Patterns and causes of long-term variation in reproductive traits of the Pied Flycatcher Ficedula hypoleuca in Finnish Lapland

Abstract

Breeding performance of a marginal northern Pied Flycatcher population, known to have been founded in 1957, was studied during 22 years (1966-1987). The number of breeding pairs varied between 8 and 57. Of this annual variation, 44% was explained by the total number of fledglings produced in the preceding year and the mean temperature of June in the current year . The annual productivity of the population was usually either `good' or `bad'. The mean temperature in May increased steadily during the study period, which resulted in an earlier date of egg-laying and larger clutches in the latter half of the study period (standardized selection differential of clutch size 1966-76 vs . 1977-87 0.40) . However, the birds did not benefit from larger clutches, since there was no corresponding increase in the number of fledglings/nest. There were no clear trends in the size of the females (1974-87) or in the egg size (1975-87). The most important factor affecting hatching success was the mean temperature during the incubation period and the most important factor affecting fledging success was the female weight . Population density, the mean date of egg-laying, the mean clutch size, and to a lesser degree nesting success varied in parallel in northern Finnish Lapland (69°N) and Swedish Lapland (66°N; Svensson 1987) in 1971-85. In Finnish Lapland theannual nesting success varied more than further south in Swedish Lapland.