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Few studies have explicitly examined avian community structure in the North American northern boreal forest. Herein we report upon the results of bird surveys in mature stands of boreal forest in the Great Clay Belt, Ontario, Canada. We related trends in avian community structure and individual bird species' abundance to two environmental gradients described in a landbase classification scheme called the Forest Ecosystem Classification — a moisture and a nutrient-richness gradient. Variation in environmental characteristics is limited in the Ontario Clay Belt and this was reflected in short environmental gradient lengths. However, major trends in avian community structure were strongly associated with the nutrient-richness gradient axis summarized by this scheme. Analyses of avian community composition indicated a continuum from moist, coniferous habitats to drier aspen-dominated mixed woodlands with several bird species occurring in varying abundances across the width of both gradients. 48 of the 58 species examined showed statistically significant associations with at least one of the two multivariate gradient axes. A smaller proportion of short-distance migrant species were associated with these gradients than were species in either the neotropical migrant or resident categories. We used a multivariate variable (“habitat breadth”) to compare degree of habitat specialization across different migratory groups. A large proportion of neotropical migrants showed a high degree of habitat specialization in mature Clay Belt forests, and neotropical species with small habitat breadths were more commonly associated with habitats dominated by broad-leaved deciduous tree species than either short-distance or permanent resident species. We discuss our findings in relation to the post-glacial history of the Clay Belt region.