edema occurs,

although the patch may remain defeathered for as long

as a month or more (until the initiation of prebasic

molt). In this postbreeding condition, prior to molt, the

female’s brood patch skin-often appears grayish and

wrinkled. If the female begins another clutch, the process is repeated.

The brood patch can be located by blowing softly to

part the feathers on the lower part of the breast and

abdomen where the skin condition can be examined.

9

FULL BREEDING dPARTIAL BREEDING NONBREEDING

Figure 5. -Different stages of brood patch development (adapted fmm Pyle and others 1987).

14
Care should be taken in looking for a brood patch

because the unwary observer can be easily misled. For

example, in hummingbirds and many juvenile songbirds, little or no down occurs on the abdomen. Therefore, they might initially look like females with a brood

patch; however, the abdomen is quite smooth with a

pink or red coloration. Also, males of some songbird

species (e.g., Myiarchus flycatchers, vireos, mimids,

and a few others [Pyle and others 19871) will incubate

or brood and partially develop a brood patch. However,

in these cases, male brood patch development is usually

incomplete, and may involve slight to moderate feather

loss, vascularization, and swelling that rarely reaches

the extent developed by females of the same species.

The following categories can be used for classifying the

brood patch condition:

1.

2.

3.

4.

No Brood Patch - Breast and abdomen with down

feathers or with traces of down feathers; skin

smooth and not puffy or wrinkled.

Smooth Skin-Breast and abdomen with down

feathers missing; skin smooth and dark red.

Edematous -Breast and abdomen without down

feathers and skin puffy or swollen with fluid and

increased vascularization evident; represents the

peak of incubation.

Wrinkled-Breast and abdomen without feathers

and skin not swollen; skin has a wrinkled or scaly

appearance.

Cloaca1 Protuberance. -With the onset of breeding, male songbirds develop external cloaca1 protuberances, or bulbs, which store sperm and facilitate

its transfer during copulation. The protuberance is

usually apparent from several weeks to several months

depending on the species and the number of clutches

produced during the breeding period. The protuberance is absent in nonbreeding males.

The cloacal protuberance can be inspected by blowing the feathers apart in the vent region. A welldeveloped protuberance in the male forms at a right

angle to the abdomen and tends to be larger at the tip

than at the base (fig. 6). Although females may occasionally show a small protuberance or at least some

swelling in the vent region, it rarely attains the size of

the male protuberance. For instance, a female in breeding condition may have a swelling that forms a gradual

slope on the abdomen (in contrast to the right angle

formed in the male) ending with the cloacal opening

directed towards the tail. However, females in this condition are usually in full breeding condition with an

active brood patch.

With experie+, an observer should be able to identify differences id,the size of the cloacal protuberance

in the male and may wish to describe the size with

subjective categories (i.e., none, small, medium, large).

Molt. -Very little is known about the sequence and

timing of molt (feather replacement) in Caribbean

birds, yet this information can be useful for determining age and sex in many species. A common molt pattern in northern temperate songbirds and some

Caribbean species involves two molts per year, just

before and just after the reproductive period. These

molts produce two plumages, the basic plumage (nonbreeding) and the alternate plumage (breeding). The

molts that bring about these plumages are the prebasic

a.

NONBREEDING

PARTIAL BREEDING

FULL BREEDING

b.

Figure 6. -Cloaca1 protuberances at different stages of development in songbirds: (a) a cloaca1

proturberance at the peak of breeding (boxed); (b) different stages of cloaca1 protruberance

development shown in profile (adapted f?om Pyle and others 1987).

15
molt, which produces the basic plumage at the end of

the breeding period, and the prealternate molt, which

produces the alternate plumage at the end of the nonbreeding period. All North American songbirds have a

prebasic molt, whereas slightly over half have a prealternate molt. Whether this pattern exists for Caribbean species remains to be documented.

Close inspection of a bird’s skin, made possible by

blowing apart the body feathers, will indicate molting

feathers by the presence of a cylindrical sheath around

the base of each new feather. The sheath surrounding

the molting feather is finally preened off when the

feather is fully mature. Thus, the documentation and

quantification of body molt simply requires an evaluation of the extent and location of sheathed feathers on a

bird’s body.

The quantification of body molt usually involves a

subjective evaluation of the amount of down or contour

feathers sheathed on the body, excluding the wings and

tail. A simple three-category scale can be used to quantify this body molt as none, moderate, or heavy. However, the molt of wing and tail feathers (designated as

flight feathers) can be evaluated with more precision

because the pattern of flight feather molt is usually

systematic. Also, it is usually easy to distinguish

between new and old flight feathers on a molting individual. To understand and document the pattern of

flight feather molt first requires knowledge of the

terms used to describe and number these feathers.

Two major groups of flight feathers are recognized on

the wing (fig. 7), the primaries and the secondaries

(“remiges”). The primaries are the long, outermost

feathers attached to the bones of the “hand” and are

numbered consecutively from the innermost at the

bend of the wing (“wrist-joint”) outward to the end of

the wing.’ Depending on the species, 9 or 10 primary

feathers occur on the wing. Pyle and others (1987)

summarize the songbirds with 9 and 10 primary feathers per wing. The pattern of primary molt proceeds one

feather at a time from the innermost primary (primary

number 1) to the outermost primary (primary number

9 or 10, depending on the species) and is symmetrical,

occurring simultaneously at the same primary position

on both wings. Therefore, primary molt can be recorded

by simply noting the primary feather number(s) missing or growing during molt. However, care should be

taken to check both wings because feathers are often

accidently lost and replaced by an adventitious molt,

which is not symmetrical and obviously does not occur

seasonally.

The secondaries are the long feathers, on the inner

part of the wing, attached to the skin covering the bone

of the forearm (“ulna”). The secondaries are numbered

in ascending order from the bend of the wing (wristjoint) inward toward the body. Molting of the secondaries also follows this sequence, one feather at a time,

except for the three innermost secondaries (“tertials”),

which tend to be molted concurrently with the longer

(NEW,

GR COVS

MID COVS \ ALULA

PI0 (IF PRESEN

GROWING SS

Figure I. -An example of a wing during molt, with p indicatingprimary feather (pp forprimaries), s for

secondary feather (ss for secondaries), terts for tertials. In this example, primary number 6

and secondary number 3 are molting (adapted from Pyle and others 1987).

16
secondaries. As with the molt of the primaries, molt of

secondaries can be recorded by noting the number of

the molting secondary feather, with a check to see if the

molt is symmetrical.

The tail feathers (“r-e&rices”) are numbered in pairs,

beginning with the innermost pair (designated as number 1) and proceeding outward in both directions to

number 5 or number 6, depending on the species. In

other words, the numbering is symmetrical from the

innermost pair of feathers ascending to the outermost

feathers. The pattern of tail molt varies among species,

with the innermost (number 1) feathers sometimes

molting simultaneously with the body molt in some

juveniles. The remaining rectrices molt symmetrically

from the innermost (number 2) to the outermost feathers (number 5 or number 6). Tail molt can be quantified

by recording the number of the molting feather.

In summary, molting can be recorded by evaluating

the extent and amount of molting feathers on the body

and the feather number of the molting flight feathers.

More details on molting and its quantification can be

found in Ginn and Melville (1983). Not only is basic

information needed on molt patterns by age and sex for

many Caribbean species, but information on the timing

of molt will contribute to the understanding of the

annual cycle of the bird species, as demonstrated in the

Jamaican _avifauna by Diamond (1974).

Body Fat. -An evaluation of the amount of body fat

on a bird can provide an indication of the bird’s condition. Low fat levels may reveal periods of stress, low

food supplies, and other conditions that indicate the

viability of an individual. Fat levels vary most in migratory species and are at their highest levels just before

migration. Fat, when present, is visibly deposited in the

abdomen and the furcula (“wishbone”). Fat deposits

can range in color from orange white, yellow white, to

off white and generally contrast with the deep red of

the pectoral muscle or the dark-colored viscera of the

abdomen.

Fat in the furcula region can be inspected by parting

the feathers in the region where the neck protrudes

from the body. A depression (furcular or interclavicular

region) is formed between the attachments of the

breast (pectoralis) muscles to the furculum and the

coracoids, forming a “V” running toward the spinal

cord. Fat in this region can be subjectively ranked into

categories from some to a lot. A scale of 0 to 4 is

adequate for most Caribbean birds, which would modify the fat categories as described in Ralph and others

(1993) (table 3).

Other Measurements. -A diversity of measurements can be made from captured birds as described in

detail by Pyle and others (1987) and summarized in

figures 8 through 11. Some measurements can be

especially valuable in helping to determine the age or

sex of certain species. For example, in many birds, the

males of a particular population will tend to be larger

Table 3.-F& categories for Caribbean birds (adapted from Ralph

and others [1993])

Fat Class Furculum Abdomen

0 No fat, region concave No fat

1 Trace of fat in scattered None or a trace of

patches, region deeply fat

concave, furculum less than

5 percent tilled

2 Thin layer of fat, furculum Trace or thin layer

less than 33 percent tilled of fat

3 Fat in small patches, Small patches of

furculum 50 percent tilled fat, not covering

some areas

4 Fat level with clavicles, Covering pad of

furculum 66 percent filled fat, slightly

mounded

5 Ftegior&ightly bulging Region well

mounded

than females of that population. Thus, measurements

of wing length, tarsal length, and weight can be quite

useful. When these measurements are included with

estimates of fat, the body size of the bird can help to

provide an indication of its health.

It is important that measurements be taken in a

standardized way, particularly when used to determine

the age and sex of individuals. Weight is probably the

simplest measurement and can be taken with a Pesolae

spring scale and a light cloth bag of known weight for

holding the bird while suspended. For other standard

measurements, refer to either Svensson (1984) or Pyle

and others 0987); at least one of these publications

should be a standard part of all netting and banding

kits, even in the Caribbean.

‘Ihpe-Recorded Playback

To attract quiet or secretive species, it is often useful

to play a tape-recording of their songs or call notes in

the appropriate habitat. This technique is particularly

valuable for counting a particular rare species. The

tape-recorded playback can be combined with point

counts or transects, although censuses that use playback cannot be compared with censuses that do not use

playback. One simple playback design involves playing a

tape of the vocalizations at the natural rate for 5 minutes followed by 5 minutes of silence in which individuals are counted. This technique could be used at

each individual point count or while walking slowly on

a transect. If playbacks are done in a consistent manner, they can be used for both survey and monitoring

purposes because they provide an index of abundance

(but not an actual measurement of density). In the

Caribbean, tape playback has been used to survey the

17
a.

b. WING CHORD FLATTENED WING

Figure 8. -Methods of measuring wing length: a. an appropriate hold for obtaining the measurement; b. two

different techniques for obtaining wing measurements (wing chord on left; flattened wing on right).

The wing chord is the most commonly used measurement (adapted from Pyle and others 1987).

Figure 9. -An accepted method of measuring the tail involves inserting a ruler between the central tail features (rectrices) and

guiding the ruler up against the body surface to obtain

the maximum reading of tail length (adapted from Pyle

and others 1987).

18
Figure lO.-The culmen and exposed c&men are two standard measures for bill length The culmen (a.) is

measured from the anterior end of the nostril to the end of the bill, whereas the exposed culmen (b.)

is measured from the edge of the feathering at the base of the bill to the bill tip (adapted from Pyle

and others 1987).

Figure 11. -Ends of the tarsus used to obtain a measurement of

tarsal length, which is best done with calipers (adapted

from Pyle and others 1987).

19
distribution of the endemic Grenada dove, _ Leptotila

wellsi, (Blockstein 1988) and to assess the populations

of several endemic bird species on Jamaica in the aftermath of a major hurricane warty 1991).

Playback can often be used to capture territorial

birds. For example, a portable speaker can be placed

beneath a net and used to attract an individual into the

net. This method requires a speaker attached to a long

cable that runs to a tape recorder operated by a hidden

observer several meters from the net. As described by

Holmes and Sherry (1989), the method was very effective for capturing wintering migrants in Jamaica.

‘Iape-recorded songs of many Caribbean species can

be obtained from the acoustical lab of the Cornell Laboratory of Ornithology, 159 Sapsucker Woods Road,

Ithaca, NY 14850, U.S.A. (telephone 607-254-2441)

Information and equipment for field recordings and

playback can be obtained from Saul Mineroff Electronics, Inc., 946 Downing Road, Valley Stream, NY

11580, U.S.A. (telephone 516-825-4702X

LITERATURE CITED

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Point Reyes Bird Observatory. [Number of pages

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Recher, Harold F. 1970. Population density and seasonal changes of the avifauna in a tropical forest

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Vilella, F.; Zwank, P.J. 1987. Density and distribution

of the Puerto Rican Nightjar in the Guayanilla Hills.

Caribbean Journal of Science. 23: 238-242.

Waide, Robert B. 1991. The effect of Hurricane Hugo

on bird populations in the Luquillo Experimental

Forest. Biotropica. 23: 475-480.

Waide, Robert B.; Narins, Peter M. 1988. Tropical forest bird counts and the effect of sound attenuation.

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Wauer, Roland H.; Wunderle, Joseph M., Jr. 1992. The

effect of Hurricane Hugo on bird populations on St.

Croix, U.S. Virgin Islands. The Wilson Bulletin. 104:

656-669.

Wunderle, Joseph M., Jr. 1985. An ecological comparison of the avifauna of Grenada and Tobago,

West Indies. The Wilson Bulletin. 97: 356-365.

Wunderle, Joseph M., Jr. 1992. Sexual habitat segregation in wintering black-throated blue warblers in

Puerto Rico. In: Hagan, J.; Johnston, D.P., eds. Ecology and conservation of NeotropicaI migrant landbirds. Washington, DC: Smithsonian Institution:

299307.

Wunderle, Joseph M., Jr.; Diaz, Ariel; Velasquez, Iris;

Scharron, Rafael. 1987. Forest openings and the distribution of understory birds in a Puerto Rican rainforest. The Wilson Bulletin. 99: 22-37.

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FOR FURTHER INFORMATION

Publications cited here are part of the library collection at the International Institute of Tropical Forestry,

U.S. Department of Agriculture, Forest Service, Call

Box 25000, Rio Piedras, PR 00928-2500, U.S.A.

21

Appendix

Examples of field data sheets are shown for fxed-raduis point count (25 m), strip

transects (25 m on each side of transect), and banding data.

23
FIXED-RADIUS POINT COUNT DATA SHEET

Date Location Weather

Observer

Species

Time Time Time Time

125m >25m 125m >25m 125m >25m 125m >25m

c

24
STRIP TRANSECT DATA SHEET

Date Location Weather

Observer

Species

Transect # _

Length

Time

I

125m >25m

Transect #

Length

Time

(25m >25m

Transect # _

Length

Time

125m ~25

25
BANDING REDXD - Mividual Species

Scientific Name

CbrmnName

BandNumber’ Color Can. Aqe !3ex Winq

lkmtion

'Ihi1 7k.rs.i Bill Wqt. Fat MoltI I

Date

*

26
Wunderle, Joseph M., Jr. 1994. Census methods for Caribbean land

birds. Gen. Tech. Rep. SO-98. New Orleans, LA: U.S. Department of

Agriculture, Forest Service, Southern Forest Experiment Station.

26 p.

Various census methods for Caribbean land birds are presented. The

method used depends upon the objective of the study. For most studies

of Caribbean land birds, the fured-radius point count method is recommended.

Keywords: Color-banding, mist netting, point counts, populations, spot

mapping, territory mapping, transects.

The use of trade, firm, or corporation names, or suggested suppliers of materials, in this paper is for the information and convenience of the reader. Such use

does not constitute official endorsement or approval by the U.S. Department of

Agriculture of any product or service to the exclusion of others that may be

available.
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should contact the USDA Office of Communications at (202) 720-5881 (voice) or

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720-l 127 (TDD). USDA is an equal employment opportunity employer.
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