Is it a mixedwood? Spatially explicit responses to gradients in habitat structure and composition in three boreal bird species

Context. Mixedwood forests are typically defined using coarse measures of stand composition and do not account for spatial configuration or scale. Such definitions may oversimplify how birds perceive habitat, potentially biasing past attempts to identify mixedwood habitat associations. Objectives. (1) Quantify how continuous gradients in tree species composition, spatial configuration, and forest age influences species’ relative abundance; (2) Identify the spatial extent and grain at which these relationships are strongest. Methods. We analyzed ~ 9200 point counts across the forested region of Alberta, Canada. We used a multi-scale design to assess the influence of forest composition, configuration, and age within 150, 500, and 1000 m extents at fine (5 m) and coarse (30 m) grains on species’ relative abundance. Results. Spatial configuration was the strongest predictor for all three species. Black-throated green and bay-breasted warbler responded most strongly to fine-grain, local (150 m) configuration, with abundance peaking in intimately mixed stands. Tennessee Warbler responded most to configuration at the broadest extent (1000 m), also favoring intimate mixtures. The proportion of conifer and forest age consistently emerged as the second and third most important predictors, respectively. Predicted abundance on synthetic landscapes showed consistently higher abundance (≈ 4–10 times higher) in intimate vs. segregated mixedwoods. Conclusions. Species- and scale-explicit mixedwood definitions that include spatial configuration better capture avian habitat use than composition alone. Preserving or restoring intimate mixtures of coniferous and deciduous species at fine spatial scales may be particularly beneficial for our focal species.