Distribution and population changes of two subspecies of Chiffchaff Phylloscopus collybita in Sweden

Abstract

We describe the breeding distribution and population trends of the two subspecies of Chiffchaff Phylloscopus collybita that occur in Sweden. Our data come from the Swedish Bird Survey: free choice point counts (1975—2006) and fixed (and habitat-representative) line transects (1996—2006). Ph. c. abietinus occurs north of about 60°N with a stronghold in central Sweden and along the coast of the Bothnian Bay. This population declined with about 75% between 1983 and 1997, but has increased again during the last decade, matching closely the changes in Finland. Ph. c. collybita, a recent immigrant, breeds mainly in southwesternmost Sweden. The population in 2006 was estimated to be 15 times larger than in 1975 but with little range expansion. The breeding distribution of the two subspecies does not yet overlap. The distribution of the northern subspecies seems to be determined by the occurrence of spruce forest, and the northern range border may be set by early autumn frost. The recent positive trend in the north may possibly be due to an increase in area of old spruce forest.