Northern Bullfinch Pyrrhula p. pyrrhula irruptive behaviour linked to rowanberry Sorbus aucuparia abundance

Abstract

Autumn Northern Bullfinch abundance at bird observatories around the Baltic Sea confirmed periodic irruptive behaviour outside of its normal wintering range, but age-ratio data suggested dispersal was not linked to reproductive success. Rowan berries are important food of Fennoscandian Northern Bullfinches and show synchronised masting in annual fruit production over large spatial scales. Norwegian and Finnish annual berry abundance indices from 1972-2004 were used to test the hypothesis that poor rowanberry production in normal wintering areas was responsible for efflux of birds to other areas. Annual Finnish wintering bird surveys and catches at local bird observatories correlated with rowanberry abundance indices, supporting the prediction that highest Bullfinch abundance would occur in normal wintering areas in years with heavy rowanberry crops. Northern Bullfinch autumn abundance at Danish and Swedish bird observatories (outside the normal winter range) showed inverse correlations with Norwegian rowanberry crops, supporting the prediction of highest irruptions in years of lowest rowanberry abundance from the normal wintering range. These data suggest that at large spatial scales, berry masting can have profound effects on the annual distribution and migratory behaviour of birds consuming the crop.