Association of Success in Interspecific Rearing of Nestlings with the Width of Trophic Niche of the Recipient Species in Hollow-Nesting Birds

Abstract

The structure of the diet of passerine bird nestlings was analyzed in order to identify possible ways to reduce food competition between these species during the breeding period. We assumed that analysis of the effects of obtaining diets of heterospecific birds by nestlings in nature can help better understand their species-specific adaptations to living conditions, and we considered the case of joint nesting of the coal tit (Periparus ater (L. 1758)) (CT) and pied flycatcher (Ficedula hypoleuca (Pallas 1764)) (PF) in Moscow oblast as a key link. A PF male attracted a female to an artificial nest box, which has been already occupied by a CT pair. The females of both species incubated a mixed clutch sitting side by side. The CT nestlings hatched earlier than the PF nestlings, and the parents of both species began to feed them. The CT nestlings started to die one by one on the 4th day of joint feeding, and the last nestling died on the 11th day. This case encouraged us to compare the diets of nestlings from 22 broods of CTs and PFs in the same region. To compare the ratios of nestlings, we collected the portions of food that were delivered to nestlings by their parents and analyzed video recordings from the nests. The diet of PF nestlings was significantly more diverse than in CT nestlings (Berger–Parker reverse indices (1/d) were 3.01 and 2.11, respectively). Caterpillars and spiders (objects with a high content of carotenoids and taurine important for the development of nestlings) dominated in the CT diet. In PF feed, these objects occupied a much smaller portion. In addition to the above, compared with coal tits, PFs brought the following to nestlings: a significant number of objects with rough, highly chitinized covers (Coleoptera, Homoptera, etc.); objects with a sharp taste, such as true bugs (Heteroptera), ladybirds (Coleoptera, Coccinellidae), and millipedes (Diplopoda, Julidae); insects containing cantharidin toxin (soldier beetles, Coleoptera, Cantharidae). The nestlings developed successfully in PF nests to which parents brought such feed objects. The dimensional ranges of objects in the CT and PF diets completely overlapped. According to our opinion, the rougher, highly chitinized food and some poisonous insects (Cantharidae) in the PF diet make fundamentally important differences in the nutrition of PFs and CTs. Most likely, PFs have resistance to cantharidin and some other toxins that are contained in insects and therefore have a wider trophic niche.