Group effect in nest defence behaviour of breeding pied flycatchers, Ficedula hypoleuca

Many group-living animals approach and mob nest predators, since grouping can increase the effectiveness of defence against predators. Some nonexperimental evidence shows that the intensity of harassment of predators increases with increasing mob size, indicating a group size effect for mobbing. In this field study we tested whether the intensity of mobbing depends on breeding group size in semicolonially breeding pied flycatchers. We recorded nest defence by parents when chicks were at least 8 days old and again 4 days later. When group size decreased naturally between the first and second trials, the intensity of mobbing a stuffed owl by the nest owners decreased. In contrast, when the number of neighbours remained unchanged the intensity of mobbing by nest owners increased significantly in the second trial. These results reveal the importance of group size in mobbing, indicating that the mobbing behaviour of the first individuals to mob is influenced by the presence of other mobbing conspecific individuals.