Feature-dependent group structures and hierarchical songbird-habitat relationships in a managed forest landscape

We examined the importance of local habitat features on forest songbird distributions using a feature-dependent group framework that organizes species in relation to variable importance of species-habitat relationships. Habitat features included vegetation structure, floristics, and environmental (i.e., topography) classes. We also examined evidence for hierarchical habitat selection by assessing feature positioning within the ensemble structural models, Random Forests. From the 44 species examined, we distinguished 14 groups, comprising two to eight species per group, and examined variation in habitat importance across the groups. Overall, floristic species turnover, shrub density, canopy height, and tree basal area were among the most important features. On average, however, no specific feature class was more important than the others across the entire collection of species. Instead, we interpreted group partitioning to occur primarily by species differentiating among specific forest types, such as bottomland riparian or oak-dominated (Quercus spp.) stands, and species responding to structural variation across successional gradients. The positioning of habitat features within individual tree models showed that for those species among specific forest types, floristics features were selected first in the models followed by environmental features. Whereas when vegetation structural features were most important, structural features were typically selected first followed by floristics. The large variability in responses likely reflects the large heterogeneity in vegetation structure and composition present in our Central Hardwood, USA, forest landscape. In such cases, avian community assembly processes incorporated the primary moisture (mesicxeric tree species) and successional (open-to closed-canopy) vegetative gradients of these forestlands, which underscores the importance of maintaining vegetation heterogeneity under accelerating environmental change. By including diverse suites of habitat types, we demonstrate how these features vary in their importance across songbird assemblages in a diverse forestland and identify key habitat features in the conservation of these bird communities.