Differences in the breeding success of Blue Tits Cyanistes caeruleus between a forest and an urban area: a long-term study

Birds have specific habitat requirements during the breeding period. The Blue Tit Cyanistes caeruleus is a species evolutionarily associated with forest areas that prefers deciduous and mixed forests, whilst its breeding in urban areas is a relatively recent phenomenon. Long-term data (2002-2015) on reproductive performance (number of hatchlings, hatching success, number of fledglings and fledging success (fledging success as the number of fledglings in relation to the number of hatchlings)) were quantified for two Blue Tit populations in two, floristically and structurally contrasting areas (a mature deciduous forest and an urban parkland) in central Poland. The principal aim of this study is to see whether the habitat type, year or the food availability affect the breeding success of Blue Tits. Forest Blue Tits produced significantly more hatchlings (9.82 +/- 2.64 (SD) in the forest vs.9.17 +/- 2.16 in the parkland) and fledglings (9.18 +/- 2.84 in the forest vs. 8.14 +/- 2.68 in the parkland) than urban Blue Tits. The number of fledglings was positively correlated with the number of hatchlings in both study areas. Both forest and urban Blue Tits shared a similar hatching success (85.9 % in the parkland and 85.5 % in the forest), while the fledging success was significantly higher in the forest (83.4 % in the parkland and 86.1 % in the forest). The amount of caterpillar frassfall was also studied (caterpillars are the optimal food for nestlings) at both study areas and it suggested that caterpillars were more abundant in the forest than in the parkland (the maximum amount of frassfall, averaged 0.21 +/- 0.11 g frass/m(2)/day in the urban parkland and 0.59 +/- 0.50 g frass/m(2)/day in the forest in 2003-2015). In the forest area, the mean number of fledglings tended to be related to the amount of frassfall but in the parkland, this relation was non-significant. The long-term dynamics of fledging success in our study sites seems to be mutually independent. Thus low fledging success in the forest site does not mean similarly low fledging success in the urban parkland site and vice versa. We suggest that food availability is one of key drivers of differences in the tits breeding success between both studied habitats.