Spatio-temporal dynamics in breeding occurrence of passerine birds in subarctic birch forest

Abstract

The three box-nesting passerine bird species pied flycatcher, great tit and common redstart breed at their distributional limit in Troms, Northern Norway. Access to food and favourable weather conditions is expected to be of importance for the birds´ breeding population dynamics. In the same area, geometrid moth larvae experience cyclic population outbreaks at approx. 10 year intervals, constituting a superabundant and potentially important source of food for insectivorous birds during the breeding season. To assess if the birds´ breeding population dynamics could be explained by moth larval densities and summer temperatures, we conducted yearly inspections of nest box occupancy and moth larval counts during a period of 13 years at 12 study sites, and obtained temperature measurements from nearby weather stations. Results from GLMM models with a logit link function suggested a moderate positive numerical response of the bird community to high moth larval densities. The response was species specific, with the great tit showing the most consistent response – indicating that this species is probably more dependent on moth larvae during breeding season than the other two bird species. The bird community also showed a positive numerical response the year after high summer temperatures, likely because it caused higher breeding success and subsequent higher return rates of breeding birds. It was concluded that the focal bird community was positively affected by high moth larval densities and summer temperatures, but that other factors were probably also limiting on breeding population densities due to the moderate response.