Long-term bird population trends at a San Francisco Bay Area preserve

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Julien M. W. Ueda, Rodolfo Dirzo and Tyler McFadden, Department of Biology, Stanford University, Stanford, CA

Background/Question/Methods
Birds provide numerous ecosystem services and intrinsic value as a part of Earth’s biodiversity, yet human impacts, such as land use and climate change, have altered and depleted bird communities worldwide. Despite recent studies describing continent wide bird population trends, most long-term studies are limited to the breeding season and thus provide insufficient data for accurately assessing changes in bird populations across the annual cycle. The Jasper Ridge Biological Preserve in Northern California contains many habitats from oak woodland to serpentine grassland that support a diversity of bird species, including thousands of migratory and breeding birds, making the Preserve an ideal study system. Here, we ask: what are the species’ trends locally, how are bird group populations changing, and how do these trends at the Preserve compare to state- and nation-wide trends? We analyzed the results of 2025 monthly bird transect surveys conducted between 1989 and 2020 in six different sectors of the Preserve. Further, we modeled the abundances of 66 species using Bayesian generalized linear mixed models while statistically controlling for temporal autocorrelation, sector of the Preserve, and observer bias. We then compared these local trends to state- and nation-wide trends reported by the North American Breeding Bird Survey.
Results/Conclusions
Five bird species presented significant population increases while 21 demonstrated significant decreases. Among bird groups, herons, shrub-nesting birds, ground foragers, and urban birds, like starlings and mockingbirds, experienced the strongest local declines. Ten local trends differed from the nationwide or statewide results, including the locally declining California Quail and the locally increasing Chestnut-backed Chickadee. Our results demonstrate how local bird populations are changing with and against nationwide populations, underscoring the complexity of anthropogenic biodiversity change. Furthermore, our detection of many declining species within a strictly managed preserve supports the need for broader conservation efforts that transcend protected areas.