Biology session

In the past decades several studies have examined how urbanization can influence the animal populations. Birds are prevalent model animals for these studies. According to recent findings, birds in urbanized populations produce less nestlings per breeding atempt than in natural sites. In spite of this, the urban populations usually have higher densities. The reason for this may be that in urban landscapes birds produce more clutch than in forest sites, either within a year or between years (i.e. birds can breed in more years). In our study we tested this hypothesis by examining four populations of great tit in two urban and two at forest sites through 3 years. We recorded 436 breedings of 201 individually identified female great tits. Our results suggest that the frequency of producing a second brood within a year does not differ between forest and urban populations. Furthermore, we did not find any habitat difference in the number of breeding seasons in which the females bred and in the total number clutch they had during the three years. In contrary, forestal females had more nestlings per clutch, so they fledged almost twice as many offspring as the urban ones during the three years. Within a year female great tits changed their breeding nestboxes more frequently in urban than in forest sites. The reason for this could be that nest competition with other species is lower in the cities. The fact that there was no significant difference in the nest fidelity between the years also supports this hypothesis. We conclude that great tits in urban sites have fewer offspring per nest and this decrease in fitness is not compensated by more breedings. Consequently, the higher density in urban sites is caused by either a better survival rate or by the migration from natural areas. The latter would mean that the great tit populations in urban landscapes are ecological sinks.