Our own common experience suggests that the adverse effects of noise on birds can be considered with regard to four potentially overlapping categories. First, noise might be annoying to birds. This may cause them to abandon a particular site that is otherwise ideal in terms of food availability, breeding opportunities, etc. Second, noise which lasts for very long periods of time can be stressful. Such noise levels can raise the level of stress hormones, interfere with sleep and other activities, etc. Thirdly, very intense noise (acoustic overexposure) can cause permanent injury to the auditory system. Finally, noise can interfere with acoustic communication by masking important sounds or sound components. The first two categories of investigation are probably best addressed by field experiments. The second two categories of effects are probably best addressed by laboratory experiments where precise control can be obtained. The results of some of these experiments are described in this paper.

Perception of song syllables taken from natural song sparrow and swamp sparrow songs was examined in five different species: song sparrows (Melospiza melodia), swamp sparrows (Melospiza georgiana), zebra finches {Taeniopygia guttata catanotis), canaries iSerinus canaria), and budgerigars (Melopsittacus undulatus). Using operant conditioning techniques, we trained these birds to discriminate among sparrow song syllables. By testing the birds on all possible pair-wise combinations of syllables, we generated "similarity" matrices for these song syllables from the birds'response latencies to detect changes in a repeating background of syllables. A number of different statistical techniques were used to examine species differences in perception including factor analysis of an inter-species correlation matrix, cluster analysis ofthe similarity matrices, and individual-differences multidimensional scaling (MDS) of the similarity matrices from each species. The spatial map of these stimuli produced by MDS revealed the perceived relations among these syllables for each species. Several acoustic measurements taken from these syllables were correlated with stimulus coordinates of the syllables in multidimensional space. These results demonstrate that natural vocal signals can be effectively used as stimuli in psychoacoustic experiments and the outcome of such experiments are likely to reveal robust species differences in perception.

Budgerigars were trained with operant conditioning procedures to discriminate among sets of calls from several species in a Same-Different task. Response latencies from this task were analyzed in several ways including multidimensional scaling (MDS) and cluster analysis. The pattern of response latencies from budgerigars reared in a large group of conspecifics was compared to that of budgerigars reared in acoustic and social isolation. Results show that budgerigars with previous experience with species-specific vocalizations and isolate-reared birds who had never heard such sounds can both discriminate among the categories of species-specific vocal signals. But, results from MDS and cluster analysis also show that rearing budgerigars in isolation has subtle effects on the perception of these categories of vocal signals.