Geographical Variation in Temperature Variability and Predictability and Their Implications for the Breeding Strategy of the Pied Flycatcher Ficedula Hypoleuca

Abstract

Between-year temperature variability was higher in northern than in southern Finland (1000 km apart). From April to June and in the second half of August there was a close correlation in mean temperature in 15-d periods between southern and northern Finland. However, within-year predictability of temperature was better in the north than in the south. The results are opposite to the conventional assumption that variable environments are unpredictable environments. Between-year variation of clutch size, hatching success and fledging success of the pied flycatcher were more pronounced in the north than in the south. Especially in the north several breeding variables correlated with mean temperature in the second half of June. It is suggested that in the north the considerable between-year variability in temperature conditions may select for a flexible breeding strategy in birds and that the considerable withinyear predictability of temperature conditions may ensure that the particular flexible strategy adopted in a given year is likely to be successful. In the south birds apparently cannot predict the favourableness of the coming breeding season from spring temperature. However, this is not a serious disadvantage since even in the coldest summers weather is relatively warm in the south. The results and ideas may apply to many animals and plants.