April 12, 2016
Thank to Perry Koehler for this guest student blog post. Perry explains how researchers created a “phantom road” to study the impact of traffic noise on birds.
Birds have little trouble flying over roads, yet studies have shown that population densities of many bird species decline in the vicinity of high traffic areas. The drop in population density can pose a problem for bird watchers; 83% of land in the United States is within a half mile of a road. Assuming you are not lucky enough to have your feeders secluded from any traffic disturbances, it is important to understand the influence of roads on birds.
Past research has shown that the presence of roads negatively affects the distribution of birds, but that research has not teased out what factors—visual disturbance, collisions, pollution, or traffic noise—impact birds the most. Recently, a group of researchers set up a “phantom road,” or a line of speakers playing traffic sounds, in an otherwise secluded and wooded area in southern Idaho. Almost all species normally observed were disturbed by the traffic noise and opted for more circuitous routes to avoid the “road.” Two species in particular, Cedar Waxwings and Yellow Warblers, were not seen at all while traffic sounds played. The research suggests that traffic noise alone negatively impacts birds and that the decrease in abundance would likely be magnified by the other factors of a road such as pollution and visual disturbance.
The location used by the scientists for their “phantom road” was a popular migratory stopover site; however, the added traffic sounds made this location unsuitable for migratory birds. The distribution of migratory birds, compared to resident breeding birds, is likely to be more heavily affected by traffic because they can easily alter their flight path to avoid roads. Resident birds are less able to avoid noisy areas, so they will still be prevalent at feeders in high traffic areas. The continued expansion of roads into suitable migratory stopover habitats compounds the difficulty of migration. This bi-annual journey is the most dangerous part of a bird’s annual cycle. For example, the Black-throated Blue Warbler now experiences 85% of its yearly mortality during migration.
But why does the sound of traffic impact birds? One explanation is that the traffic masks important communication from members of the same species, different species, and predators. Increased predator vigilance is necessary in noisier areas leading to a higher stress environment and decreased food intake. Some species are also more disrupted by traffic because their vocalizations are within the same frequency as the traffic noise. Perhaps Cedar Waxwings and Yellow Warblers cannot overcome this interference by shifting their vocalizations to a higher frequency, as some species of flycatcher have been known to do.
Why didn’t the bird cross the road? It turns out the road was too noisy, hindering its communication, so it adapted and chose a more secluded route to the other side. Next time you are bird watching, keep in mind that many birds are sensitive to noisy traffic and look for them where they feel more comfortable.
McClure, C. J. W., Ware, H. E., Carlisle, J., Kaltenecker, G., and Barber, J. R. (2013), An experimental investigation into the effects of traffic noise on distributions of birds: avoiding the phantom road. Proceedings of the Royal Society B, 280. doi: 10.1098/rspb.2013.2290