Roads are a major hazard to wildlife, as roadkill often testifies. However, it is generally assumed that this only applies to creatures forced to walk or slither across the highway. Birds, other than the flightless varieties, can get enough altitude to cross in safety, so why should roads be a problem? Yet an investigation of this belief has found it wanting. Even many species of flying birds are troubled by roads.

Christopher Johnson, a Griffith University graduate student, conducted a study of forest-dwelling birds and roads. Griffith's largest campus, at Nathan, is almost surrounded by the bird-rich Toohey Forest, making it an ideal place to start such research. Johnson extended his studies to sites around southern Brisbane, an area that hosts around 30 percent of Australia's bird species, carefully recording bird numbers both close to roads and 100 meters (330 feet) away.

"For this study, we decided to try something new, by looking at the influence of different road widths (two, four and six-lanes) on bird crossings,” Johnson said in a statement. “In addition, we analyzed the road-crossing ability of birds of different body sizes and whether the type of bird, for example, small forest dependent, large forest dependent, honeyeater or urban tolerant species, had an effect.” Johnson and his colleagues also kept track of seasonal variations.

In Frontiers in Ecology and Evolution, Johnson reports that the wider the road, the fewer birds that crossed it, with the difference being greatest among small (less than 20 centimeters or 8 inches long), forest-dependent species.

Johnson and his co-authors are not sure of the reasons roads have such an effect. Some birds may be intimidated by any open space where they could be spotted by predators. Others may simply be using the roads as handy boundaries to delineate their territories. Johnson also observed that highly aggressive species of birds were often heavily represented in the open roadsides, possibly scaring off more timid creatures. Previous studies have shown birds experience stress from traffic noise, which is likely to be greater near multi-lane highways.

Road avoidance may lower the risk of being hit by a car, but the habitat fragmentation it induces can interfere with breeding and the capacity to migrate when local food resources are scarce.

"People use the road transport system to get from point a to b. Unfortunately, this has a negative impact on wildlife movement, particularly within urban environments," said co-author Daryl Evans. As Evans points out, these can often be addressed through overpasses, underpasses, and fencing to keep the less intelligent animals from stopping mid-road. Nevertheless, these are usually designed for creatures that walk rather than those that fly, and little work has been done to find the most effective way to rectify this.