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Traffic Noise and Birds
 Figure 1. Conceptual relationship between the distance from the noise source and the overlapping effects of noise on hearing and behavior. When the bird is close to the noise source, all four effects (see text for details) are likely to occur. As the animal moves further away, the effects become systematically less problematic. When the noise source is far enough away, only behavioral and/or physiological effects remain as possible responses to noise. PTS, permanent threshold shift; TTS, temporary threshold shift.
a decrease in hearing sensitivity; an increase in stress and steroid hormone levels; changes in foraging location and behavior; interference with acoustic communication between conspecifics; and failure to recognize other important bio- logical signals such as the sounds of predators and/or prey. Any of these effects could have long-term consequences and enduring impacts that would come from the concomitant interference with breeding by individuals and populations, thereby threatening the survival of individuals or species.
Noise Can Have Several Effects
Typically, the negative effects of noise are directly related to the level of the noise that usually decreases with the distance of the listener from the noise source. This leads to a simpli- fied scheme for understanding and predicting the complex, overlapping effects of noise that can occur in real world envi- ronments (Figure 1).
The effects that noise can have include hearing damage and permanent threshold shift (PTS); a temporary threshold shift (TTS) such as when our hearing is temporarily worse after leaving a loud rock concert; masking such as humans experi- ence when conversing in a noisy restaurant; and various other
kinds of physiological effects, stress effects, and distraction that we experience from a TV constantly on in the back- ground. All four of these categories of effects could occur simultaneously if the noise exposure was sufficiently intense.
Less intense exposures, or being located further away from the sound source, might eliminate permanent hearing loss from noise exposure, but the other effects such as a TTS, masking, and other behavioral and physiological effects may remain. At an even greater distance from the sound source, the noise eventually becomes barely audible. But even barely audible sounds (e.g., gun shots, fireworks, lawn mowers) might still be capable of causing stress if the human or animal has had previous negative experience with them.
Extensive laboratory data show that birds are much more resistant to hearing loss, auditory damage, and decline in vocal quality from acoustic overexposure than are humans and other mammals (e.g., Ryals et al., 1999; Saunders and Dooling, 2018). This is in part because birds can regenerate the auditory hair cells of the inner ear that are responsible for hearing even after they have been damaged by intense noise exposure. Birds, unlike mammals (including humans; e.g., Lewis et al., 2016), with damaged auditory hair cells subsequently recover a good deal of their hearing and vocal precision when the damaged hair cells are naturally replaced
with new hair cells (Figure 2; Dooling et al., 2008).
Thus, continuous traffic and urban noise, even if the bird were exposed continuously for extremely long durations (i.e., 72 hours) at extreme levels (i.e., over a 100 dB sound pressure level), is unlikely to cause much of a PTS, hear- ing loss, or permanent auditory damage in birds, whereas such damage is highly likely in humans and other mam- mals (e.g., Murphy, 2016). Figure 2 tracks the changes in the birds’ thresholds in the laboratory during and after exposure to 4 different intensity levels of continuous noise for 72 hours. Within a few minutes of exposure, threshold shift is apparent. After about 12-24 hours, threshold shift reaches an asymptote and no further changes in thresh- old are observed. When the noise stops, hearing begins to recover. Hearing recovers completely within a few weeks for exposure levels of 76, 86, and 96 dB (i.e., it was a TTS) but not for 106 dB, which may have resulted in a PTS of a few decibels, although the birds were not followed for longer than several weeks. Subsequent hair cell regeneration may have resulted in full recovery. There are also species differences in the damaging effects of noise. Canaries and
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