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

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. LAY SUMMARY Temperature is thought to be an important “supplemental cue” used by female birds to time breeding. It is widely assumed that late spring temperatures, within ∼1 month of egg laying, are most predictive of laying date, consistent with the idea that this fine-tunes timing of breeding to year-specific local conditions at the time of laying. Here, we show, in European Starlings, that a relatively broad temperature window (January 2 to April 4) best predicts laying date and that a long-term “mid-winter” temperature window (January 8 to February 22) was also strongly correlated with laying date. We found no evidence that short-term, immediate prelaying temperatures predict laying date. These results contrast markedly with the widely held view that immediate, prebreeding temperatures best predict variation in timing of laying. Birds must have mechanisms that allow integration of long-term temperature information even though these are currently poorly characterized. RESUMEN La temperatura, particularmente dentro de aproximadamente el primer mes antes de la puesta de huevos, se cree que es una señal importante a corto plazo que utilizan las aves hembra para ajustar el momento de la reproducción a las condiciones locales. Aquí mostramos que una ventana de temperatura relativamente amplia a largo plazo (del 2 de enero al 4 de abril, 92 días; r2 = 0.73) predijo mejor el momento de la puesta de huevos en Sturnus vulgaris. Una ventana de temperatura de “mediados del invierno” también estuvo fuertemente correlacionada con la fecha de puesta (r2 = 0.58), pero no encontramos apoyo para una influencia de las temperaturas a corto plazo inmediatamente antes de la puesta de huevos. Evaluamos la relación entre la temperatura ambiente y el momento de la puesta de huevos utilizando 3 enfoques complementarios: (1) un análisis exploratorio “sin restricciones”; (2) un enfoque de ventana deslizante tradicional; y (3) ventanas de temperatura específicas, basadas en información biológica. Nuestros resultados contrastan con la opinión ampliamente aceptada de que las temperaturas a corto plazo antes de la reproducción son las que mejor predicen la variación en la puesta porque permiten que las aves ajusten el momento de la reproducción a las condiciones locales alrededor del momento de la puesta de huevos. Esto significa que deben existir mecanismos que permitan la integración de la información de temperatura a largo plazo en las aves—quizás implicando de manera más parsimoniosa efectos indirectos de la temperatura en el crecimiento de las presas de insectos ectotérmicos de las aves—aunque estos mecanismos están actualmente mal caracterizados.