Reproduction of Pied Flycatchers (Ficedula hypoleuca) in Good and Bad Breeding Seasons in a Northern Marginal Area

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Abstract
We studied relationships between female size, date of egg laying, clutch size, egg size, and breeding success in Pied Flycatchers (Ficedula hypoleuca) in a marginal area in northwestern Finnish Lapland (69°N). Average June temperatures in 1975-1981 were used to classify years into "good" (warm) and "bad" (cold) breeding seasons. In general, early and warm springs were followed by "good" breeding seasons: females were heavier, laying was earlier, clutches were larger and contained larger eggs, and hatching and fledging success were better than in late and cold springs. Mainly due to low nesting success in "bad" breeding seasons, the local population could not maintain itself; immigration from the south was needed. We suggest that spring weather may provide information concerning the character of the coming breeding season, which may help birds to maximize their lifetime production of young.

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