Vocal responses of two songbirds to human-generated noise

(Rattlesnake Canyon, New Mexico)

Anyone who has spent any time in a city knows that humans, and human activities, can be loud. This isn't just the case in urban centers, but also in more remote environments--for example, northwest New Mexico's Rattlesnake Canyon Habitat Management Area, which contains natural gas wells that make noise in excess of 95 dB(A)--well past the level that is considered potentially dangerous for employees in noisy occupations.

Just as that noise can prove hazardous for humans, it may also have negative impacts on wildlife, in some surprising ways. Researchers from the National Evolutionary Synthesis Center, the San Juan Institute of Natural and Cultural Resources, and the University of Colorado, Boulder have investigated the effects of gas well noise on two closely related species of songbird--the plumbeous vireo (Vireo plumbeus), and the grey vireo (Vireo vicinior).

(Plumbeous vireo, Vireo plumbeus)

(Grey vireo, Vireo vicinior)

Males of these species use song to attract potential mates and warn off rivals, so living in a noisy environment where their songs may be "masked" (acoustically obscured by) anthropogenic noise could challenge their ability to maintain a territory (which provides trees to roost in and food to eat) and to acquire a breeding partner. Thus, researchers predicted that habitat occupancy should decrease with increasing levels of gas well noise. However, among birds remaining in the noisier areas, the researches expected to observe some changes to standard song parameters--variations in song pitch, duration, and singing rate--indicating that the birds were attempting to improve their ability to communicate in the midst of noise pollution.

Unexpectedly, numbers of both plumbeous and grey vireos were similar across all habitats surveyed, regardless of noise level. In other words, in contrast to some of the other bird species examined in previous studies, the vireos put up with intense levels of noise in their environment. Both types of bird were found to vary their song parameters with respect to background noise levels, suggesting that perhaps they did not need to relocate because they could mitigate the amount of masking done by the noise. While plumbeous vireos increased their minimum song frequency by over 700 Hz, grey vireos increased their maximum song frequency by over 900 Hz. Plumbeous vireos also sang shorter songs in response to louder ambient noise, while grey vireos sang songs that were nearly 1.5 times longer.

Perhaps the most encouraging interpretation of these results is that at least some species are capable of altering their behaviors in response to potentially damaging human activity. However, it remains to be seen whether these alterations are actually useful--it is possible that the adjustments do not improve the singers' detectability enough to allow them to maintain "normal" relationships with other birds. It is intriguing that two species that are so closely related display such different responses to environmental noise. Unfortunately, that means it may be difficult to extrapolate these results to other species living in other types of noisy environment.

Luckily, this sort of study is currently underway on a variety of species in several countries, meaning that researchers may soon have enough data to identify general patterns that apply to all birds--and perhaps even to other acoustically-communicating animals, as well, including (but not limited to!) insects, mammals, and amphibians.

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Francis, Clinton D., Ortega, Catherine P., and Cruz, Alexander. Different behavioural responses to anthropogenic noise by two closely related passerine birds. Biology Letters. Online advance publication.

Thanks to the following websites for the images used in this post: http://www.birdseyeviewgis.com/blog/2011/5/15/a-day-of-hiking-and-birding-after-a-long-week-of-gis.html
http://ibc.lynxeds.com/photo/grey-vireo-vireo-vicinior/small-tress-trailhead
http://www.summitpost.org/bandelier-national-nm-guadalupe-np-and-carlsbad-np-photo-trip-report/607872