Higher degree-days at the time of breeding predict size of second clutches in the barn swallow

Failure to adjust timing of biological events to ongoing rapid climate change may incur negative fitness consequences for organisms and may have severe negative consequences for population viability. We studied long-term trends in breeding phenology and reproductive success of a barn swallow Hirundo rustica population during 1971−2009 in relation to spring accumulated temperature (degree-days). Degree-days at the time of a phenological event constitute a more accurate and meaningful measure of the biological environment than mean temperatures over fixed time intervals, because accumulated temperature determines seasonal development, emergence and reproduction of many organisms, including the invertebrates on which species at higher trophic levels feed. Although laying dates showed an inverted U-shaped trend over time, degree-days at laying of first and second clutches increased linearly, suggesting that barn swallows are becoming increasingly ecologically mismatched relative to spring events. In addition, higher degree-days at laying were associated with smaller second clutches. These findings suggest that barn swallows are currently delaying reproduction relative to spring phenology, with negative consequences for their reproductive output. This delay may thus contribute to the population decline observed over the last decades. Degree-days at the time of reproduction may therefore provide a mechanistic link between phenology, breeding success and demographic trends of migratory bird populations.