USDA Forest Service

Pacific Southwest Research Station


Pacific Southwest Research Station
800 Buchanan Street
West Annex Building
Albany, CA 94710-0011

(510) 559-6300

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Research Topics Ecosystem Processes

^ Main Topic | Tropical Ecosystems | Sierra Nevada Ecosystems


Abundance, distribution, and population trends of oak woodland birds.

Study Plan
Research Project Summary

The Research

View of oak woodlands at the San Joaquin Experimental RangeOak woodlands support the richest wildlife community of any habitat in California, with over 330 species of birds depending on them at some state in their life cycle, yet little avian research has been done in this habitat. Oak woodlands have been severely impacted by humans. Only two-thirds of California's original oak woodland habitat remains and much of that has been altered and/or degraded. Since Anglo-American settlement, oak woodlands have been impacted by clearing for fuelwood, agriculture and range, livestock grazing, mining activities, and altered fire regimes. More recently, increasing urban development and agricultural encroachment have fragmented and decreased the extent of oak woodlands. The effects of these changes on bird species are poorly understood.

Photo of Oak titmouseLong-term studies are necessary to detect even fairly large changes in abundance of wildlife species. We have collected point count data at 210 counting stations throughout the San Joaquin Experimental Range since 1985. We are currently analyzing population trends for species breeding at SJER. We will also assess the importance of weather variables, including precipitation and temperature, on annual variability in bird numbers. Due to the high annual variability in counts of breeding species that we have observed, we have also examined the number of years needed to establish a useful baseline period. Our results suggest that 11-22 years of count data are needed to obtain baseline data for 34 breeding species in our data set. Uncommon or rare species require longer periods of time.

We are using this dataset to investigate issues of statistical power to detect population trends and response to treatment effects with different effect sizes, and to make recommendations regarding the number of counting stations and the number of years needed.

Data on nest success and nest site selection have also been collected for most breeding species. Ongoing studies are investigating the breeding and foraging ecology of European starlings (Sturnus vulgaris) at SJER. Starlings are an exotic species that have increased markedly in the foothill woodlands. Starlings are known to compete for and usurp nest sites from other cavity-nesting birds, whose nest sites are commonly considered to be limiting. Other than habitat loss, starlings may be the biggest threat to cavity-nesting birds in oak woodlands in California.

Other ongoing analyses include examination of arrival dates and patterns of abundance of migrant bird species and the response of the bird community to historical fire, prescribed fire, and fire suppression.


blue arrrow Monitor population trends of bird species using oak woodlands
blue arrrowExplore patterns and possible underlying causes for population trends, including climate change.
blue arrrowMake recommendations for improvements in the protocols for monitoring landbirds, including the number of years needed to detect a given change with adequate statistical power.
blue arrrowExamine factors affecting the abundance of European starlings and the effects of starlings on native cavity-nesting birds.

Methods and Design

Photo of point count stationThe methods described here follow guidelines suggested in Verner (1985) and Verner and Milne (1989). A total of 210 counting stations (7 lines with 30 counting stations each) have been established throughout SJER. Counting stations are at least 200 m apart along the same line and between the separate lines. Although this is closer spacing than ideal for independent samples, it is a reasonable separation for most species and our intent here is to obtain only an index of relative abundance for comparison across years. By following the same protocol each year, we believe any potential biases resulting from a lack of independence in the counts are consistent from year to year. All counting stations are clearly identified by placement of yellow cattle ear tags wired to trees, shrubs, fences, and occasionally to steel fence posts, providing consistency in location across years, and numerous red tags placed between stations help guide observers from point to point along the lines. With this system, observers unfamiliar with the lines can follow them quickly and locate the counting stations.

Four observers complete unlimited distance point counts during the peak breeding period of most species that nest at SJER, from the last week of March through the end of April. Observers are randomly assigned to lines (lattice design) and each observer will eventually sample all seven lines on different mornings, completing one count at each of the 210 stations. Recording of birds at counting station on a line will begin at 10 minutes after official sunrise and will continue at successive stations at 10-min intervals. By adhering to this schedule, an observer will record birds at six stations per hour, with all 30 stations on a line sampled within 5 hours, thus controlling for time-of-day variability (Verner and Ritter 1986). Counts will not be done during rainy mornings or when wind velocity consistently exceeds 32 km/hr (Beaufort scale).

Data are recorded on pre-designed data forms that list all common and uncommon species, and most rare species. This method eliminates problems with illegible writing and aberrant four letter bird codes. It also provides a guide to likely species that can prompt observers to be sure that they record all that they detect.

Data on precipitation and temperature are collected from a weather station maintained at SJER since 1934. We will also examine the effects of the El Niño Southern Oscillation (ENSO) using annual mean monthly values of the standardized Southern Oscillation Index to represent ENSO conditions for each calendar year (

Vegetation measurements were collected at each of the 210 counting stations in 2001 to examine habitat relations of oak woodland bird species and are now available for inclusion in analys. Variables include percent cover of the primary tree species, shrubs, rock, and the presence of standing or flowing water during the count period.

Statistical Analyses:

The metric for abundance will be the total number of each species at each point summed over observer and route each year. We will use nonparametric Poisson regression models as a subclass of generalized additive models (GAM) as a first (exploratory) statistical approach to examine relations between species' abundance and the independent variables. This approach will be used due to lack of prior knowledge of the functional shapes of relations between abundance and explanatory variables. Smoothing functions (LOESS) of annual or other temperature functions, annual or other precipitation functions, sampling day (Julian day), and location are tried as explanatory variables to best fit the data. Linearity is not assumed; instead the data will be used to visualize the simultaneous relations between explanatory variables and abundance. Analysis of deviance and F-tests will be used to select the explanatory variables to be included in the model. Parametric functional shapes for the explanatory variables will be obtained by trying polynomial, logarithmic, or other parametric functions suggested by the output of GAM. For the parametric approach, the overdispersed term will be estimated to account for between-observer variability. The Poisson regression model as a subclass of the generalized linear models will be used for parametric analysis of trends. S-Plus will be used for fitting GAM and the GENMOD procedure of SAS (2000) will be used for the generalized linear models.

Application of Research Results

Oak woodlands support the richest wildlife community of any habitat in California, with over 330 species of birds depending on them at some stage in their life cycle. In addition to resident and migrant breeding birds, oak woodlands provide wintering and migratory stopover habitat. Despite their importance to wildlife, oak woodlands have been severely impacted by humans. Only two-thirds of California's original oak woodland habitat remains and much of that has been altered and/or degraded. Since European settlement of California, foothill oak-woodlands have been managed primarily for livestock production, while urban or suburban development, agricultural encroachment, especially vineyards and orchards, and firewood harvesting are more recent threats. The effects of these changes on bird species are poorly understood.

These data will be useful in exploring possible reasons underlying the variability in annual counts of species. SJER has long term weather data, and data on precipitation and temperature are likely to be important factors for many species. Although we cannot “manage” for weather conditions, identification of significant weather variables and critical seasons is of interest to predict species’ vulnerability to extreme conditions such as El Niño Southern Oscillation events and global warming. After accounting for variability in bird numbers due to observer bias and variation in weather variables, long-term population trends will be examined. Patterns such as concurrent trends in foraging and nesting guilds, migratory status, and vulnerability to interspecific competition may provide information on the causes underlying these changes.

Our results suggest that a minimum of 15 years of point count data are needed to detect trends in bird populations with confidence. Twenty years are needed for many study designs (for example those with few count stations or few visits per season) and will allow analysis of a greater number of species. Our results further point out that species with low abundance or high variability cannot be effectively monitored using point counts in most cases, even with large sample sizes.

Starlings were first documented at SJER in the late 1960s and by 1970 SJER was home to several nesting pairs. Point count data show that numbers of European Starlings have increased at SJER since 1985. This increase in abundance may be related to grass height and grazing intensity, as short grasses and forbs may allow starlings to forage more easily on the ground. Consequently, the successful invasion of starlings into foothill oak pine woodlands may negatively impact cavity nesters, and monitoring of these species is needed. Many species of secondary cavity nesters breeding in oak woodlands are decreasing and/or experiencing local extirpations which maybe related to interspecific competition with starlings for nest sites.

Western bluebird femaleWe have found that starlings use nest cavities similar in size and shape to those used by native species, especially Acorn Woodpeckers (Melanerpes formicivorus), Western Bluebirds (Sialia mexicana), Violet-green Swallows (Tachycineta thalassina), and to a lesser extent White-breasted Nuthatches (Sitta carolinensis), raising questions about nest site availability for these species with the increase in abundance of starlings. Behavioral studies show that these species recognize starlings as potential aggressors.


map of the Sierra Nevada Ecoregion
The San Joaquin Experimental Range (SJER), Madera County, CA, in the western foothills of the Sierra Nevada.

Lead Scientists/Collaborators

1) Purcell, K.L.; 1) Verner, J.; 1) Drynan, D.A.; 2) Mori, S.R.; 3) Olsen, R.G.; 3) Wible, V.P.

1) USDA Forest Service,
Pacific Southwest Research Station
Sierra Nevada Research Center
2081 E. Sierra Ave
Fresno, CA 93710

2) USDA Forest Service,
PSW Research Station, Statistics Group
800 Buchanan Street
Albany, CA 94710

3) California State University
Department of Biology
Fresno, CA

Publications and Reports

Purcell, K.L., S.R. Mori, and M.K. Chase. 2005. Design considerations for monitoring avian abundance using point counts: Examples from California oak woodlands. Condor 107:305-320.

Purcell, K.L., and S.L. Stephens. 2005. Changing fire regimes and the avifauna of California's oak woodlands. Studies in Avian Biology No. 30. Fire and Avian Ecology in North America.

Purcell, K.L., and S.L. Stephens. In press. Natural and anthropogenic fire regimes, vegetation effects, and potential impacts on the avifauna of California oak woodlands. Proceedings from the Third International Partners In Flight Conference: A Workshop on Bird Conservation Implementation and Integration. USDA Forest Service Gen. Tech. Rep. PSW-GTR-191. Albany, CA.

Tietje, W., K. Purcell, S. Drill. In press. Oak woodlands as wildlife habitat. Chapter 3 in A Planner's Guide for Oak Woodlands, Second Edition. University of California, Division of Agriculture and Natural Resources Publication no. xx.

R.B. Standiford, D.D. McCreary, and K.L. Purcell, technical coordinators. 2002. Oaks in California's Changing Landscape. USDA Forest Service Gen. Tech. Rep. PSW-GTR-184. Albany, CA.

Kunzman, M.R., K. Ellison, K.L. Purcell, R.R. Johnson, and L.T. Haight. 2002. California Towhee (Pipilo crissalis). In A. Poole and F. Gill, editors. The Birds of North America. No. 632. The Academy of Natural Sciences, Philadelphia, PA, and The American Ornithologists' Union, Washington D.C.

Purcell, K.L., J. Verner, and S. R. Mori. 2002. Factors affecting the abundance and distribution of European Starlings (Sturnus vulgaris) at the San Joaquin Experimental Range in R.B. Standiford, D.D. McCreary, and K.L. Purcell, technical coordinators, Oaks in California's Changing Landscape. USDA Forest Service Gen. Tech. Rep. PSW-GTR-184.

Olsen, R.G. 2001. Nest-site competition between the European starling and native cavity-nesting birds. M.S. Thesis, California State University, Fresno.

Purcell, K.L. and J. Verner. 1999. Nest predators of open and cavity nesting birds in oak woodlands. Wilson Bulletin 111:251-256.

Verner, J. and K. L. Purcell. 1999. Fluctuating populations of House Wrens and Bewick's Wrens in foothills of the Western Sierra Nevada of California. Condor 101:219-229.

Purcell, K.L. and J. Verner. 1998. Density and reproductive success of California Towhees. Conservation Biology 12:442-450.

Purcell, K.L., J. Verner, and L.W. Oring. 1997. A comparison of the breeding ecology of birds nesting in boxes and tree cavities. Auk 114 (4):646-656.

Verner, J., K. L. Purcell, and J. G. Turner. 1997. Bird communities in grazed and ungrazed oak-pine woodlands at the San Joaquin Experimental Range. Pp. 381-390 in N. H. Pillsbury, J. Verner, and W. D. Tietje, technical coordinators, Proceedings of a Symposium on Oak Woodlands: Ecology, Management, and Urban Interface Issues, 19-22 March 1996, San Luis Obispo, CA. USDA Forest Service Gen. Tech. Rep. PSW-GTR-160.

Verner, J., K. L. Purcell, and J. G. Turner. 1996. Monitoring trends in bird populations: Addressing background levels of annual variability in counts. Transactions of the Western Section of the wildlife Society 32:1-7.

Last Modified: Apr 19, 2011 08:03:30 PM