Climate's Impact on Bird Distributions in California

Carly Robbins

Abstract

This study examines the effect of Earth’s rising temperatures on the species distributions of Hirundo rustica (barn swallow), Melozone crissalis (California towhee), and Piranga ludoviciana (western tanager) in California between 1980 - 1997 and 1998 - 2017. We evaluated the change in bird distributions by applying spatial statistics to measure geographic distributions and to detect and analyze spatial clusters of bird counts3. In addition, we conducted a negative binomial regression to determine if monthly mean temperature4 could predict bird counts. Results indicated a northward shift in species distributions between the two time intervals for all species. A comparison of barn swallow and western tanager hot spot analysis maps revealed spatial clusters of high bird counts between 1998 - 2017 north of California, suggesting a change in environmental conditions that enabled these areas to support the two species. Negative binomial regression results showed that increases in temperature would lead to increases in both barn swallow and California towhee counts. Barn swallows were the species most sensitive to temperature changes, as temperature was a statistically significant predictor variable only for barn swallows. Inclusion of additional explanatory climate variables into the negative binomial regression models may improve results.