Long-term, not short-term, temperatures predict timing of egg laying in European Starling

Abstract

Abstract Temperature, particularly within ~1 month of egg laying, is thought to be an important, short-term cue used by female birds to calibrate timing of breeding to local conditions. Here, we show that a relatively broad, long-term, temperature window (January 2 to April 4, 92 days; r2 = 0.73) best predicted timing of egg laying in European Starlings (Sturnus vulgaris). A “mid-winter” temperature window was also strongly correlated with laying date (r2 = 0.58), but we found no support for an influence of short-term temperatures immediately before egg laying. We assessed the relationship between ambient temperature and timing of egg laying using three complimentary approaches: (1) an “unconstrained,” exploratory analysis; (2) a traditional sliding window approach; and (3) specific, biologically informed temperature windows. Our results contrast with the widely held view that short-term, prebreeding temperatures best predict variation in laying because they allow birds to adjust timing of breeding to local conditions around the time of egg laying. This means that mechanisms that allow integration of long-term temperature information must exist in birds—perhaps most parsimoniously involving indirect effects of temperature on growth of the bird’s ectothermic insect prey—even though these are currently poorly characterized.