Warm winters lead to increased reproductive effort, but lower reproductive success: Hidden costs of climate warming in a threatened bird

ABSTRACT 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 breeders. We leveraged a 37-year dataset on the demography of Aphelocoma coerulescens (Florida Scrub-Jay) to investigate not only whether winter weather affects scrub-jay phenology and reproductive success, but also whether winter weather affects overall reproductive effort. Scrub-jays bred early but fledged fewer offspring in springs following warm winters. Surprisingly, this reduced reproductive success came at the expense of increased reproductive effort (number of nests built, number of eggs laid, and length of the breeding period across all attempts) by the breeders. Given the well-known trade-off between reproductive effort and survival, we highlight an important but typically unrecognized cost of climate warming on birds, as revealed by a long-term study of a large, well-protected population.