Monday, 28 November 2011

Anthropogenic noise prompts both short- and long-term changes in birdsong

As increasingly more bird species are found to alter their vocalizations in the presence of anthropogenic noise, researchers are developing more detailed questions about the process by which this behavioral modification occurs. One of these questions focuses on the time scale over which these noise adjustments happen: Do birds in noisy areas only vary their vocalizations during specific noisy events, or do noise-disturbed birds always sing different songs than their undisturbed counterparts? Results of a recent study by collaborators from the University of Ottawa and the University of Windsor suggest that, in one species at least, the answer is “both.”

That species is the red-winged blackbird, Agelaius phoeniceus, which is often found living in marshes alongside noisy roads. Singing male blackbirds produce songs across a 1-5 kHz frequency range, the lower end of which overlaps with the frequencies dominated by anthropogenic noise. Songs comprise a few introductory syllables and a broadband trill; the latter song element appears to be the most important part of the song, as it facilitates species recognition. In the current study, the researchers investigated whether, and how, anthropogenic noise influences the structure of these trills.

(A male red-winged blackbird, Agelaius phoeniceus)

The study involved two separate experiments. In the first, the researchers located males in marshes that are not exposed to human noises. After placing a set of speakers near each territorial male, the scientists then recorded songs produced against a background of no noise (a silent control treatment) and 89-dB, low-frequency white noise (approximating traffic). This allowed them to investigate whether individual birds were capable of “real-time” shifts in response to increasing levels of background noise. In the second experiment, the researchers compared songs produced by males in undisturbed, quiet marshes with those performed by roadside males singing in temporarily quiet periods. This part of the study was aimed at determining whether all blackbirds sing similar songs during quiet conditions, or whether long-term exposure to generally louder ambient noise environments leads disturbed birds to make permanent shifts in their vocalizations.

Previous studies have shown that birds may alter a variety of song parameters in response to noise. Thus, the authors of the current study measured several different variables associated with each recorded trill: duration, minimum frequency, maximum frequency, and energy distribution of the spectrum (in other words, which frequency the birds produce at the highest amplitude). They also performed two types of validation in order to make sure that their study techniques were sound. The first was a basic comparison of average environmental noise levels at roadside versus “undisturbed” marshes; the former were verified as louder than the latter (~65.8 dB compared to ~51.7 dB). The second validation investigated whether the experimental playback condition may have artificially influenced results because of the difficulty of accurately measuring birdsong against a background of white noise; the experimental setup again passed the test, as the results indicated that the patterns measured here were not merely an artifact of study design.

(Roadside marshes provide habitat for red-winged blackbirds--and may also provide considerable levels of background noise)

For the first experiment, the researchers analyzed 140 songs from 20 male blackbirds. Of all the song characteristics measured, only one was influenced by the introduction of experimental noise: entropy, which decreased in response to noise. This variable is a measure of song “randomness;” purer tones have a value close to 0, while white noise has a value closer to 1. This entropic shift resulted from the birds’ concentrating their spectral energy at lower frequencies. The second experiment, which analyzed 344 songs from 63 males, found very similar patterns as those observed in the first half of the study. The only variable associated with habitat type was trill entropy, as trills at roadside marshes were more tonal than trills at undisturbed sites, and, again, this was caused by a concentration of spectral energy at lower frequencies.

Taken together, the results of these two experiments indicate that red-winged blackbirds exposed to anthropogenic noise are capable of both short-term behavioral plasticity and maintenance of long-term behavioral differences, the latter of which may even be a sign of adaptation. Although this is the first report that tonality is influenced by human noise pollution, previous studies have shown that natural noise has similar effects (for instance, in large-billed leaf warblers, Phylloscopus magnirostris and several species of frog, all living near loud running water in the Himalayas).

(Large-billed leaf warbler, Phylloscopus magnirostris--another species that adjusts the tonality of its vocalizations in response to ambient noise)

Given all of the ways in which the blackbirds could have altered their songs, it is interesting to ponder why they focused only on entropy. Some possibilities are associated with the detectability of the signal: Higher frequencies attenuate more rapidly when transmitting through the environment, so the redistribution of spectral energy to the lower frequencies may enhance signal transmission. Additionally, narrow-band signals (such as pure tones) are easier to discriminate from background noise, making more tonal songs more receiver-friendly. Other possibilities are associated with the potential costs and constraints associated with non-entropy-related alternatives: Perhaps the birds’ morphologies prevent them from achieving some song qualities, or maybe it requires too much energy to produce certain types of songs. Of course, it is also possible that the manipulations measured here were merely a side effect of some other, unmeasured, variable.

Other important costs may be those associated with signal transmission and receiver discrimination. Even for the birds that manipulated their songs in the presence of noise, it is possible that these vocal acrobatics were not enough to prevent their songs from being masked by anthropogenic noise. If so, we might expect to see that males in noisier locations have more difficulty wooing females and/or producing successful broods. Depending on how the birds choose mates and habitats, it is feasible that noise could eventually drive the formation of different subspecies of blackbirds. All of these possibilities have serious conservation and management implications, causing the authors of the current research to recommend follow-up studies in the future.

Hanna, D., Blouin-Demers, G., Wilson, D.R., Mennill, D.J. 2011. Anthropogenic noise affects song structure in red-winged blackbirds (Agelaius phoeniceus). The Journal of Experimental Biology 214:3549-3556.

Thanks to the following websites for providing the images used in this post:

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