Nest design and the abundance of parasitic Protocalliphora blow flies in two hole-nesting passerines

Ectoparasites dwelling in bird nests regularly reduce reproductive success and condition of breeding birds. Thus, establishing the factors that determine the abundance of ectoparasites is important for better understanding of reproductive trade-offs and life history evolution in birds. A recent hypothesis states that interspecific differences in the abundance of ectoparasites may be caused by nest composition. For example, great tits (Parus major) have nests made of mosses and fur, whereas Ficedula flycatchers have nests made of grasses, bast, and bark, and tits are more infested by nest-dwelling ectoparasites than flycatchers. We swapped nests between pairs of great tits and collared flycatchers (F albicollis) during egg-laying or early incubation and counted parasitic Protocalliphora blow flies at the end of breeding to test this hypothesis experimentally. We controlled statistically for habitat (oak versus spruce forest), brood size, season, year, and mean nestling weight before fledging. We found a significant effect of bird species (tit > flycatcher), habitat (oak > spruce), and year. There was no effect of nest type. Consequently, the hypothesis ascribing the different abundance of ectoparasites in great tits and collared flycatchers to different nest composition was not supported by our study.