This study investigated the masking of pure tones by anthropogenic noises in humans and birds. Bird experiments were conducted in the laboratory using operant conditioning and psychophysical procedures but with anthropogenic noises rather than white noise. Humans were tested using equivalent psychophysical procedures in the field with ambient background noise. Results show that for both humans and birds published critical ratios can be used to predict the masking thresholds for pure tones by these complex noises. Thus, the species' critical ratio can be used to estimate the effect of anthropogenic environmental noises on the perception of communication and other biologically relevant sounds.

All environments are noisy, and auditory systems have evolved to cope with this noise. Indeed all sensory systems employ mechanisms that facilitate the separation of relevant signals from irrelevant noise. Interestingly, most of what we know about hearing comes from tests conducted in the near absolute quiet of an acoustic test booth. Because of their tractability in the laboratory, their complex vocal repertoires, and their elaborate acoustic communication systems, birds have proven valuable models for understanding the effects of noise on hearing and acoustic communication in part by bringing laboratory and field studies together. Noise can have at least four different kinds of effects occurring either alone or together. These four categories of effects are hearing damage and permanent threshold shift (PTS) from acoustic overexposure, temporary threshold shift (TTS) from acoustic overexposure, masking of acoustic communication signals (or other biologically relevant sounds), and a host of other physiological and behavioral responses including effects on attention. Here we consider masking as separate from these other effects of noise on hearing and acoustic communication. Furthermore, we take an 'auditory-centric' point of view and consider masking exclusively from the point of view of the listening bird. We review the behavioral and auditory strategies that birds use to maximize communication in a noisy environment and suggest an approach to assessing the risk posed by noise, whether natural or anthropogenic.

Anthropogenic noises can cause a variety of adverse effects on birds and other wildlife. These effects include stress and physiological changes, auditory system damage from acoustic overexposure, and masking of communication and other important biological sounds. A second reason is that, while all humans have similar auditory capabilities and sensitivities, the same is not true for all animals. Still another issue is separating the various effects of noise. There are well documented adverse consequences of elevated noise on humans including hearing loss, masking, stress, physiological and sleep disturbances, and changes in feelings of well-being, and it would not be too surprising to find a similar range of effects in animals. The simplest kind of masking experiment is to measure the sound detection thresholds for pure tones in the presence of a broadband noise.

Animals, like humans, frequently communicate using long-range acoustic signals in networks of several individuals. In socially and acoustically complex environments, however, communication is characterized by a variety of perceptual challenges that animals strive to overcome in order to interact successfully with conspecifics. Species differences in auditory sensitivity and the characteristics of the environment are major factors in predicting whether environmental noise limits communication between animals or interferes with detection of other biologically important sounds. Working with both birds and humans and using both synthetic and natural noises in both laboratory and field tests, we have developed a model for predicting the effects of particular masking noises on animal communication. Moreover, by comparing birds listening to bird vocalizations in noise with humans listening to speech in noise, we gain a novel intuitive feel for the challenges facing animals in noisy environments. This approach of considering communication from the standpoint of the receiver provides a better approach for understanding the effects of anthropogenic noises that exceed ambient levels. For instance, in determining risk to a particular species, effective communication distances derived from this model might be compared to other aspects of the species biology such as territory size.

In reverberant environments, the auditory system processes complex signals that are distorted and overlapped by tails of echoes of preceding sounds. These reverberation effects can be used to discriminate between different types of acoustic environments and to locate a sound source, but it also limits the transfer of biologically important information. To examine the positive impact of reverberation on acoustic scene analysis, we used psychophysical methods to test the ability of canaries, zebra finches, and budgerigars to discriminate between reverberation differences. Negative effects were investigated by asking whether birds can segregate reverberated songs from multiple individuals despite degraded auditory grouping cues. Birds were able to hear small changes in reverberation, although this ability was somewhat dependent on both the species and, in particular, the type of signal. Species-specific vocalizations as well as broadband signals posed the smallest difficulty. Interestingly, the non-oscine, flock-living budgerigar out-performed the other two oscine species. Moreover, birds were less able to “hear out” a target song among a cacophony of other songs when all vocalizations were reverberated. The results suggest both positive and negative impact of reverberant environments on the perception of vocal signals and the analysis of complex acoustic scenes in the tested species.

Birds in natural habitats using long-range acoustic signals experience substantial acoustic alterations of their signals when transmitted from a sender to a receiver. The main degradation comes from reflections of the emitted sounds from surfaces in and around the transmission pathway. These reverberations cause tails of echoes and affect frequency and amplitude modulation within the sounds’ original time frame. Humans discriminate among reverberant and nonreverberant speech segments and use the nature of reverberation in determining the location of the sound source. Using operant conditioning and psychophysical methods, we investigated the ability of several species of small birds to discriminate between nondegraded sounds and sounds artificially reverberated to varying degrees, while holding other sound aspects constant. Birds were able to easily discriminate between nonreverberant sounds and sounds reverberated to various extents. By comparing the birds’ performance on stimuli varying along different dimensions of reverberation, we can make predictions as to the birds ability to use reverberations as acoustic cues in distance estimation and locating sound sources. [Work supported by NICD DC000198 (to R.J.D.) and DC004664.]

Birds often sing from high perches referred to as song posts. However, birds also listen and keep a lookout from these perches. We used a sound transmission experiment to investigate the changes for receiving and sending conditions that a territorial songbird may experience by moving upwards in the vegetation. Representative song elements of the blackcap Sylvia atricapilla were transmitted in a forest habitat in spring using a complete factorial design with natural transmission distances and speaker and microphone heights. Four aspects of sound degradation were quantified: signal-to-noise ratio, excess attenuation, distortion within the sounds determined as a blur ratio, and prolongation of the sounds with “tails” of echoes determined as a tail-to-signal ratio. All four measures indicated that degradation decreased with speaker and microphone height. However, the decrease was considerably higher for the microphone than for the speaker. This suggests that choosing high perches in a forest at spring results in more benefits to blackcaps in terms of improved communication conditions when they act as receivers than as senders.

Birdsong is degraded as it propagates through the habitat. This affects its use as a signal in communication networks, depending on song type as well as sender and receiver location. Nest holes constitute special receiver locations to females of many hole-nesting species. Although such locations are frequently used by females during their fertile period, for instance at dawn when information gathering from singing males may be essential, the conditions for receiving sounds inside nest holes are currently unknown. We investigated these conditions with a sound transmission experiment, in which great tit songs were broadcast in a deciduous forest and re-recorded from both outside and inside a nest box. Several aspects of sound degradation encompassing signal attenuation and distortion were quantified. Attenuation was strongest inside the nest box for both song and background noise. However, the signal-to-noise ratio, which affects song detection and discrimination, was only slightly reduced inside the nest box. Signal distortion, which has implications for both information transfer and ranging, provided conflicting results for the two types of song notes, the highly modulated buzz notes and the less modulated pure notes. These results provide the first evidence that entering nest boxes complicates song reception conditions for female songbirds. We suggest that this may ultimately affect vocally mediated information gathering in the network.

Songbirds living in temperate forests experience great seasonal changes in habitat acoustics during the part of the breeding season when singing activity is high. These changes, which are brought about by accelerated vegetation growth and leaf burst in spring, affect sound propagation and potentially render vocal communication more difficult as the total number of scattering and absorbing obstacles increases. We investigated this in a sound transmission experiment in which representative great tit (Parus major) songs were broadcast in a typical forest habitat before and after foliation. Speaker and microphone were placed at natural separation distances and in typical sender and receiver positions. For each song note we quantified several aspects of sound degradation and found that they all increased considerably when leaves were present. Before foliation the same amount of degradation would only be obtained by doubling the transmission distance, i.e. foliage shortens the active space of are. at tit song. This inevitably alters distance information, provided that distance-dependent, structural changes of received songs are used as ranging cues. Moreover, sender and receiver positions within the canopy become unfavourable compared to heights just below the canopy when the aim is to maximise song propagation distances. Altogether, the presence of foliage greatly affects the potential for vocal information transfer in great tits and requires behavioural and/or perceptual adjustment of the communicating individuals to counteract or reduce the impact of foliage on signal degradation.