Urban Birds Sing Shorter Songs When Traffic Is Loud
A songbird changes its song when traffic is loud, according to a recent study, and this could affect its ability to attract mates and defend its territory from rivals
Traffic noise creates a variety of problems for people, and it also poses problems for wildlife. This is especially true for birds who rely on song to attract a mate and to defend a territory from rivals. So what do songbirds do when confronted with traffic noise?
According to a newly published study by Katherine Gentry of George Mason University in Virginia, excessive traffic noise causes songbirds to sing a different tune — a tune that is shorter in duration and comprises a lower frequency range of sounds.
Traffic noise typically is a low frequency sound but it also can interfere with and mask high-frequency sounds, particularly certain sorts of birdsongs. Most birds deal with this problem by nesting in areas that “match” their vocal performances, allowing their songs to be most effective. For example, previous studies have found that songbirds with shorter songs can establish territories in noisier areas whereas birds that sing higher-pitched songs do better in quieter areas.
But when songbirds nest in urban areas, they must instead change their songs to “match” their environment — and that includes automobile traffic. Urban birds deal with background noise by adjusting the amplitude, range of frequencies, timing, or duration of their song so it is easier to hear. This ability to change one’s song in response to environmental sounds is referred to as song plasticity, and this acoustic flexibility is typically associated with oscine songbirds.
There is another major group of songbirds, known as suboscines, whose song-producing organ, the syrinx, has a different anatomic structure to that of oscines. Many suboscines sing a simpler tune than do oscine songbirds, and suboscines are more likely to show up in areas that are poorly matched acoustically to their song types. This observation raises the question: can suboscines somehow adjust their song structure so it is heard over background noise?
The study birds adjusted their songs as traffic noise levels fluctuated
To conduct this research, Dr. Gentry and a team of her colleagues focused on a drab little North American flycatcher known as the Eastern wood pewee. The Eastern wood pewee, Contopus virens, is visually indistinguishable from a closely-related species, the Western wood pewee — unless you hear its vocalizations: the Western wood pewee sings a high-pitched descending song whereas the Eastern wood pewee whistles an entirely different, ascending song (video):
Based on the information already mentioned, most people would predict that the Eastern wood pewee’s high-pitched song would be masked by the constant background racket created by big-city traffic.
The study birds live in three urban parks in the greater Washington DC metro area. Dr. Gentry and her team recorded Eastern wood pewee songs at sites where traffic was either constant or was reduced on a weekly basis due to 36-hour road closures. They measured and analyzed the bandwidth, duration, and maximum, peak and minimum frequencies of the birds’ songs, as well as the low-frequency background noise within 20 seconds of each song, and the overall full-spectrum background noise levels.
“We found that Eastern wood pewees modified their songs to optimize transmission depending on traffic noise levels,” said Dr. Gentry in a press release.
The birds sang shorter songs with a lower frequency to improve acoustic transmission as traffic noises increased. But when the roads were closed, the study birds produced more natural songs with broader bandwidths, lower minimum frequencies and longer duration.
Since the study birds adjusted their songs so they could be heard above traffic noise, it is possible that these altered songs were less likely to generate a response from nearby birds listening in. This, of course, would likely affect these songs’ effectiveness in attracting a mate or defending a territory. But, the study’s authors noted, temporarily closing a road could provide nearby birds with opportunities to briefly “strut their stuff” with optimized vocal performances.
In view of the fact that the Eastern wood pewee population in the Washington DC area has declined by more than 50% over the last 70 years, temporary road closures during strategic times in the breeding season might help to slow or stop the decline of this species. Further, temporary road closures could benefit other songbirds in the area that also must adjust their vocal performances to “match” fluctuating traffic noise levels.
“The results confirm that temporary measures to reduce traffic noise, such as weekend road closures, can benefit animals and is a feasible and effective option for managing traffic noise,” concluded Dr. Gentry.
And don’t forget that temporary road closures would also benefit people who live nearby.
Katherine E. Gentry , Megan F. McKenna and David A. Luther (2017). Evidence of suboscine song plasticity in response to traffic noise fluctuations and temporary road closures, Bioacoustics, published online on 18 April 2017 ahead of print. doi:10.1080/09524622.2017.1303645
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