Colonial and single breeding in fieldfares,Turdus pilaris L.: a comparison of nesting success in early and late broods

Summary Pairs of fieldfares breed singly or in colonies. Their breeding season was subdivided into an early part, when the trees were still leafless, and a late part, when the trees had leaves. Early colonial pairs has a significantly higher nesting success than single pairs. However, amongst late broods the nesting success of both categories of pairs was the same. Colonies of only two pairs show a weak trend suggesting that it could be advantageous for early broods to have a neighbour nearby, while the nesting success of late broods seems to be highest if the neighbour is farther away. The number of neighbours is more important than the distance to the nearest neighbour when colonies of different sizes are examined. The nesting success of colonial broods is also influenced by the time of breeding. Success increases with colony size, in early colonies, but the opposite applies for the late breeding season.A little owl was placed near single and colonial nests to show how communal defence contributed to the greater success of early colonial nests. Predators entering a colony risk being contaminated by significantly more faeces due to the higher rate of attacks by fieldfares using aimed defecation near colonial nests and these could affect the predators' flying ability.The possibility of fieldfare colonies serving as information centres for food finding is also discussed.