Where you live matters: extreme weather conditions amplify territory quality effects on island bird reproduction

Abstract

Aspects of territory quality and environmental variation (e.g. rainfall, temperature) are key predictors of reproductive success in birds. However, the interaction between variation in territory quality and extreme weather events on reproduction has seldom been explored. The Island Scrub-jay (Aphelocoma insularis), a species endemic to Santa Cruz Island, California, is a highly territorial species where breeding pairs maintain year-round territories that vary in vegetation composition and structure. Using a 15-year dataset (2008-2023), we examined how physical metrics of territory quality and extreme weather conditions affect reproductive output. We found that high-quality territories are characterized by more chaparral/woodland habitat, while low-quality territories have a higher proportion of scrub habitat. In addition, during drought and abnormally hot years, pairs in lower-quality territories experienced significantly reduced reproductive output, often failing to fledge any young at all. Conversely, years of high rainfall and cooler temperatures had a uniformly positive effect, boosting reproductive output for all breeding individuals regardless of territory quality, likely due to increased resource availability. Our findings highlight the importance of how variance in territory quality interacts with drought and high temperatures to disproportionately reduce reproductive output within poor quality territories, but also how wet, cool years can buffer populations by facilitating reproductive success across all territories. As droughts and extreme heat are projected to become more frequent, severe, and prolonged under future climate change scenarios, the reproductive variance among breeding pairs risks skewing the effective population size and reducing the genetic diversity of this already vulnerable, range-restricted species. This study emphasizes the importance of quantifying variation in territory quality and protecting structurally diverse, high quality habitat types as they may play an outsized role in long-term persistence of territorial species amidst increasing climatic stress.