Trade-off between reproduction and moult - a comparison of three Fennoscandian pied flycatcher populations

Organisms that reproduce at high latitudes are assumed to have evolved seve ral adaptations to the short summer. For birds, and especially for long-dis tance migrants, there is a time constraint because both reproduction and mo ult must be completed before autumn migration. It has therefore been assume d that birds at northern latitudes must initiate their moult during reprodu ction more often than birds at low latitudes. To investigate how passerine birds breeding at different latitudes allocate their time between reproduct ion and moult, we compared timing of these activities during three consecut ive breeding seasons in three widely separated populations of the pied flyc atcher Ficedula hypoleuca. Our results show that the frequency of individua ls with moult-breeding overlap, and moult initiation in relation to breedin g stage, varied considerably among populations and years. In all three popu lations, female moult initiation was restricted to the late nestling period . The males had a more pronounced moult-breeding overlap than the females, but its duration was similar in all three study areas. Thus, there was no e vidence for a more pronounced moult-breeding overlap at high compared with low latitudes. These results suggest that pied flycatchers sometimes accept a moult-breeding overlap, but that the time gained by having too extensive an overlap between reproduction and moult does not outweigh the associated costs. Long distance migrants breeding at northern latitudes apparently ex perience a trade-off between reproduction and somatic investment during mou lt. We therefore suggest that a pronounced moult-breeding overlap is not a typical strategy used by long-distance migrants to adjust to the short bree ding season at northern latitudes.