Multiscale assessment of the influence of habitat structure and composition on bird assemblages in boreal forest

The relative contributions of habitat structure and composition to biodiversity are often scale-dependent. Although bird communities in boreal forest have been largely altered and threatened by forest harvesting, bird habitat selection in this ecosystem has not been fully understood. Our study aimed to assess the relative contributions of habitat structure and composition on the assemblages of boreal birds at multiple spatial scales characterized by radii ranging from 100 to 1,000 m. We recorded bird species occurrence at 96 stations located in an old-growth forest in the Côte-Nord region of Québec, Canada. We characterized habitat structure using the proportion of dense, open, and sparse stands, and habitat composition using the proportions of coniferous, mixedwood, and deciduous stands. We used partial canonical correspondence analyses and hierarchical variance partitioning to assess the relative contribution of habitat structure and composition on bird assemblage, and logistic regression to model the probability of occurrence for individual species in response to habitat variables. Our results revealed that habitat structure and composition explained similar proportions of the variance in bird assemblage (21.7 vs. 21.6 %), regardless of spatial scale. Whilst logistic regression yielded fair predictions in the occurrence of individual species (i.e., area under the receiver-operating characteristic curve >0.70 for 90 % of the species), it further confirmed our findings in community level analysis. Our study indicates that habitat structure and composition are both important in shaping bird assemblages, but spatial scale draws little influence on their relative contributions.