Tits, Warblers, and Finches: Foliage-Gleaning Birds of Nearctic and Palearctic Boreal Forests

Abstract

We describe two major patterns of abundance of different types of canopy foliage-gleaning birds at four sites (northwestern Canada, eastern Canada, European Russia, and central Siberia) in boreal forest. First, because of the extreme numerical dominance and breadth of habitat use of the Chaffinch (Fringilla coelebs), the European Russian sites stand as outliers to the relationships between the number of species and the abundance of all foliage-gleaning birds, as well as to more detailed patterns of the composition of the foliage-gleaner guild. The Chaffinch is one of the few canopy foliage-gleaning birds in temperate forests that is abundant in agricultural habitats during the nonbreeding season, which probably allowed a rapid expansion of its range and growth in abundance with the advent of farming in Europe. The second pattern emerges when the European Russian sites are excluded from analyses: the proportional abundance and number of species of warblers or warbler-like birds were positively related to overall abundance of foliage gleaners. This suggests that warblers prosper when there is high resource productivity during the breeding season. A possible resolution between the patterns found in European Russia and the other boreal forest sites may be that Fringilla are functionally equivalent to warblers and have essentially replaced warblers to achieve their high abundance. The patterns of relative abundance give support to both the Resource Seasonality Hypothesis (warblers versus tits), and the Ecological Replacement Hypothesis (Fringilla versus warblers) as explanations for shifts in the numerical dominance of different types of foliage gleaners in different regions.