DEVELOPMENTAL STABILITY AND IDEAL DESPOTIC DISTRIBUTION OF BLACKBIRDS IN A PATCHY ENVIRONMENT

Previous studies of the distribution of breeding blackbirds Turdus merula in a patchy environment of tree lots in a matrix of farmland have shown that nestling mortality due to nest predation and starvation decreased with increasing habitat patch size. The net annual reproductive success increased with increasing patch size and accordingly a larger fraction of yearling blackbirds appeared to be forced to settle in small habitat patches. In this study I independently assess habitat quality as perceived by the blackbirds using a measure of developmental stability as a criterion. Fluctuating asymmetry in wing length and tail length of blackbird nestlings and fledglings decreased with increasing patch size. Nestlings with high levels of asymmetry also had high levels of asymmetry as independent fledglings, and local post-fledging survival probability was inversely related to the level of fluctuating asymmetry. Adult blackbirds breeding in small habitat patches did not have higher asymmetry than those breeding in large patches, and this observation suggests that natural selection had reduced the level of asymmetry among adults. Developmental stability is concluded to be a potentially useful independent measure of performance in studies of conservation biology and landscape ecology.