Age-related difference in return rate of Willow Warblers Phylloscopus trochilus at two breeding sites in Sweden

Abstract

We studied breeding and natal site fidelity of Willow Warblers Phylloscopus trochilus in 1983 to 1993 at two ringing sites, Ammarnäs in Swedish Lapland and Kvismaren in South Central Sweden. The yearly trapping effort was rather similar at the two study sites . We used pooled data from all study years in the analyses . The overall recapture rate was almost two times higher at Kvismaren than at Ammarnäs. This difference was caused by an eight times higher recapture rate of Willow Warblers ringed as juveniles at Kvismaren as compared to Ammarnäs, whereas the recapture rate of birds ringed as adults did not differ between study sites. When we restrict the analysis to include mainly local birds, the between year recapture rate ofjuveniles was still more than 20 times higher at Kvismaren than at Ammarnäs. We discuss two possible explanations for this difference in recapture rate of juveniles between the two study sites. One explanation is that juvenile Willow Warblers at Ammarnäs may suffer higher mortality, either because they have a longer migration route or as a result of adverse weather conditions in the breeding areas in late summer. This is, however, contradicted by the fact that the breeding densities of Willow Warblers in the mountain birch forest are much higher than in other habitats where the subspecies (P.t. acredula) occurs in Sweden. Thus a more likely explanation is that natal site fidelity is lower at Ammarnäs, possibly as a result of a homogeneous habitat (mountain birch forest) and because rich food patches (e .g . Epirrita autumnata) are unpredictably distributed between years.