1. Birds use increasing photoperiod during spring as the major cue to time gonadal maturation and breeding so that nestlings are growing during peak food (normally invertebrate) abundance. Climate warming will advance invertebrate abundance, so birds relying entirely on photoperiod will breed too late. Can temperature modulate responses to photoperiod? 2. Common Starlings, Sturnus vulgaris, were kept in two indoor aviaries, in which photoperiod tracked natural changes, but temperature was held at either 20 degrees C or 5 degrees C (year 1), or at 18 degrees C or 8 degrees C (year 2). 3. Despite the large differences in temperature (15 degrees C and 10 degrees C), this had no effect on the rate or timing of testicular maturation. 4. However, unexpectedly, testicular regression occurred significantly earlier at the higher temperatures. Post-nuptial moult also started significantly earlier in both males and females. 5. Therefore, the degree to which birds can advance the timing of egg-laying in response to increasing spring temperature may be constrained. At the same time, increasing spring temperature will advance the end of the breeding season (fewer breeding attempts). The reported advances in median egg-laying dates may, in part, be a consequence of this rather than an indication of successful adaptation. Adaptation to climate change may require evolutionary changes in the physiological responses to photoperiod.
Dawson, A
The effect of temperature on photoperiodically regulated gonadal maturation, regression and moult in starlings - potential consequences of climate change
Functional Ecology