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Pacific Southwest Research Station
800 Buchanan Street
Albany, CA 94710-0011
Research Topics Ecosystem Processes
About this Research:
Sierra Nevada Ecosystems
Abundance, distribution, and population trends of oak woodland birds
Oak 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.
Monitor population trends of bird species using oak woodlands
Methods and Design
The 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.
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.
We 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.
1) Purcell, K.L.; 1) Verner, J.; 1) Drynan, D.A.; 2) Mori, S.R.; 3) Olsen, R.G.; 3) Wible, V.P.
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.
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