Temperature and timing of egg-laying of European Starlings

Many small passerine species breed earlier after a warm rather than a cold spring (long-term effect), and then start egg-formation after a clear incre ase in ambient temperature (T-a) (short-term effect). We investigated the r ole of T-a on timing of breeding and the exact time of egg-laying in both a free-living and captive breeding population of European Starlings Sturnus vulgaris. The start of the breeding season of free-living starlings in sout hern Germany was highly correlated with T-min from March. In captivity unde r ad libitum feeding conditions, low T-a during February retarded reproduct ive development and behavior. thereby delaying the onset of breeding. Egg-f ormation occurred during a period with elevated temperatures. By increasing or decreasing the temperature of the nestbox by 2-3 degrees C. from late M arch onwards, differences were found in the number of breeding females and the start of egg-laying between groups with heated, unmanipulated, and cool ed nestboxes. Most females with cooled nestboxes started egg-formation in t he week after the cooling stopped. Experiments under temperature controlled conditions in captivity showed that almost all starlings delayed egg-layin g during a period when T-a was kept at 7 degrees C, then started laying one week after a 5 degrees C increase in T-a. The sensitivity of the reproduct ive system of starlings to spring temperatures seems to be an adaptive resp onse, timing the period for laying (short-term effect) and for raising the young (long-term effect) to periods with high food availability.