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 increase in ambient temperature (Ta) (short-term effect). We investigated the role of Ta 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 southern Germany was highly correlated with Tmin from March. In captivity under ad libitum feeding conditions, low Ta during February retarded reproductive development and behavior, thereby delaying the onset of breeding. Egg-formation occurred during a period with elevated temperatures. By increasing or decreasing the temperature of the nestbox by 2-3°C, from late March onwards, differences were found in the number of breeding females and the start of egg-laying between groups with heated, unmanipulated, and cooled nestboxes. Most females with cooled nestboxes started egg-formation in the week after the cooling stopped. Experiments under temperature controlled conditions in captivity showed that almost all starlings delayed egg-laying during a period when Ta was kept at 7°C, then started laying one week after a 5°C increase in Ta. The sensitivity of the reproductive system of starlings to spring temperatures seems to be an adaptive response, timing the period for laying (short-term effect) and for raising the young (long-term effect) to periods with high food availability.