RELATIONSHIPS OF BIRD COMMUNITY STRUCTURE AND SPECIES DISTRIBUTIONS TO 2 ENVIRONMENTAL GRADIENTS IN THE NORTHERN BOREAL FOREST

Few studies have explicitly examined avian community structure in the North American northern boreal forest. Herein we report upon the resul ts 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 Ecosy stem Classification a moisture and a nutrient-richness gradient. Varia tion in environmental characteristics is limited in the Ontario Clay B elt and this was reflected in short environmental gradient lengths. Ho wever, major trends in avian community structure were strongly associa ted with the nutrient-richness gradient axis summarized by this scheme . Analyses of avian community composition indicated a continuum from m oist, coniferous habitats to drier aspen-dominated mixed woodlands wit h several bird species occurring in varying abundances across the widt h of both gradients. 48 of the 58 species examined showed statisticall y significant associations with at least one of the two multivariate g radient axes. A smaller proportion of short-distance migrant species w ere associated with these gradients than were species in either the ne otropical migrant or resident categories. We used a multivariate varia ble (''habitat breadth'') to compare degree of habitat specialization across different migratory groups. A large proportion of neotropical m igrants 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 tree species than ei ther short-distance or permanent resident species. We discuss our find ings in relation to the post-glacial history of the Clay Belt region.