Birds advancing lay dates with warming springs face greater risk of chick mortality

Significance The advancement of breeding behavior is a well-documented response to changing climate conditions, as timing reproduction with resource availability is critical for success in many species. However, the relationship between cues that prompt reproduction and resource availability can become decoupled, reducing success. Tree swallows have advanced reproduction in response to warming springs, but now chicks are exposed to twice the risk of inclement weather conditions, resulting in high rates of chick mortality. Mass mortality events appear to be driven by decreasing insect availability at low daytime temperatures. Our findings highlight the complex effects of climate change on animal life cycles and demonstrate the urgency of understanding how animals balance information from the environment when making crucial life history decisions.