LB 4-36 - Climate change is concurrent with migratory shifts for Yellow-rumped Warblers arriving to the San Francisco Bay Area

Climate change in recent decades has led to dramatic declines in bird populations in the US and Canada. Short-distance migrants such as Yellow-rumped Warblers (Setophaga coronata, YRWA) are particularly vulnerable as climate change alters a substantial portion of their breeding and wintering ranges. Two subspecies of YRWA occupy the US: Audubon's Warblers (AUWA) and Myrtle Warblers (MYWA). Although generally occupying distinct northerly breeding ranges, the two subspecies intermix in parts of their wintering ranges, including California’s San Francisco Bay Area. The Bay Area thus represents an informative region in which to study subspecific migration trends of YRWAs in relation to changes in climate and food availability over time. We analyzed YRWA winter capture data from 1986 through 2021 from Coyote Creek Field Station (CCFS) in Milpitas, CA. Trends in winter arrival timing for both subspecies were investigated alongside weather data including: the timing of the first freezing period, monthly precipitation, and monthly minimum temperature. We also created generalized linear models using iNaturalist data of fruiting toyon (Heteromeles arbutifolia), pink honeysuckle (Lonicera hispidula), and blue elder (Sambucus cerulea) from 2015 through 2021 to compare peak fruiting week with YRWA winter arrival.We found that peak YRWA winter arrival to CCFS for both subspecies occurred in mid-October, with particular years seeing delayed winter movements. These years included 2014 through 2018 as peak winter arrival occurred in December. The latter half of our study period (2004 through 2021) also saw delayed migration for MYWAs compared to the first half; peak MYWA arrival occurred four weeks later from 2004 through 2021 than from 1986 through 2003. We detected a shift toward later freezing temperatures in much of the YRWA breeding range from 2004 through 2021, a potential explanation for our observed delays in peak winter movements. This timeframe also saw a dramatic decline in local YRWA abundance at CCFS, particularly for AUWAs. This decline co-occurred with increased precipitation from August to October in much of the western portion of the YRWA range. Lastly, we found a correlation between peak toyon fruiting and YRWA winter arrival. Our preliminary findings suggest that the effects of climate change such as later freezing have altered YRWA winter arrival to the Bay Area. Future work incorporating data from additional banding stations will further elucidate whether changes to YRWA migration or abundance patterns are general across their range.