DIFFERENCES IN THE ONSET OF LAYING BETWEEN ISLAND AND MAINLAND MEDITERRANEAN BLUE TITS PARUS-CAERULEUS - PHENOTYPIC PLASTICITY OR GENETIC-DIFFERENCES

Blue tits Parus caeruleus breeding in deciduous or in evergreen woodla nds on the mainland of Europe start to lay 3-4 weeks earlier than Blue Tits on the island of Corsica breeding in evergreen woodlands. A simi lar difference in average laying date between the two Blue Tit populat ions has been found in a relatively small sample of captive birds held in outdoor aviaries on the mainland. To provide data on the habitat c ontrast in Corsica for comparison with that on the mainland, a new stu dy area was started in one of the few deciduous woodlands on Corsica. In addition, new data on laying dates of captive Blue Tits were gather ed to increase the sample size of independent pairs in aviaries. This study shows that the habitat effect (deciduous v evergreen woodland) o n the average laying date of Blue Tits is relatively small in comparis on with the geography effect (European mainland v Corsica). The laying date differences between mainland and insular Blue Tits persisted wha tever the type of habitat (deciduous, evergreen or outdoor aviaries on the mainland). The data support the hypothesis that the observed diff erences in the onset of laying between the mainland and insular Blue T its were mainly influenced by genetic effects and relatively little by non-genetic maternal effects of by genotype-environment interactions.