There are striking differences in life-history traits associated with reproduction, between an insular population of the blue-tit and one on the nearby mainland. In order to test the extent to which these differences are genetic, two samples of birds, one from Corsica and one from mainland southern France, were hand-raised. These birds bred subsequently in aviaries at Montpellier during three consecutive years, at the same date and in the same way as did the natural populations from which they came. It is concluded that the difference in laying-date between the two samples has a genetic basis. The adaptation to each particular set of environmental conditions is discussed.