A natural experiment on the impact of overabundant deer on songbird populations

Declines in songbird populations have been identified both in North America and in Europe. Several explanations have been proposed but few studies have evaluated the possibility that deer overabundance might affect songbird populations, and none have identified general rules to predict such an impact. We used a group of islands in the Haida Gwaii archipelago (British Columbia, Canada), where islands without deer co-exist near islands with deer, as a natural experiment to test if the dependence of each species on understorey vegetation was a good predictor of deer impact. Forest bird assemblages were compared on six islands that either had no deer, had deer for less than 20 years or for more than 50 years, and on an enlarged set of 31 islands for which vegetation data and an index of deer impact were available. In the six islands data-set, songbird abundance on islands browsed for more than 50 years was 55-70% lower than on deer-free islands. There was a significant decrease in alpha diversity on islands browsed by deer, but gamma diversity remained unchanged. Bird species with the highest dependence on understorey vegetation were most affected and their abundance decreased by 93%. Bird communities flipped from being 73% dependant on understory vegetation on deer-free islands to 79% not dependant on understory vegetation on islands with deer for more than 50 years. A canonical correspondence analysis on the 31 island data-set allowed us to further separate the interactions between bird abundance and distribution, vegetation features and deer presence. We propose that deer overabundance results in a decrease in songbird habitat quality through decreased food resources and nest site quality and may explain part of current continental-scale decreases in songbird populations.