Abundance and Productivity of Birds 

Over an Elevational Gradient1 

Kathryn L. Purcell2 

Abstract 

This study is investigating the abundance and productivity of birds breeding in four 

forest types over an elevational gradient in conifer forests of the southern Sierra Nevada 

of California to identify the most productive habitats for each species, and to examine 

elevational shifts in abundance, especially as they relate to temperature and precipitation. 

Species richness and abundance decreased with increasing elevation, although higher 

elevations were important for cavity-nesting species. Abundance and nest success varied 

across years and forest types. Dark-eyed juncos (Junco hyemalis) were most abundant at 

the lowest and highest elevations and had their highest nest success in the lodgepole pine 

(Pinus contorta) forest type. Excluding the ponderosa pine (Pinus ponderosa) forest type, 

where they were rare, dusky flycatchers (Empidonax oberholseri) were least abundant in 

true fir sites but had their highest nest success and productivity there. Abundance vs. 

nest success and number of young fledged were both negatively correlated, suggesting 

that dusky flycatchers were unable to assess the probability of successfully nesting in a 

given habitat prior to settling. In accordance with expectations, 15 species exhibited 

downslope elevational shifts following severe winters in 1995 and 1998, and six species 

exhibited upslope shifts. 

USDA Forest Service lands include many forest types, broadly distributed 

along elevational and latitudinal gradients. Specific management activities are 

often concentrated in certain elevational bands and forest types, and wildlife 

species are affected according to their distribution and response. Identification of 

species’ requirements for survival and reproduction and knowledge of their 

responses to forest management practices are crucial to maintaining biodiversity. 

To manage for a species, we need to know what limits its abundance and 

distribution, whether it is primarily interactions between species, abiotic factors 

such as temperature and precipitation, or factors relating to historical and 

evolutionary constraints. Individuals of a species do not survive independently 

of individuals of other species. Predation, competition, and parasitism are often 

cited as potential factors regulating the organization of bird communities 

(Hairston and others 1960, Hudson and Dobson 1991, MacArthur 1958, Price 

1986). Interspecific competition for resources has received much attention in this 

regard, especially in birds, although the importance of competition has been 

challenged (for review see Wiens 1989). Disproportionate predation on prey 

species may favor differences in relative abundance of coexisting species. Nest 

predation is recognized as a major cause of reproductive failure in birds (Ricklefs 

1969), and it may be an important selective force affecting the distribution of 

species across habitats (Martin 1988a). 

1 An abbreviated version of this 

paper was presented at the 

Symposium on the Kings River 

Sustainable Forest Ecosystems 

Project: Progress and Current 

Status, January 26, 1998, Clovis, 

California. 

2 Research Wildlife Biologist, Pacific 

Southwest Research Station, USDA 

Forest Service, 2081 E. Sierra Ave., 

Fresno, CA 93710. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 121 
Purcell Abundance and Productivity of Birds Over an Elevational Gradient 

Weather can also be an important factor influencing bird abundance in 

temperate montane forests (Hejl and Beedy 1986, Hejl and others 1988, Raphael 

and White 1984). The response of a particular species may be mediated by 

migration status or nest type, with neotropical migrants and cavity nesters 

generally less negatively impacted by harsh winters (Bock and Lynch 1970). 

Weather conditions can exert an influence on both the upper and lower 

elevational limits of species, with some species moving upslope in years with 

light precipitation and downslope in years with heavy precipitation (DeSante 

1990). Extreme weather conditions can also exert a strong influence on bird 

abundance, and the resulting changes in abundance do not necessarily reflect 

habitat choice (Hejl and Beedy 1986, Hejl and others 1988). To include the range 

of variability encountered, long-term studies are required to examine the effects 

of yearly variations in weather on bird abundance and productivity (Verner and 

Purcell 1999). 

Monitoring programs based solely on census data may detect population 

problems only long after they have occurred. Healthy populations are those in 

which reproduction is sufficient to maintain population size. Unhealthy 

populations can be maintained only by immigration from healthy populations, 

causing some population problems to go undetected for years. Presence of a 

species may not reflect a population’s health because data on species abundance 

may not be a reliable indicator of the value of a habitat (Purcell and Verner 1998, 

Van Horne 1983). Identification of productive or source habitats is crucial when 

managing for species persistence. When bird abundance and productivity shift 

in response to weather conditions, source habitats may shift as well and an 

understanding of these relations and the ability to predict this response is also 

needed. Relations between bird abundance and productivity can help elucidate 

the mechanisms by which birds select breeding habitat, whether those choices 

are adaptive, and, if not, what the mechanism for settling on breeding territories 

might be. 

Long-term studies, involving multiple species at large spatial scales are 

needed to assess the response of a bird community to silvicultural methods, with 

results being used to improve future planning (so-called adaptive management). 

Nongame birds are particularly appropriate for monitoring environmental 

health. Breeding productivity is more easily monitored for birds than for any 

other group of vertebrate taxa. 

The objectives of this study were to assess the abundance and productivity of 

bird species in four forest types over an elevational gradient in the southern 

Sierra Nevada of California; to identify species’ breeding habitat requirements, 

including identification of the most productive habitats for each species; to 

assess current population health of a wide range of species, including the 

identification of source and sink habitats; to examine relations between 

abundance and productivity across the four forest types; to develop models of 

habitat needs for healthy populations of coexisting species and to predict species’ 

vulnerabilities to habitat change; and to examine elevational shifts in bird 

abundance, especially as they relate to the severity of the previous winter’s 

weather, and to examine whether, in years of unusual weather conditions, 

variations in productivity are associated with these shifts. This paper reports 

preliminary results from the first 4 years of a planned 10-year study. 

Methods 

Eighteen study sites in four forest types were selected on the King’s River Ranger 

District of the Sierra National Forest in 1994 (fig. 1). Sites at the lowest elevations 

are in ponderosa pine (Pinus ponderosa) stands (elevation 1,024-1,372 m), followed 

by mixed-conifer stands (elevation 1,707-2,012 m), and true fir stands (elevation 

2,170-2,347 m), with lodgepole pine (Pinus contorta) stands at the highest 

122 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
Abundance and Productivity of Birds Over An Elevational Gradient Purcell 

Figure 1—Locations of study 

sites in the Sierra National Forest, 

California: yellow = ponderosa 

pine (elevations 1,04 1,372 m); 

green = mixed conifer (elevations 

1,701–2,012 m); red = true fir 

(elevations 2,170–2,347 m); and 

purple = lodgepole pine 

(elevations 2,469–2,774 m). 

elevations (2,469-2,774 m). Each forest type has four replicates, except mixed 

conifer, which has six replicates. Of the six mixed-conifer sites, three lie in each 

watershed of the Kings River Sustainable Forest Ecosystems Project area. All 

sites are protected from major disturbance for 10 years, including timber harvest, 

road construction, and major fuel breaks, after which they will be incorporated 

into the adaptive landscape treatment appropriate to their watershed. All sites 

consist of at least 60 ha of mature forest with relatively high canopy cover. A 40-

ha gridded plot has been established in each of the 60-ha sites to allow censusing 

and to facilitate mapping and relocation of nests. 

From 1995 to 1998, field crews censused birds on 8 (1995) or 16 (1996-1998) 

plots each year, using a timed transect method. Transects were 1000 m long and 

observers walked at a rate of 50 m per 3 min. Observers recorded all birds seen or 

heard <50 and >50 m from the transect line. Censusers were trained during a 2-

week period at the beginning of the field season, with additional training as they 

moved into higher forest types and encountered new species; each observer’s 

hearing was tested yearly. Each transect was counted six times during the 

breeding season, with two visits by each of three observers. The order of census 

visits and starting points of observers were randomly selected, with the constraint 

that visits were evenly divided between the two starting points. Censuses began 

at 07:00 PDT in all forest types except ponderosa pine, where they began at 07:30 

PDT to accommodate the shorter day lengths earlier in the season. All censuses 

were completed within 1.5 hr. In 1996 through 1998, field crews recorded the 

percent cover and depth of snow in a 1-m-radius circle around the census 

markers on each plot following each census. 

We searched for nests of all bird species and monitored nests every 3 to 4 

days, following the methods of Martin and Geupel (1993). Open nests were 

checked directly, where possible, or with a mirror on a pole or a small video 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 123 
Purcell 

Figure 2—Operational use of a 

fiberscope to check the contents of a 

white-headed woodpecker’s cavity 

nest. 

Figure 3—Climbing a large pine tree 

by using a stacked set of Swedish 

climbing ladders that are secured to 

the bole of the tree. 

Abundance and Productivity of Birds Over an Elevational Gradient 

camera mounted on an extendable fiberglass pole. Cavity nests were checked 

with a fiberscope (Purcell 1997) (fig. 2). When nests were too high to reach from 

the ground and nest substrates were sturdy, field crews climbed to nests by 

using a variety of climbing techniques (fig. 3). Prior to field operations, field 

assistants received training in tree climbing techniques and were certified to 

national Forest Service standards. 

Habitat measurements were taken at the nest site and at random sites on 

each plot each year. The variables recorded described the nest site and the 

habitat surrounding the nest. They included, but were not limited to: basal area 

124 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
Abundance and Productivity of Birds Over An Elevational Gradient 

of trees and snags; number and species of trees and snags by size class; canopy 

cover; litter depth; slope; aspect; length, diameter, and decay class of logs; and 

percent cover of grasses, forbs, rock, soil, litter, logs, shrubs, and trees in the 

understory. Variables specific to the nest included nest height; species, height, 

and diameter of the nesting substrate; variables describing placement of open 

nests; and dimensions and orientation of cavity nests. 

I estimated nest success and daily mortality rates based on Mayfield’s 

methods (1961, 1975), with variances calculated following Hensler and Nichols 

(1981). Differences among daily mortality rates were tested using program 

Contrast (Hines and Sauer 1989), as described by Sauer and Williams (1989). I 

used two-way analysis of variance to test differences in abundance across years 

and forest types. 

Results 

From 1995 through 1998, crews detected a total of 92 bird species and monitored 

1954 nests of 66 species. Thirty-five percent of the species recorded during 

censuses occurred in all four forest types. Eighteen percent, or 17 species, 

occurred in only one forest type; of these, 14 species only in the ponderosa pine 

forest type. Many of these 14 species are typical of drier, low-elevation habitats, 

such as ash-throated flycatcher (Myiarchus cinerascens), western scrub-jay 

(Aphelocoma californica), bushtit (Psaltriparus minimus), blue-gray gnatcatcher 

(Polioptila caerulea), Hutton’s vireo (Vireo huttoni), Bullock’s oriole (Icturus 

bullockii), and Lawrence’s goldfinch (Carduelis lawrencei). 

Species richness declined with increasing elevation (fig. 4a), and abundance 

followed the same pattern (fig. 4b). Abundance of the cavity-nesting guild, however, 

A 

B 

Purcell 

Figure 4—Yearly species richness 

(number of species detected at 

unlimited distance) and relative 

abundance (total count per plots, 

detected within 50 m.) in four 

forest types over an elevational 

gradient (open bars = ponderosa 

pine, stipled = mixed conifer, 

cross-hatched = true fir, solid = 

lodgepole pine). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 125 
Purcell 

Figure 5—Relative abundance 

(total count per plot) of cavitynesting birds in four forest types 

over an elevational gradient (PP = 

ponderosa pine, MC = mixed 

conifer, TF = true fir, LP = lodgepole 

pine). HAWO = hairy woodpecker, 

NOFL = northern flicker, PIWO = 

pileated woodpecker, RBSA = redbreasted sapsucker, WHWO = 

white-headed woodpecker, WISA = 

Williamson’s sapsucker, BRCR = 

brown creeper, MOCH = mountain 

chickadee, RBNU = red-breasted 

nuthatch. 

Abundance and Productivity of Birds Over an Elevational Gradient 

showed the reverse pattern, with all but two species attaining their highest 

abundance in true fir or lodgepole pine sites (fig. 5). The two exceptions were 

northern flickers (Colaptes auratus), which decreased in abundance with increasing 

elevation, and pileated woodpeckers (Dryocopus pileatus), which reached their 

maximum abundance in mixed-conifer sites (fig. 5). Abundance of open nesters 

showed no clear trend, as might be expected in such a diverse group. 

Abundance of dark-eyed juncos (Junco hyemalis) was variable across years 

(Type III sums of squares, F3,40 = 9.04, P = 0.0001) and across habitats (Type III 

sums of squares, F3,40 = 6.40, P = 0.0012). Abundance was low in 1997 and 1998, 

and in mixed-conifer and true fir sites (fig. 6a). Nest success did not differ 

significantly across years (P = 0.17), although variability was high. Nest success 

increased with increasing elevation (P = 0.02), with the highest nest success in 

lodgepole pine forests (P = 0.0045; fig. 6b). 

The abundance of dusky flycatchers (Empidonax oberholseri) was lower in 

1997 and 1998 than in other years (Type III sums of squares, F3,30 = 9.28, P = 

0.0002; fig. 7) and lowest in true fir habitat (Type III sums of squares, F3,30 = 9.53, P 

= 0.0006; fig. 7). Nest success, on the other hand, was highest in true fir habitat (P 

= 0.0359; fig. 7), yielding a negative, although nonsignificant, relationship 

between abundance and nest success (r = -0.41, P = 0.19; fig. 8). 

Clutch size of dusky flycatchers did not differ across forest types (fig. 9), but 

the number of young fledged per attempt was highest in true fir (fig. 9). After 

removing small sample sizes for year/forest type combinations (n < 8), the 

correlation between abundance and clutch size was nonsignificant (r = -0.57, n = 

6, P = 0.24; fig. 10), but abundance and number fledged per attempt was 

significantly negatively correlated ( r = -0.78, n = 8, P = 0.02; fig. 10). 

126 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 

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Abundance and Productivity of Birds Over An Elevational Gradient Purcell 

Figure 6—Relative abundance (total 

count per plot) and nest success of 

dark-eyed juncos by year and forest 

type (PP = ponderosa pine, MC = 

mixed conifer, TF = true fir, LP = 

lodgepole pine), from low to high 

elevation. Sample sizes for nest 

success are shown above columns. 

Figure 7—Dusky flycatcher 

abundance (cross hatched columns) 

and nest success (solid columns) by 

forest type (PP = ponderosa pine, MC 

= mixed conifer, TF = true fir, LP = 

lodgepole pine) from low to high 

elevation. Sample sizes for nest 

success are shown above columns. 

Figure 8—Relative abundance (total 

count per plot) vs. nest success of 

dusky flycatchers. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 127 
Purcell 

Figure 9—Clutch size (stippled 

columns) and number of young 

fledged per nesting attempt (solid 

columns) by dusky flycatchers by 

forest type (PP = ponderosa pine, MC 

= mixed conifer, TF = true fir, LP = 

lodgepole pine) from low to high 

elevation. Sample sizes are shown 

above columns. 

Figure 10—Relative abundance 

(total count per plot) vs. clutch size 

(open circles) and number of young 

fledged per attempt (closed circles) 

for dusky flycatchers. Small sample 

sizes (n < 8) for year/forest type 

combinations were deleted. 

Figure 11—Snow depth and snow 

water content recorded 1 April each 

year at Courtright Reservoir 

(elevation 2,570 m). 

128 

Abundance and Productivity of Birds Over an Elevational Gradient 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
Abundance and Productivity of Birds Over An Elevational Gradient 

The 1995 and 1998 breeding seasons followed severe winters, with snow 

remaining on the ground until well into the breeding season. Snow-depth data 

from Courtright Reservoir (elevation 2570 m) showed high snow depths on 1 

April of 1995 and 1998, compared to 1996 and 1997 (fig. 11). Field personnel did 

not record snow data in 1995, but 38-46 percent of the census markers had snow 

during the census period in mixed-conifer, true fir, and lodgepole pine forest 

types in 1998. No snow was recorded at the census markers in 1996 or 1997. 

We expected to observe downslope shifts in populations of some species in 

years with heavy precipitation. Looking only at presence/absence data and 

examining species that either disappeared from habitats where they had 

occurred or appeared in habitats where they were absent in 1996 and 1997, I 

found 15 downslope shifts of species in 1995 and 1998 compared with six 

Table 1—Downslope and upslope shifts of species of species in the breeding season after the severe winters of 1994-95 

and 1997-98, based on presence/absence data and disappearance of species from forest types where they had occured in 

the previous year, or their appearance in forest types where they were absent in the previous year 

Purcell 

Forest type Bird species Year Direction of

shift

Ponderosa pine 

Mixed-conifer 

True fir 

Lodgepole pine 

Ash-throated flycatcher (Myiarchus cinerascens)


Western scrub-jay (Aphelocoma californica)


Winter wren (Troglodytes aedon)


Bewick's wren (Thryomanes bewickii)


Song sparrow (Melospiza melodia)


Fox sparrow (Passerella iliaca)


Cassin's finch (Carpodacus cassinii)


Mourning dove (Zenaida macroura)


Wrentit (Chamaea fasciata)


Brown-headed cowbird (Molothrus ater)


Brown-headed cowbird (Molothrus ater)


Williamson's sapsucker (Sphyrapicus thyroideus)


Pacific-slope flycatcher (Empidonax difficilis)


Ruby-crowned kinglet (Regulus calendula)


Lincoln's sparrow (Melospiza lincolnii)


White-headed woodpecker (Picoides albolarvatus)


Western wood-pewee (Contopus sordidulus)


Golden-crowned kinglet (Regulus satrapa)


Hermit warbler (Dendroica occidentalis)


White-crowned sparrow (Zonotrichia leucophrys)


Purple finch (Carpodacus purpureus)


1998 down 

1995 down 

1995 up 

1998 up 

1998 up 

1998 down 

1995, 1998 up 

1995, 1998 down 

1995, 1998 down 

1998 down 

1995, 1998 down 

1995, 1998 down 

1995, 1998 up 

1998 down 

1995 down 

1998 down 

1998 down 

1995 down 

1995 up 

1998 down 

1995, 1998 down 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 129 
Purcell Abundance and Productivity of Birds Over an Elevational Gradient 

upslope shifts (table 1). Abundance of dusky flycatchers in ponderosa pine 

habitat in 1995 and 1998 was more than twice that in 1996 and 1997. The spotted 

towhee (Pipilo maculatus), a species found only in ponderosa pine and mixedconifer types, was only half as abundant in the mixed-conifer type in 1995 and 

1998 as it was in 1996 and 1997. Dusky flycatchers nested in ponderosa pine 

habitat for the first time in 1998, and one nest of the mountain chickadee (Poecile 

gambeli) was found in a ponderosa pine site in both 1995 and 1998. 

Discussion 

A consensus has emerged in recent years that the appropriate focus for wildlife 

issues by the Forest Service is on maintaining native biodiversity. Conservation 

of biological diversity depends on identification and preservation of habitat 

conditions that sustain healthy populations of coexisting species. Results 

obtained from this research are important to resource specialists and agency 

biologists in managing for and maintaining biodiversity. Identification of source 

and sink habitats and understanding their dynamics are crucial for maintaining 

healthy populations of these species. 

Among the four forest types under study, ponderosa pine provides habitat 

for the most species and the most individuals. Many of the species found there 

are more typically found at lower elevations in vegetation types that are not 

well represented on public lands. 

Abundance and nest success have varied significantly across years and 

forest types, underlining the need for long-term studies. Some of the variability 

in abundance and nest success appeared to be related to winter weather 

conditions preceding the breeding season. The lower nest success at lower 

elevations found in some species might be compensated by the longer breeding 

seasons at lower elevations and the possibility of multiple breeding attempts. 

A lack of correlation between abundance and nest success suggests that 

individuals are not able to judge the probability of nesting successfully in a 

particular habitat. A negative relation suggests inappropriate choices. This 

pattern is consistent with density-dependent predation, where, at high densities, 

predators can specialize on the nests of a particular species, resulting in low nest 

success. At low densities, nest predators cannot specialize on the more rare 

nests, leading to low rates of nest predation (Major and others 1994; Martin 

1988a, 1988b, 1996). If density-dependent predation is acting, dusky flycatchers 

cannot make appropriate choices regarding nesting habitat, as they will 

experience low nest success wherever they settle in high densities. Social 

interference might also cause low nest success at high densities (Jones and 

Leopold 1967, Tompa 1964), if adults are forced to spend more time away from 

their nests defending territories and mates, leaving the nest undefended and 

exposed to predation. Poor post-fledging survival in true fir sites could also 

account for a lack of recruitment in high-quality habitat, although no a priori 

reason exists to expect lower survival in fir than in other habitats. 

Although I did not report analyses of habitat data here, the accumulating 

vegetation and nest-site data will be important for describing habitat 

characteristics of nest sites used by birds, it will allow identification of variables 

important to successful nesting, and it will allow prediction of the effects of 

forest management practices on forest birds. 

After 10 years, when these sites are to be managed according to the 

silvicultural prescription appropriate to each of the two watersheds, the efficacy 

of these approaches for maintaining biodiversity will be assessed. The hope is 

that, by maintaining a mosaic of forest stand structures using uneven-aged 

management practices and reintroducing fire into the ecosystem, all species will 

be retained. “To keep every cog and wheel is the first precaution of intelligent 

tinkering” (Leopold 1966). 

130 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
Abundance and Productivity of Birds Over An Elevational Gradient 

Acknowledgments 

Many field assistants participated in the various phases of this study: Karen E. 

Bagne, Kenneth K. Bush, Carol L. Campbell, Christopher S. Caris, Douglas J. 

Cubanski, Scott W. Deal, Douglas A. Drynan, Bryan A. Gibson, James R. Jacobson, 

John P. Lindsey, Kevin M. Mazzocco, Rolf B. Miller, Bonnie M. Nielsen, Levin T. 

Nason, Rodney G. Olsen, Natasha A. Sherman, Rebecca A. Steffensen, Michael C. 

Seely, Susan K. Sutton-Mazzocco, Randy L. Swanberg, Elena M. Talamantez, 

Katherine A. Thumlert, Jennifer G. Turner, Kendrick C. Weeks, Mark E. 

Westehuebe, and Richard J. Young. Yvonne Cougoulat, Susan J. Sferra, and 

Jared Verner provided constructive suggestions on the draft manuscript. To all I 

extend my sincere appreciation. 

References 

Bock, Carl E.; Lynch, James F. 1970. Breeding bird populations of burned and unburned conifer 

forest in the Sierra Nevada. Condor 72: 182-189. 

DeSante, David F. 1990. The role of recruitment in the dynamics of a Sierran subalpine bird 

community. American Naturalist 136: 429-445. 

Hejl, Sallie J.; Verner, Jared; Balda, Russell P. 1988. Weather and bird populations in true fir forests 

of the Sierra Nevada, California. Condor 90: 561-574. 

Hejl, Sallie J.; Beedy, Edward C. 1986. Weather-induced variation in the abundance of birds. In: 

Verner, Jared; Morrison, Michael L.; Ralph, C. John, eds. Wildlife 2000: modeling habitat 

relationships of terrestrial vertebrates. Madison, WI: University of Wisconsin Press; 241-244. 

Hairston, Nelson G.; Smith, Frederick E.; Slobodkin, Lawrence B. 1960. Community structure, 

population control, and competition. American Naturalist 94: 421-25. 

Hensler, Gary L.; Nichols, James D. 1981. The Mayfield method of estimating nesting success: a 

model, estimators and simulation results. Wilson Bulletin 93: 42-53. 

Hines, James E.; Sauer, John R. 1989. Program contrast: a general program for the analysis of 

several survival or recovery rate estimates. Washington, DC: U.S. Department of the Interior. 

Fish and Wildlife Service. Fish and Wildlife Technical Report 24; 7 p. 

Hudson, Peter J.; Dobson, Andrew P. 1991. The direct and indirect effects of the caecal nematode 

Trichostrongylus tenuis on red grouse. In: Loye, Jenella E.; Zuk, Marlene, eds. Bird-Parasite 

Interactions: Ecology, Evolution, and Behaviour. Oxford, England: Oxford University Press; 49-68. 

Jones, R. E.; Leopold, A. S. 1967. Nesting interference in a dense population of wood ducks. 

Journal of Wildlife Management 31: 221-228. 

Leopold, Aldo. 1953. Round River. Oxford, England: Oxford University Press, Inc.; 173 p. 

MacArthur, Robert H. 1958. Population ecology of some warblers of northeastern coniferous 

forests. Ecology 39: 599-619. 

Major, R. E.; Pyke, G. H.; Christy, M. T.; Gowing, G., and Hill, R. S. 1994. Can nest predation explain 

the timing of the breeding season and the pattern of nest dispersion of New Holland 

honeyeaters? Oikos 69: 364-372. 

Martin, Thomas E. 1988a. On the advantage of being different: nest predation and the coexistence 

of bird species. Proceedings of the National Academy of Sciences, USA 85: 2196-2199. 

Martin, Thomas E. 1988b. Processes organizing open-nesting bird assemblages: competition or 

nest predation? Evolutionary Ecology 2: 37-50. 

Martin, Thomas E. 1996. Fitness costs of resource overlap among coexisting bird species. Nature 

380: 338-340. 

Martin, Thomas E.; Geupel, Geoffrey R. 1993. Nest-monitoring plots: methods for locating nests 

and monitoring success. Journal of Field Ornithology 64: 507-519. 

Mayfield, Harold F. 1961. Nesting success calculated from exposure. Wilson Bulletin 73: 255-261. 

Mayfield, Harold F. 1975. Suggestions for calculating nest success. Wilson Bulletin 87: 456-466. 

Price, Mary V. 1986. Structure of desert rodent communities: a critical review of questions and 

approaches. American Zoologist 26: 39-49. 

Purcell, Kathryn L. 1997. Use of a fiberscope for examining cavity nests. Journal of Field 

Ornithology 68: 283-286. 

Purcell, Kathryn L.; Verner, Jared. 1998. Density and reproductive success of California towhees. 

Conservation Biology 12: 442-450. 

Purcell 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 131 
Purcell Abundance and Productivity of Birds Over an Elevational Gradient 

Raphael, Martin G.; White, Marshall. 1984. Use of snags by cavity-nesting birds in the Sierra 

Nevada. Wildlife Monographs 86: 1-66. 

Ricklefs, Robert E. 1969. An analysis of nesting mortality in birds. Smithsonian Contributions to 

Zoology 9: 1-48. 

Sauer, John R.; Williams, Byron K. 1989. Generalized procedures for testing hypotheses about 

survival or recovery rates. Journal of Wildlife Management 53: 137-142. 

Tompa, Frank S. 1964. Factors determining the number of song sparrows, Melospiza melodia 

(Wilson), on Mandarte Island, B.C., Canada. Acta Zoologica Fennica 109: 3-73. 

Van Horne, Beatrice. 1983. Density as a misleading indicator of habitat quality. Journal of Wildlife 

Management 47: 893-901. 

Verner, Jared; Purcell, Kathryn L. 1999. Fluctuating populations of House Wrens and Bewick’s 

Wrens in foothills of the Western Sierra Nevada of California. Condor 101: 219-229. 

Wiens, John A. 1989. The ecology of bird communities, vol. 2. Cambridge: Cambridge University 

Press; 316 p. 

132 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002.