Broods of Pied Flycatcher nesting in natural tree holes and nest-boxes in Białowieża Forest (E Poland) were compared. Natural holes in primeval stands of the Białowieża National Park were located by following singing males, then monitored several times during the season. Nest-boxes situated in the managed part of the forest were inspected weekly. Flycatchers breeding in natural holes started laying eggs on average two days later (15 May) and laid smaller clutches (6.4 eggs) than birds breeding in nest-boxes (13 May and 6.7 eggs). The predation rate was significantly lower in natural holes (av. 47%) than in nest-boxes (av. 65%). This result indicates that generalisations regarding the evolution of adaptations to predation by nest-box populations should be treated with caution.
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