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zebra finches recover completely from a 120 dB continuous exposure within a few weeks, whereas budgerigars still have a 10 dB threshold shift several weeks into recovery. A con- siderable amount is known from such laboratory studies on the growth and recovery of TTS from noise exposure (Ryals et al., 1999; Saunders and Dooling, 2018). Taken together, it is quite clear from these laboratory studies that even in rare instances where birds may remain close to high levels of traffic or urban noise sources for a few hours, it is unlikely to cause permanent hearing loss or auditory damage.
Masking Effects on Communication Can Easily be Underestimated
As the distance from the traffic or anthropogenic noise source increases, the level of the noise exposure usually decreases, and, therefore, birds will have less risk of a TTS from noise. It is tempting to think that there is no risk at these lower noise levels. But there still can be considerable risk if the added noise from traffic is above the natural ambient-noise level. By masking critical sounds, this added noise could seriously interfere with a bird’s ability to detect prey, assess its acoustic environment (i.e., auditory scene), and communicate with
other birds. Fortunately, there are ways to estimate this risk. Highway engineers can precisely estimate and measure the noise caused by various highways at different distances from the highway. And birds can have their hearing tested in the laboratory, with their thresholds measured precisely in the quiet of an auditory testing booth. Figure 3 shows a bird’s hearing capabilities, displayed as an audiogram, in relation to a typical spectrum of traffic noise.
There is both bad news and good news in Figure 3. The bad news for humans is that much of the energy in traffic noise is in the frequency regions important for human speech communication. The good news for birds is that most of the energy in bird vocalizations is at higher frequencies than traf- fic noise. Still, traffic noise can often include enough energy in the bird’s region of best hearing that at close distances, it can have a significant impact on how well birds can hear their species-specific vocalizations.
To return to the noisy restaurant example, the speech from a talker must be at a certain overall level in relation to the over- all level of the background sounds produced by other talkers in order to be heard. This is called the signal-to-noise ratio
Figure 2. Growth and decay of threshold shift (TS) from a 72-hour continuous noise exposure in birds. Birds recover their hearing completely within 8 hours for the lowest exposure levels and within 2 days for the 96 dB exposure level. They are left with a 5-10 dB PTS weeks after cessation of exposure to a 106 dB noise.
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