Some common woodland songbirds go to great lengths to avoid crossing open spaces created by loggers and farmers. Instead, they stick to the trees that might hide them from a hungry hawk, even if it means flying roundabout routes. The finding, reported in this month's issue of Conservation Biology, suggests that forest fragmentation contributes to songbird losses by forcing birds to waste precious energy. It also suggests that creating forested corridors between woodland patches might help some birds, although some biologists dispute that idea.
Over the last few centuries, logging and land clearing have fragmented many of North America's great forests into ragged patchworks of fields, clear-cuts, and disconnected woodlands. Many biologists believe this fragmentation has contributed to the dramatic declines in some forest-nesting birds. Some researchers, for example, have found evidence that birds nesting in smaller forest patches are more exposed to predators, such as house cats, than birds dwelling in larger forest reserves. But the elusive habits of many species have made it difficult for researchers to observe how birds actually behave when confronted with crossing a gap in the trees.
Now, Andre Desroches of the Université Laval in Sainte-Foy in Canada, and Susan Hannon of the University of Alberta in Canada, have found a way to study how some birds react to forest gaps. In their study, conducted in southern Quebec, the researchers tried to coax five types of birds to cross 7- to 160-meter gaps by attracting them with taped birdcalls. Overall, the nuthatches, chickadees, warblers, kinglets, and vireos showed little reluctance to fly across gaps less than 30 meters wide to reach the tape recorder. As the gaps grew larger, however, the birds were increasingly likely to take longer, roundabout routes that didn't require them to fly out into the open. For instance, they were eight times more likely to remain under cover than fly across a 100-meter gap. The researchers theorize that the energetic cost of a bird's longer trip is worth the benefit of avoiding predators, such as bird-eating hawks, but could be stressful over the long run.
Hannon says similar studies could eventually be of practical use to loggers and land managers by providing guidelines on how big forest gaps can be before they become an obstacle to birds. She also says the study suggests that preserving forested corridors could allow the birds to move unimpeded across the landscape.
But several researchers, while lauding the study for its creative approach to studying bird behavior and highlighting one effect of fragmentation, do not believe its findings should be interpreted as proof that wildlife corridors will help birds. Carola Haas of Virginia Polytechnic University in Blacksburg and Dan Simberloff of the University of Tennessee, Knoxville, say that the benefits of corridors remain unproven. Says Simberloff: "Corridor advocates may say this otherwise interesting study shows the value of corridors, but it is an issue the authors didn't really address."