Three Decades of Crimes and Misdemeanours in the Nest Box Life of European Pied Flycatchers Ficedula hypoleuca
Pied Flycatchers Ficedula hypoleuca are known to be victims of nest killings in contexts of competition for nest boxes. However, there is only anecdotal information on their opposite role as perpetrators of nest takeovers and occasional killings of other songbirds. Over 31 years we examined whether competition with Great Tits Parus major over nest box ownership is a significant source of mortality for Pied Flycatchers and whether the increase in Pied Flycatcher populations affected the use of nest boxes by the smallest tit species, the Blue Tit Cyanistes caeruleus and Coal Tit Periparus ater, in two forests in the central Iberian Mountain Range. We found 31 Pied Flycatchers killed inside nest boxes. The Great Tit was the most frequent species to which the killings were attributed. Followed by Pied Flycatchers themselves and Nuthatches Sitta europaea. We confirmed the killing by Pied Flycatchers of at least two conspecific males and one incubating female Coal Tit, with one killer male identified. Large increases in population densities of flycatchers after nest box deployment in the two study areas seem to have triggered an intensification of agonistic interactions among their potential occupants, resulting in an increase of takeovers by Pied Flycatchers of nest boxes initially owned by the smallest tit species. Great Tits are not a serious threat for Pied Flycatchers breeding in the study area, most likely due to their preference for natural holes over the nest box types most frequently installed. The interactions of Pied Flycatchers with Blue and Coal Tits for the ownership of nest boxes do not seem to cause significant mortality in the species involved. However, aggressive usurpations of nest boxes by Pied Flycatchers surely have negative impacts on the reproductive success of the victims through their eviction from suitable nesting sites.