Selection for the Timing of Great Tit Breeding in Relation to Caterpillar Growth and Temperature

Abstract

1. We investigated the relation between the timing of great tit breeding, measured as the mean date of laying the first egg in each clutch, the timing of caterpillar availability, measured as median pupation dates of winter moth, selection for laying dates, measured from the recruitment into the local breeding population in subsequent years, and temperature in the period after egg laying. 2. There was a significant positive correlation between the timing of the great tit's laying and the timing of the caterpillars. 3. In most years there was selection for earlier laying in this great tit population. The selection differentials were usually the same for male and female recruits. 4. The selection differential for laying date was strongly correlated (r = 0.84, n = 21, P <0.0001) with the difference in timing between birds and caterpillars. This difference in timing was in turn strongly correlated with temperatures in the period after egg laying. 5. When the period after the laying date was subdivided, the selection differential was strongly correlated with the mean temperature during the period when most birds were incubating. 6. We discuss six alternative hypotheses explaining a consistent selection differential for earlier laying. One of these is new and is based on the fact that the birds can delay their breeding more succesfully than they can speed it up, once they have started laying. [KEYWORDS: Caterpillar pupation; insectivorous birds;natural selection; timing of breeding; wytham wood Parus-major; clutch size; incubation; heritability;reproduction]