Egg Size and Related Reproductive Traits in a Southern Passerine Ficedula hypoleuca Breeding in an Extreme Northern Environment

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Abstract
Reproductive traits of the pied flycatcher, F. hypoleuca, were studied at 3 biogeographically different localities in Finland: Helsinki (south boreal), Oulu (mid-boreal) and Kilpisjarvi (sub-arctic area, where the species is a newcomer). Clutch size decreased with the date of laying, being smallest in the north. Average female size and weight and egg size were the same in the south and the north. Environmental variance in egg size was greater in the north, but the female component was still considerable (about 50%). Egg size correlated negatively with clutch size in the south, but the correlation changed to positive towards the north. At Kilpisjarvi female wight (fatness) rather than size determined egg size and hatching success. The short and small eggs did not hatch well in the north; this is probably because small eggs are laid by lightweight females, which cannot incubate for as long periods as heavier females, especially during the frequent cold spells, and lightweight females lay inferior eggs, which are more sensitive to cold weather. Selection pressure for large egg size seemed strong in the north.