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Meta-analysis shows that overabundant deer (Cervidae) populations consistently decrease average population abundance and species richness of forest birds

ABSTRACT Local-scale studies have shown that an overabundance of Cervidae species (deer, elk, moose) impacts forest bird communities. Through meta-analysis, we provide a generalized estimate of the overall direction and magnitude of the indirect effects overabundant cervids have on avian species. We conducted 2 distinct meta-analyses that synthesized data on 130 bird species collected from 17 publications. These analyses compared bird species' population abundance and/ or species richness at sites with overabundant cervids to sites with lower cervid abundance or without cervids. We evaluated whether the impacts of overabundant cervids are generally in the same direction (positive, negative) across avian species and locations and if effects vary in magnitude according to avian nesting location and foraging habitat. We found that where cervids were overabundant, there was a significant decrease in mean bird population abundance and species richness. Species that nest in trees, shrubs, and on the ground showed the largest decreases in abundance, as did species whose primary habitat is forest and open woodland and species that are primarily insectivores or omnivores. We did not find significant decreases in abundance for avian species that nest in cavities, whose primary habitat is grassland or scrub, nor for species that mainly eat seeds. Our results indicate that overabundant cervids, likely through their direct effects on vegetation and indirect effects on insects and forest birds, negatively impact individual bird populations and decrease overall avian species richness. LAY SUMMARY Local-scale studies suggest that when cervid species (deer, elk, moose) are overabundant, forest bird populations and communities are negatively impacted. We synthesized data from 17 publications to provide a global summary of the average impacts that overabundant cervids have on bird species abundance and richness. Forest bird abundance and richness decline where cervid species are overabundant, likely because they alter vegetation that birds use as components of habitat and the resources that vegetation provides birds. The highest magnitude abundance declines are seen in populations of tree, shrub, and ground-nesting avian species, species whose primary habitat is forest and open woodland, and species that are insectivores or omnivores. Our results suggest that, to maintain forest bird biodiversity, wildlife and land managers should consider actions to keep cervid species from becoming overabundant or reduce populations where they are already overabundant.