Spatial and Temporal Variation in Reproductive Traits of Adjacent Northern Pied Flycatcher Ficedula hypoleuca Populations

Abstract

Spatial and temporal variation in the number of breeding pairs, site-tenacity of adult females, laying date, clutch size and reproductive success of the Pied Flycatcher was studied in 1987-1989 in two areas 35 km apart in northern Scandinavia and related to weather conditions, plant phenology and predation. One study area was situated in a relatively productive and mild forest zone (Skibotn, 69020'N), the other further south in a harsh zone of mountain birch forest (Kilpisj~irvi, 69003'N). The hypothesis of no spatial difference was rejected for number of pairs, sitetenacity, laying date, clutch size and breeding success. Both areas showed considerable between-year variation in several breeding variables. At Skibotn, egg-laying started 7 d after birch-leafing; at Kilpisjirvi, 5 d before. Birch-leafing occurred 18 d earlier at Skibotn than at Kilpisjirvi, to be compared with the difference of only 6 d in laying date. At Skibotn, clutch size averaged 6.2 and number of fledglings per nest 3.6; at Kilpisjirvi 5.5 and 2.9. At Kilpisjirvi, breeding success seemed to be limited by cold weather; at Skibotn by predation. Site-tenacity of adult females was relatively low in both areas: 5% at Skibotn and 11% at Kilpisjdirvi.