Warm winters lead to early breeding, increased breeding effort, but lower reproductive success, in a threatened habitat specialist

Climate warming is affecting the phenology and life-history of animals around the world. In birds, although warm winters have been shown to advance breeding dates and affect reproductive success in a diverse range of taxa, few studies document whether changes in breeding phenology are associated with variability in reproductive effort by the adults. We leverage a 40-year dataset on the demography of Florida Scrub-Jays (Aphelocoma coerulescens, FSJ) to investigate not only whether winter weather affects FSJ phenology and reproductive success, but also whether FSJs invest more effort in reproduction following warm winters. We found that FSJs bred early but fledged fewer offspring in springs following warm winters. This reduced reproductive success came at the expense of increased reproductive effort (N nests built, N eggs laid, and length of the breeding period across all attempts) by the breeders. Given the well-known tradeoff between reproductive effort and survival, we highlight an important but typically unrecognized cost of climate warming on birds, in a large, well-protected population of a threatened habitat specialist, as revealed by a long-term study.