Low fledging success of urban Great Tits

Abstract

Studies at various localities in Europe have shown that nest-box populations of tits in urban habitats produce fewer eggs and fledglings than do birds in rural habitats . The results of the present 7-year study on dense urban and moderately dense rural tit populations also show that the average clutch size was significantly lower in the urban than in the rural populations. The difference was 17% in the Great Tit Parus major and 10% in the Blue Tit P. caeruleus. The fledgling production of tits calculated per egg laid, per breeding attempt, or per successful nesting was lower in the urban populations than in the rural ones. However, the opposite was true when the young production per habitat area was considered. During the breeding season, the absolute amount ofinsect food in the environment did not seem to differ considerably between the urban and rural habitats, suggesting that food supply as such did not limit the young production of breeding pairs. The variation in the density and clutch size of tits was also largely determined by other factors than the absolute amount of food. Densities of tits were determined by the environmental conditions in the winter before the breeding season. In the Great Tit, habitat (the degree of urbanisation), population density, and the weather conditions in spring (March and April) affected the clutch size significantly, while fledgling production was affected by late spring weather and Blue Tit density. In the Blue Tit, habitat was the only factor studied having a significant effect on the clutch size . The clutch size, in turn, as well as the weather conditions in May, significantly affected fledgling production . The poor quality of food before the breeding season in urban habitat's maybe partly responsible for the small clutch size of tits, while the low quantitative share of food due to high densities may be primarily responsible for the low egg and fledgling production of breeding pairs.