Avian population trends based on 27 years of data from California oak woodlands

Abstract

With 27 years of data (1986–2012), we examined long-term patterns of variation in abundance and population trends for 35 species of oak woodland birds. Following a standardized protocol, from three to eight observers counted birds annually at 210 count stations at the San Joaquin Experimental Range, Madera County, California. We used both linear regression and nonparametric regression (spline-smoothing the year effect) to evaluate population trends. The abundance of most species varied greatly over the study period, emphasizing the importance of long-term monitoring. Overall, 12 species increased: the California Quail (Callipepla californica), Anna’s Hummingbird (Calypte anna), Red-tailed Hawk (Buteo jamaicensis), Acorn Woodpecker (Melanerpes formicivorus), Nuttall’s Woodpecker (Picoides nuttallii), American Kestrel (Falco sparverius), Common Raven (Corvus corax), Violet-green Swallow (Tachycineta thalassina), White-breasted Nuthatch (Sitta carolinensis), Bewick’s Wren (Thryomanes bewickii), European Starling (Sturnus vulgaris), and House Finch (Haemorhous mexicanus). Five species decreased: the California Scrub-Jay (Aphelocoma californica), Blue-gray Gnatcatcher (Polioptila caerulea), Western Bluebird (Sialia mexicana), Western Meadowlark (Sturnella neglecta), and Bullock’s Oriole (Icterus bullockii). Of these, the Blue-gray Gnatcatcher, Western Meadowlark, and Bullock’s Oriole present cause for concern. We recommend further research to elucidate the causes for the declines of these three species. We also recommend continued monitoring of the California Scrub-Jay and Western Bluebird to verify recovery to previous levels of abundance. Our results emphasize the need for long-term monitoring of avian populations, which will provide insights into population dynamics and may reveal negative trends that may require conservation action.