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Now hear this! The gray vireo makes itself heard above humanmade noise by increasing the maximum frequency of the sounds in its songs and by making them last longer.

Clinton Francis

C'mon Feel the Noise

In response to noise pollution, some songbirds change their tunes. But two closely related species have surprised scientists by editing their calls differently, a new study finds.

Birds rely on calls and songs, but human-generated noise threatens to interfere with such communication. There are more than 10,000 known bird species, making it impossible for scientists to test how each will respond to having humans as noisy neighbors. So they try to predict a bird’s response based on how a closely related species reacts. But there’s a catch. “If there are no clear patterns regarding how bird songs change in response to noise among closely related species, it will greatly limit our ability to predict how a species may change its song in new acoustic environments,” says Clinton Francis, an ecologist at the National Evolutionary Synthesis Center in Durham, North Carolina.

To compare closely related species living in noisy environments, Francis and colleagues turned to plumbeous and gray vireo birds in woodland surrounding 26 gas-treatment sites—a mix of sites with noisy compressors as well as quiet control sites—in Rattlesnake Canyon Habitat Management Area in New Mexico in mid-2009. The setting was ideal because the sound made by the gas compressors, like most human-generated noise, is mostly in the lower frequency range that overlaps with those used by birds for communication. Furthermore, the number of people and the types of vegetation present were similar in both the noisy and the quiet locations, eliminating the chance these factors could skew any results. “Because these two vireos are closely related, we expected them to respond in the same way to the noise disturbance,” Francis says.

However, the two birds responded to the noise of the pumps in dramatically different ways, the scientists report in Biology Letters. The plumbeous vireo increased the minimum frequency of the sounds in its songs, reducing the overall range of frequencies used. In contrast, the gray vireo increased the maximum frequency of its cheery tunes, increasing its frequency range. Furthermore, the researchers found that song duration changed with increasing noise for both species but in opposite ways. The duration of the plumbeous vireo’s songs decreased with increasing noise, whereas the gray vireo’s songs were nearly 1.5 times longer than usual in the noisiest areas. Based on previous research into performance constraints for bird songs, the plumbeous vireo may not be able to sing as long when it increases the minimum frequency of its songs, the scientists speculate.

However, behavioral ecologist Jeffrey Podos of the University of Massachusetts, Amherst, says he is skeptical that both species like the noisy and quiet sites equally. “Yes, the two species are from the same genus, but the authors' own data suggest that these species differ in their willingness to live near the noisy compressors,” Podos says. “I thus don't follow the initial prediction that the two species should react similarly to masking noise.” Francis says that his team has previously found that gray vireos avoid nesting in noisy environments, as Podos mentions. But he believes the smaller number of nests could be due to the birds failing to secure a long-term mate in the noisy surroundings and that they still happily inhabit these environments, just like the plumbeous vireos.