RELATIONSHIP BETWEEN THE SUCCESS OF INTERSPECIFIC REARING OF NESTLINGS WITH THE WIDTH OF THE TROPHIC NICHE OF THE RECIPIENT SPECIES IN HOLE-BREEDING BIRDS

We analyzed the diet of nestlings in two hole-breeding passerine birds in order to identify the ways to reduce food competition between these species during the breeding period. We suggest that an analysis of the consequences of an unusual behavior in nature can be useful for a better understanding of their species-specific adaptations. We found a case of joint nesting of the Coal Tit, Periparus ater (CT), and the Pied Flycatcher Ficedula hypoleuca (PF), in the Moscow Region. A PF male attracted the female to the nest box which had already been occupied by a CT pair. The females of both species incubated a mixed clutch sitting side-by-side. CT chicks had hatched before PF ones and the parents of both species started feeding them. On the 4th day of joint feeding, CT chicks began to die one by one, the last nestling perishing on the 11th day. This case encouraged us to compare the diets of nestlings from 22 broods of PH and CT in the same local region by collecting food portions delivered to the nestlings and analyzing the video recordings. According to our neck-collar data, the diet of PF was more diverse than that of CT. In PF, the most abundant groups of prey were flying insects (adult Diptera, Hymenoptera and Lepidoptera etc.), vs. caterpillars and spiders in CT. These two latter invertebrate groups are known to be very important for growing chicks as sources of carotenoids and taurine. The size of prey items delivered to nestlings by PF and CT strongly overlapped. In contrast to the diet of CT nestlings, that of PF nestlings contained a significant proportion of species with a tough chitinous cuticle (Coleoptera, Homoptera, Diplopoda etc). They were present even in the diet of young chicks. In addition, there were species with acrid taste, such as bugs (Heteroptera), ladybugs (Coleoptera, Coccinellidae), millipedes (Diplopoda: Julidae) and soldier beetles (Cantharidae), the latter prey containing cantharidin, an irritant chemical. We suggest that the presence of a large number of rough, heavily chitinized food items and some poisonous insects (Cantharidae) revealed in the PF diet, vs. their absence in the CT diet is an essential characteristic of their foraging. The resistance to cantharidin and some other possible insect toxins could be one of the key characteristics of a broader food niche in PF.