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Wesołowski T. 2001. Host-parasite interactions in natural holes: marsh tits (Parus palustris) and blow flies (Protocalliphora falcozi). Journal of Zoology 255: 495-503.

Infestation of marsh tits (Parus palustris, Paridae, Passeriformes) broods by bloodsucking larvae of Protocalliphora falcozi Séguy 1928 (Calliphoridae, Diptera) was studied over an eight year period in a population breeding in natural holes, in the primaeval forest (Białowieża National Park, E Poland). Overall 54% of 222 marsh tit nests were parasitized. Prevalence (27-88%) significantly changed across years. This variation did not depend on timing of marsh tit breeding season, winter, or spring temperatures. Frequency of infestation did not depend on forest type or hole attributes. Infestation intensity was rather low (median 8, max. 75 flies/nest, 85% of nests with <3 flies/young). Higher intensities tended to coincide with higher prevalences. Larger broods contained significantly more blow flies, per nestling load did not depend on brood size. No effect of infestation on nestlings was recorded - their mortality did not increase, nor fledging was delayed. In response to the presence of blow flies parents apparently attempted to increase their feeding rate. There was a clear reproduction cost: 60-63% of females and 68-69% of males having only 0-8 blow flies but only 34% of females and 44% of males with more than 8 flies/brood survived till the next spring. It is suggested that low clutch size of marsh tit could have evolved, inter alia, to reduce the fitness costs of ectoparasites.

Key words: Protocalliphora falcozi, Parus palustris, primaeval forest, cost of reproduction