ИСТИНА

The analysis of consequences of unusual behavior of birds in nature could be useful for better understanding of their specific adaptations. We found the case of joint nesting of the Coal Tit Periparus ater (CT) and the Pied Flycatcher Ficedula hypoleuca (PF) in the Moscow region. PF male had attracted the female in the nest box that was already occupied by CT pair. Females of both species incubated mixed clutch sitting side-by-side. CT chicks hatched earlier than PF chicks and parents of both species started to feed them. At the 4th day of joint feeding CT chicks began to die one-by-one and at the day 11 last nestling was dead. This case pushed us to compare the diets of nestlings of PH and CT at the same local region by collecting food portions delivered to nestlings and by analysis of video recordings. Prey compositions and size of food items are discussed. We suppose that the revealed presence of poisonous insects (Cantharidae, Melyridae etc.) in FH diet and its absence in CT diet is the essential characteristic of their foraging. The resistance to cantharidin and some other possible insect toxins could be one of the causes of broad food niche in PF.