Friday, 27 January 2012

How do noisy miners cope with noise?

Vocal differences between urban and rural birds have frequently been documented, as have variations in the vocalizations of birds living in different types of "natural" habitat. Based on these findings, it seems logical to assume that birds in different types of urban habitat should have measurably different vocal characteristics from each other, but this possibility has only rarely been studied. In fact, no research at all has addressed whether birds in quieter and louder territories within an urban setting respond to local noise regimes by delivering quieter and louder vocalizations, respectively. This particular vocal manipulation is known as the "Lombard effect," and is thought to be one of the most effective ways to prevent being masked by ambient noise.

(A noisy miner, Manorina melanocephala. Alternative and more poetic names include "garrulous honey-eater," "black-headed miner," "snakebird," "cherry-eater," "mickey," and "squeaker.")

Researchers from Australia's Monash University recently investigated use of the Lombard effect among noisy miners (Manorina melanocephala) living in two types of urban habitat in Melbourne: in vegetation along either major roadways or residential streets. Miners, native Australian honeyeaters, can easily be recognized by the loud alarm calls they use to warn each other of potential danger. Miners live in large aggregations, or colonies--80 of which were examined in the current study. Each was visited on a weekday morning when traffic was heaviest and birds were most vocally active. One researcher, in the company of a potential "predator" (her dog Persephone), approached a focal bird until it began to produce alarm calls. These, along with the local ambient noise regime, were then recorded. Analysis of the recordings revealed the minimum, maximum, and mean amplitude of each; by measuring the distance between the microphone and the vocal bird, the researchers were able to standardize their amplitude measurements so they reflected volumes at 1 m from each bird. While visiting each site, the scientists also observed other behaviors related to miner communication, including how high the birds perched and whether they delivered calls while in flight.

(The Melbourne skyline, with arterial roads in the foreground)

Comparisons of the ambient noise data showed that environmental noise levels differed significantly between arterial and residential sites; specifically, arterial roads were an average of 15 dB louder than those in residential neighborhoods (66 vs. 51 dB). Correspondingly, noisy miners in colonies along arterial roads delivered alarm calls that were an average of 9 dB louder than those of their residential road counterparts (89 dB vs. 80 dB). Analyses of all calls across all noise regimes revealed that alarm call volume was significantly correlated with background noise level; in other words, no matter where the miners lived, they tailored their calls to suit their habitat.

The birds did not alter call-related behaviors such as choice of perch height and use of in-flight calling. However, there were some interesting relationships between habitat type and choice of alarm call. In arterial road colonies, miners were more likely to use "alarm call 2", while birds in residential areas were more likely to use "alarm call 3." Both calls signal the approach of a terrestrial predator and consist of loud, repeated single notes; the main difference is that the latter sounds more "husky." It is not yet clear why miners in different areas should prefer to use different calls, since acoustic analyses suggest that they are equally likely to be masked by ambient noise.

 (A small flock of noisy miners)

In Melbourne and elsewhere in Australia, miners are a very successful colonizer of anthropogenic habitats. The results of the current research demonstrate that this may, in part, result from the birds' adaptability to human noise regimes. By altering their call amplitudes when necessary, the birds can prevent their warning signals from being masked by the sounds of nearby traffic. Interestingly, a follow-up analysis revealed that signal-to-noise ratios (SNR) in arterial roadway sites were lower than those in residential areas, indicating that the arterial birds may merely be lessening the negative impacts of noise rather than completely preventing them. Perhaps the reduction in SNR is not harmful given the distances over which these birds are communicating, or maybe the miners have simply reached the limits of what amplitudes they can achieve. Further work will be needed to investigate these, and other, possibilities, as well as the consequences of the reduced SNR.

Previous studies on other species have documented a close link between body size and the energetic cost of vocalizing at high amplitudes. While many small birds can produce calls and songs at even louder amplitudes than those recorded here for miners, it is more metabolically demanding for little species than for the relatively sizable miner. As a result, the authors of the current study suggest that smaller birds may be particularly susceptible to anthropogenic noise regimes--another possibility that will need to be examined in future research.

For supplemental images related to this post, please visit the Anthrophysis pin board at Pinterest

Lowry, H., Lill, A., and Wong, B.B.M. 2012. How noisy does a noisy miner have to be? Amplitude adjustments of alarm calls in an avian urban "adapter." PLoS ONE: 7(1):e29960.

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