Over the last few decades many migratory bird species in the United States have fallen on ever tougher times as development has devoured natural landscapes. But not all species have suffered equally--some have held their own, or even increased in number. This pattern has baffled scientists. But a new study--the first to consider population changes across the entire home range for several bird species--reveals a surprisingly simple explanation: Species that tend to nest in fragmented areas are hit harder than ones that seek out unbroken forest.
The grim message of previous studies was that habitat loss generally hinders reproduction, particularly in migratory birds. But biologists Therese Donovan of the U.S. Geological Survey's Vermont Cooperative Fish and Wildlife Research Unit in Burlington and Curtis Flather, with the U.S. Forest Service's Rocky Mountain Research Station in Fort Collins, Colorado, suspected the authors of these studies might have been, well, too close to the trees to see the forest. Donovan and Flather used survey data for 10 bird species across roughly the eastern third of the United States and aerial maps to estimate the fraction of each species' population that occupied fragmented landscapes over a decade. Populations stayed steady if only a small proportion nested in forest fragments, but fell if a larger proportion did, Donovan and Flather report in the April issue of Ecological Applications.
The findings suggest that "populations are affected by processes that happen over much larger scales than ecologists are used to dealing with," Flather says. He contends that looking only at the local situation can be misleading. For example, juvenile birds can disperse widely from their birth sites, and bumper crops of fledglings can repopulate bird-depleted areas and even out the numbers across the species' range.
The study is an "original and important contribution toward understanding forest bird population trends," says biologist Pierre Drapeau of the University of Quebec in Montreal. He says the findings underscore the importance of large tracks of unfragmented forest: By selecting these areas as nesting sites, some birds may be able to survive habitat fragmentation in other areas. However, he says more work is needed to establish whether the trend applies to species beyond the 10 included in this study.