Fluctuation of a breeding population of Brambling Fringilla montifringilla during 33 years in a subalpine birch forests

Abstract

The breeding density of a Brambling Fringilla montifringilla population studied during 1966-1998 in a subalpine birch forest area covering 3 .5 km2 in central Norway varied between two and 52 territories/km2 with a mean of 28 .4 . The population fluctuated synchronously with the abundance of larvae ofthe geometrid moth Epirrita autumnata (rs= 0.82, P < 0.001) with population peaks in 1975-1976, 1985-1986 and 1996. The birds were largest (wing length as index of body size) in peak years. Although nest predation did not markedly influence the breeding success of Bramblings, it was negatively correlated (rs = - 0.42, P < 0.05) with the density of small rodents, probably because of increased predation by mustelids in years after the population crash of rodents. In summers with long periods of cold, rainy weather, several pairs abandoned their nests with eggs or nestlings when the abundance ofE. autumnata was low, but not when it was high. It is suggested that the Brambling, due to its widespread movements and lack of breeding site tenacity, may be less adapted to varying environmental conditions and resources than other breeding species in the subalpine passerine community, and that Bramblings breeding in subalpine forests are dependent on E. autumnata.