Moose winter browsing affects the breeding success of great tits

Abstract

In many areas, ungulates may have a large impact on the landscape due to their large body size and wide distribution. Moose (Alces alces) winter feeding has been carried out for a decade in parts of Hedmark County, southeast Norway. Previous studies have documented a gradual decline in browsing pressure away from the feeding stations. We utilized this gradient to study the indirect effects of moose browsing on a passerine bird, the great tit (Parus major). The downy birch (Betula pubescens) trees surrounding the feeding stations are subject to an intense browsing pressure gradually decreasing outwards. We put up nest boxes at feeding stations (i.e., sites of intense browsing pressure), with nearby controls with low visible browsing. The number of tall birch trees and the birch canopy cover was lower in browsed compared to control plots. Due to the reduced birch canopy cover, the total biomass of arthropods available around the nest boxes was lower in browsed compared to control plots. Great tit breeding pairs produced 1.3 fewer fledglings in browsed compared to control plots. We suggest this difference to be caused by lack of food in the browsed plots. Hence, moose browsing may reduce the breeding success of great tits through a series of knock-on effects through two other components of the boreal forest community.