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Traffic Noise and Birds
 Figure 4. Effects of anthropogenic noise (in this case, traffic noise) on four different auditory behaviors based on the median bird critical ratio function. In the case illustrated for a hypothetical noise level, the distance between a comfortable communication distance and the distances required for recognition, discrimination, and detection are quite large.
represented schematically in Figure 4 showing how the appli- cation of these principles provides a view of how well birds can communicate in a noisy environment. The key to making this work is to know, or be able to estimate, the level of the com- munication signal and the level of the noise at the bird’s ear.
The model described above should be useful for establish- ing quantitative guidelines for the effects of traffic and urban noise on acoustic communication in birds. Again, the required input data are the level of the signal and the noise at the bird’s ear. These thresholds also depend on knowing the spectral characteristics of vocalizations, the distance over which conspecific acoustic communication (e.g., the terri- tory size) normally occurs, and the existing levels of ambient noise. Noise levels that limit the maximum communication distances to a distance that is less than the diameter of the bird’s territory size (or known communication distances in ambient noise) may have serious biological consequences.
The level of natural ambient noise already present in the bird’s environment is a key factor in determining whether additional noise from traffic and other urban activities would have any effect. And variation in territory size, the size of the critical ratio among birds, and natural ambient-noise levels are key variables that make it impossible to use a single noise level as
a one-level-fits-all level in terms of estimating whether traffic or urban noise is limiting communication distance by causing additional masking. As noted above, critical ratios have been measured in songbirds and nonsongbirds and there are species differences. These species differences in critical ratios, whether the birds are singing from treetops in relative open areas, down in the canopy of a dense forest, or at ground level where there is ground absorption, are all factors that would affect these distance estimates.
Can Birds Compensate for Noisy Environments?
Returning again to the noisy restaurant example, humans adopt a variety of strategies to hear better in a noisy envi- ronment (Roy and Siebein, 2019). From our own experience, these include speaking louder, turning the head, moving closer, changing location, or only vocalizing during pauses in the noise. Like humans, birds also exhibit the Lombard effect, which means that they increase vocalization levels in the presence of background noise (Lane and Tranel, 1971). The Lombard effect has been demonstrated in the field in various bird species (Brumm and Todt, 2002; Brumm and Zollinger, 2011) where it has been shown that birds can raise the level of their vocalizations in response to noise by as much as 10 dB (Manabe et al., 1998).
Both laboratory and field data show that birds use remark- ably similar strategies for maximizing communication in noisy environments (California Department of Transportation, 2016; Dooling and Leek, 2018). It is estimated that European black- birds (Turdus merula) and great tits (Parus major) could receive an improvement in the SNR equivalent to the benefit from closing the interbird distance in half by simply moving upward about 9 m to a higher perch (Dabelsteen et al., 1993; Blumenrath and Dabelsteen, 2004).
It is also the case that a receiver moving from a lower posi- tion to a higher position had a greater impact on whether a vocalization was heard than when the sender moved from a lower position to a higher position. Beyond just detecting a sound, there is every reason to think that the other levels of hearing (i.e., discrimination, recognition, and comfort- able communication) would also show similar degrees of enhancements when these various strategies are employed. In a sense then, the distance estimates between communicat- ing birds obtained by applying critical ratios to the problem of two birds communicating in a natural environment represent somewhat of a worst-case scenario. Two birds can utilize a suite of compensatory strategies to enhance the SNR during
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