Forest fragmentation in the temperate zone and its effects on migratory songbirds

Although much attention has been paid to the impacts of tropical deforestation on populations of Neotropical migrants, fragmentation of breeding habitat may be an equally serious problem for many of these birds. Populations of many migrant songbirds have been declining in recent decades, especially within small woodlots. Censuses from woodlots of different sizes also consistently show that many migrant songbirds are area-sensitive, i.e. they are absent from all but the largest woodlots in a region. In contrast, long-term censuses from large, unfragmented forests show few consistent patterns of decline in Neotropical migrants. Population declines are therefore linked to forest fragmentation because they are most pronounced in small, isolated woodlots Fragmentation leads to significant increases in nest predation and cowbird Molothrus spp. parasitism, the two most important causes of population declines and area-sensitivity. Predation and nest-parasitism rates are higher in small woodlots and along the edges of larger tracts than in the interior of large tracts. Data from fragmented forests in the American Midwest show that reproductive rates of several forest species are probably well below levels necessary to compensate for adult mortality. Among Wood Thrushes Hylocichla mustelina nesting in central and southern Illinois, for example, 89-100% of nests contain cowbird eggs (average of 2.2-4.6 cowbird eggs/nest) and nest-predation rates range from 50 to 96%. For the Wood Thrush and other forest songbirds, fragmented landscapes may be population sinks with populations sustained by immigration from larger, unfragmented forest tracts. These data emphasize the importance of protecting large, unfragmented forests for breeding habitat. We need far better data on dispersal rates and distances, fecundity and survival rates before we can determine what levels of predation and parasitism migratory birds can tolerate. The effects of silvicultural practices such as clear-cutting and selective logging on migratory songbirds may depend upon the landscape context. Preliminary evidence from a fragmented national forest in the Midwest suggests that selective logging can have relatively little impact on forest songbirds. We tentatively propose that low-volume selective logging be used as an alternative to clear-cutting. Logging roads should be closed and revegetated soon after harvest, and rotation times should be lengthened to permit regeneration of large, old trees.