The boreal forest, about 11 percent of the world's forest, stretches across the continents of Canada, Europe, and Asia. Regardless of continent, the vegetation of the boreal forest, dominated by conifers, is fairly consistent. But the bird communities are quite different.
Studies at sites in European, Russian, and Canadian boreal forests revealed that the birds in North American forests spend a lot more time foraging in foliage, especially spruce or pine needles. European and Asian birds spend more time on branches, twigs, and other substrates.
While it is true that North American forests simply have more conifers, the birds foraged in them even more often than one would expect given their abundance. So why would the bird communities differ across continents and why would their foraging strategies differ?
The answer may lie in the geologic past. During the height of the Ice Age glaciers extended from the North Pole to cover much of Northern Hemisphere. The boreal forest retreated south, but the extent of the remaining forests were quite different.
North American boreal forests were still quite extensive at this time, but European and Asian boreal forests appear to have survived only in isolated, small pockets or in narrow bands.
So birds that specialized in foraging in conifers in Europe and Asia may have gone extinct during the Ice Age. They just had nowhere to go.
Another possibility is that favored prey items might have gone extinct around at this time. North American birds feast on the irruptive spruce budworm, a caterpillar that is periodically found in huge numbers in parts of its range. Such irruptive caterpillars are not found now in Europe and Asia. They may not have been able to survive in small pockets as their life history strategy is to get so common in an area that the birds cannot eat all of them.
This article summarizes the information in this publication:
Greenberg, Russell S., Pravosudov, V., Kozlenko, A. and Kontorschikov, V. 1999. Vegetation history and foraging behavior of foliage gleaning birds in the Canadian and Russian boreal forests. Oecologia, 120: 451-462.
We compared foraging behavior of foliage-gleaning birds of the boreal forest of two Palaearctic (central Siberia and European Russia) and two Nearctic (Mackenzie and Ontario, Canada) sites. Using discriminant function analysis on paired sites we were able to distinguish foliage-gleaning species from the Nearctic and Palaearctic with few misclassifications. The two variables that most consistently distinguished species of the two avifaunas were the percentage use of coniferfoliage and the percentage use of all foliage. Nearctic foliage-gleaner assemblages had more species that foraged predominantly from coniferous foliage and displayed a greater tendency to forage from foliage, both coniferous and broad-leafed, rather than twigs, branches, or other substrates. The greater specialization on foliage and, in particular, conifer foliage by New World canopy foliage insectivores is consistent with previously proposed hypotheses regarding the role of Pleistocene vegetation history on ecological generalization of Eurasian species. Boreal forest, composed primarily of spruce and pine, was widespread in eastern North America, whereas pockets of forest were scattered in Eurasia (mostly the mountains of southern Europe and Asia). This may have affected the populations of birds directly or indirectly through reduction in the diversity and abundance of defoliating outbreak insects. Loss of habitat and resources may have selected against ecological specialization on these habitats and resources.
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