Speech perception by budgerigars (Melopsittacus undulatus): spoken vowels

Discrimination of natural, sustained vowels was studied in 5 budgerigars. The birds were trained using operant conditioning procedures on a same-different task, which was structured so that response latencies would provide a measure of stimulus similarity. These response latencies were used to construct similarity matrices, which were then analyzed by multidimensional scaling (MDS) procedures. MDS produced spatial maps of these speech sounds where perceptual similarity was represented by spatial proximity. The results of the three experiments suggest that budgerigars perceive natural, spoken vowels according to phonetic categories, find the acoustic differences among different talkers less salient than the acoustic differences among vowel categories, and use formant frequencies in making these complex discriminations.     

Perceptual organization of acoustic stimuli by budgerigars (Melopsittacus undulatus): III. Contact calls

We trained budgerigars by operant conditioning to discriminate among a set of contact calls in a same-different task and analyzed response latencies from this task by using multidimensional-scaling (MDS) and cluster-analysis procedures. Humans listened to the same calls and indicated the similarity between pairs of calls by a direct rating procedure. An MDS program (sindscal) was used to arrange these complex acoustic stimuli in multidimensional space reflecting perceptual organization. Multiple regression techniques were used to identify the acoustic characteristics of contact calls that were correlated with the perceptual dimensions obtained from MDS. A number of spectral characteristics (e.g., peak frequency, rate of frequency modulation, and concentration of spectral energy) emerged as important for both budgerigars and humans, but the relative salience of these cues differed for the two species. Additional tests with two groups of budgerigars--cagemates and noncagemates--showed that experience with calls can change the salience of various acoustic characteristics used for perceptual organization and individual recognition.     

Perceptual organization of acoustic stimuli by budgerigars (Melopsittacus undulatus): II. Vocal signals

Operant conditioning and multidimensional scaling procedures were used to study auditory perception of complex sounds in the budgerigar. In a same-different discrimination task, budgerigars learned to discriminate among natural vocal signals. Multidimensional scaling procedures were used to arrange these complex acoustic stimuli in a two-dimensional space reflecting perceptual organization. Results show that budgerigars group vocal stimuli according to functional and acoustical categories. Studies with only contact calls show that birds also make within-category discriminations. The acoustic cues in contact calls most salient to budgerigars appear to be quite complex. There is a suggestion that the sex of the signaler may also be encoded in these calls. The results from budgerigars were compared with the results from humans tested on some of the same sets of complex sounds.     

Perception of conspecific faces by budgerigars (Melopsittacus undulatus): I. Natural faces.

Perception of faces by 4 budgerigars (Melopsittacus undulatus), a species of small parrot, was studied with a same-different discrimination task. Reaction times were taken as a measure of the similarity between pairs of faces and analyzed with multidimensional scaling to reveal patterns of similarity among the faces. The perception of natural faces was tested to determine which characteristics were perceptually salient. Color, patterns of markings, darkness of the iris, and size of the pupil corresponded to the observed patterns of similarity among the faces. Differences among budgerigar faces were more salient than differences among zebra finch faces, and budgerigar faces were perceptually distinct from the faces of other avian species. The results from these experiments provide a basis for understanding the ways in which these signals function in the coordination of social behaviors.     

Perceptual organization of acoustic stimuli by budgerigars (Melopsittacus undulatus): I. Pure tones

A new combination of operant conditioning and psychophysical scaling procedures was used to study auditory perception in a small bird. In a same-different discrimination task, budgerigars learned to discriminate among pure tones that varied along one or more acoustic dimensions. Response latencies were used to generate a matrix of interstimulus similarities. Multidimensional scaling procedures were used to arrange these acoustic stimuli in a multidimensional space that supposedly reflects the bird's perceptual organization. For tones that varied in intensity, duration, and frequency simultaneously, budgerigars were much more sensitive to frequency changes. From a set of tones that varied only in intensity, it was possible to calculate the growth of loudness with intensity for the budgerigar. For tones that varied only in frequency, budgerigars showed evidence of an "acoustic fovea" for frequency change in the spectral region of 2-4 kHz. Budgerigars and humans also differed in their perceptual grouping of tone sequences that rise, fall, or remain constant in pitch. Surprisingly, budgerigars were much less responsive to pitch contour than were humans.     

Perception of warble song in budgerigars (Melopsittacus undulatus): evidence for special processing

The long, rambling warble song of male budgerigars is composed of a large number of acoustically complex elements uttered in streams lasting minutes a time and accompanied by various courtship behaviors. Warble song has no obvious sequential structure or patterned repetition of elements, raising questions as to which aspects of it are perceptually salient, whether budgerigars can detect changes in natural warble streams, and to what extent these capabilities are species-specific. Using operant conditioning and a psychophysical paradigm, we examined the sensitivity of budgerigars, canaries, and zebra finches to changes in long (>6?min) natural warble sequences of a male budgerigar. All three species could detect a single insertion of pure tones, zebra finch song syllables, budgerigar contact calls, or warble elements from another budgerigar’s warble. In each case, budgerigars were more sensitive to these changes than were canaries or finches. When warble elements from the ongoing warble stream were used as targets and inserted, out of order, into the natural warble stream so that the only cue available was the violation of the natural ordering of warble elements, only budgerigars performed above chance. When the experiment was repeated with all the ongoing warble stream elements presented in random order, the performance of budgerigars fell to chance. These results show species-specific advantages in budgerigars for detecting acoustic changes in natural warble sequences and indicate at least a limited sensitivity to sequential rules governing the structure of their species-specific warble songs.     

Perception of distance calls by budgerigars (Melopsittacus undulatus) and zebra finches (Poephila guttata): assessing species-specific advantages

Budgerigars (Melopsittacus undulatus) and zebra finches (Poephila guttata) share a common functional class of vocalizations called distance calls. The perception of species-specific distance calls by both species was measured with a habituation-dishabituation operant paradigm. Changes in discrimination performance were noted as listening conditions were modified or stimulus properties were altered. Both species showed better performance for calls of their own species. For zebra finches this tendency increased slightly when a background noise was added to the testing environment. Shifting the frequency region of the calls affected the discrimination performance of male budgerigars but not females or zebra finches. Reversing the temporal order of the calls affected the perceptual advantage for conspecific vocalizations in zebra finches but not in budgerigars. These results highlight species differences in perceiving acoustic communication signals.     

Acoustic and perceptual categories of vocal elements in the warble song of budgerigars (Melopsittacus undulatus)

The warble songs of budgerigars (Melopsittacus undulatus) are composed of a number of complex, variable acoustic elements that are sung by male birds in intimate courtship contexts for periods lasting up to several minutes. If these variable acoustic elements can be assigned to distinct acoustic-perceptual categories, it provides the opportunity to explore whether birds are perceptually sensitive to the proportion or sequential combination of warble elements belonging to different categories. By the inspection of spectrograms and by listening to recordings, humans assigned the acoustic elements in budgerigar warble from several birds to eight broad, overlapping categories. A neural-network program was developed and trained on these warble elements to simulate human categorization. The classification reliability between human raters and between human raters and the neural network classifier was better than 80% both within and across birds. Using operant conditioning and a psychophysical task, budgerigars were tested on large sets of these elements from different acoustic categories and different individuals. The birds consistently showed high discriminability for pairs of warble elements drawn from between acoustic categories and low discriminability for pairs drawn from within acoustic categories. With warble elements reliably assigned to different acoustic categories by humans and birds, it affords the opportunity to ask questions about the ordering of elements in natural warble streams and the perceptual significance of this ordering.     

Auditory disturbances promote temporal clustering of yawning and stretching in small groups of budgerigars (Melopsittacus undulatus)

Yawning may serve both social and nonsocial functions. When budgerigars (Melopsittacus undulatus) are briefly held, simulating capture by a predator, the temporal pattern of yawning changes. When this species is observed in a naturalistic setting (undisturbed flock), yawning and also stretching, a related behavior, are mildly contagious. On the basis of these findings, we hypothesized that a stressful event would be followed by the clustering of these behaviors in a group of birds, which may be facilitated both by a standard pattern of responding to a startling stressor and also contagion. In this study, we measured yawning and stretching in 4-bird groups following a nonspecific stressor (loud white noise) for a period of 1 hr, determining whether auditory disturbances alter the timing and frequency of these behaviors. Our results show that stretching, and to a lesser degree yawning, were nonrandomly clumped in time following the auditory disturbances, indicating that the temporal clustering is sensitive to, and enhanced by, environmental stressors while in small groups. No decrease in yawning such as found after handling stress was observed immediately after the loud noise but a similar increase in yawning 20 min after was observed. Future research is required to tease apart the roles of behavioral contagion and a time-setting effect following a startle in this species. This research is of interest because of the potential role that temporal clumping of yawning and stretching could play in both the collective detection of, and response to, local disturbances or predation threats.     

Effects of deafening on the development of nestling and juvenile vocalizations in budgerigars (Melopsittacus undulatus).

The effects of complete and partial cochlear extirpation at ages 9-11 days posthatch were assessed in 5 nestling budgerigars (Melopsittacus undulatus) to determine if auditory feedback is necessary for the production of nestling vocalizations. Although early deafening had no effect on the production of food-begging calls produced during the first 2 weeks posthatch, deafening did disrupt the expected transition from these early calls to the longer and more complex frequency-modulated, patterned food-begging calls normally appearing 3-4 weeks posthatch. All birds sustaining either complete or partial cochlear extirpation failed to develop stereotyped contact calls around the time of fledging at 5 weeks. These results are consistent with previous research showing that deafened nestlings do not develop normal contact calls (R. J. Dooling, B. F. Gephart, P. H. Price, C. McHale, & S. E. Brauth, 1987) and also indicate that a form of sensorimotor learning is involved in the production of mature, patterned food-begging calls in budgerigars.     

The discrimination of natural movement by budgerigars (Melopsittacus undulates) and pigeons (Columba livia)

Three experiments examined the ability of birds to discriminate between the actions of walking forwards and backwards as demonstrated by video clips of a human walking a dog. Experiment 1 revealed that budgerigars (Melopsittacus undulates) could discriminate between these actions when the demonstrators moved consistently from left to right. Test trials then revealed that the discrimination transferred, without additional training, to clips of the demonstrators moving from right to left. Experiment 2 replicated the findings from Experiment 1 except that the demonstrators walked as if on a treadmill in the center of the display screen. The results from the first 2 experiments were replicated with pigeons in Experiment 3. The results cannot be explained if it is assumed that animals rely on static cues, such as those derived from individual postures, in order to discriminate between the actions of another animal. Instead, this type of discrimination appears to be controlled by dynamic cues derived from changes in the posture of the demonstrators.     

Nonsimultaneous auditory masking in the budgerigar (Melopsittacus undulatus)

The frequency resolving power of budgerigars and that of humans were compared on several nonsimultaneous masking procedures in which one pure tone is used to mask another. Similar patterns of frequency selectivity were found for all three masking procedures (forward, backward, and combined forward-backward) for both species. Budgerigars showed considerably greater frequency resolving power on all three procedures than humans. Budgerigars also showed differences in frequency resolving power across masking conditions, but human listeners did not. These results indicate that the budgerigar auditory system may be even more highly tuned than was previously thought and suggest fundamental differences between the mechanisms of frequency selectivity of bird and those of humans.     

Budgerigars (Melopsittacus undulatus) copy virtual demonstrators in a two-action test

Juvenile budgerigars (Melopsittacus undulatus) observed thin film transistor video playback of a virtual conspecific demonstrator using its beak to remove a stopper from a food box, either by pulling the object upward or by pushing it downward. Simultaneously (Experiment 1) or subsequently (Experiment 2), the observers were offered a similar stopper box and rewarded with access to food for each removal response, regardless of its direction. Observers of upward movement made a greater proportion of up responses in total and showed a stronger tendency to increase the proportion of up responses over test trials than observers of downward movement. These findings provide the first demonstration of which the authors are aware that birds are able not only to detect and respond to a moving video image but also to copy its movements.     

Speech perception by rhesus monkeys: The voicing distinction in synthesized labial and velar stop consonants

Monkeys were presented with synthetic speech stimuli in a shock-avoidance situation. On the basis of their behavior, perceptual boundaries were determined along the physical continua between /ba/ and /pa/, and /ga/ and /ka/, that were close to the human boundaries between voiced and voiceless consonants. As is the case with humans, discrimination across a boundary was better than discrimination between stimuli that were both on one side of the boundary, and there was generalization of the voiced-voiceless distinction from labial to velar syllables. Unlike humans, the monkeys showed large shifts in boundary when the range of stimuli was varied.     

Frequency selectivity in the parakeet (Melopsittacus undulatus) studied with psychophysical tuning curves

Seven parakeets were trained to avoid shock during pure-tone stimulation. A modified method of limits was used to measure detection thresholds of the pure tones. The intensity of masker tones, at numerous frequencies, was varied in order to measure the masked threshold of a probe-tone signal set at a fixed frequency and intensity. Masking curves were obtained for three probe tones (.63, 1.6, and 2.5 kHz) at each of five sensation levels (10, 20, 30, 40, and 50 dB). The masking curves from this procedure are frequently referred to as "psychophysical tuning curves" and provide an indication of frequency selectivity. The results are compared with analogous data in other species and suggest that frequency analysis in the parakeet ear is somewhat less accurate than in the mammalian ear.     

Phonological perception by birds: budgerigars can perceive lexical stress

Metrical phonology is the perceptual “strength” in language of some syllables relative to others. The ability to perceive lexical stress is important, as it can help a listener segment speech and distinguish the meaning of words and sentences. Despite this importance, there has been little comparative work on the perception of lexical stress across species. We used a go/no-go operant paradigm to train human participants and budgerigars (Melopsittacus undulatus) to distinguish trochaic (stress-initial) from iambic (stress-final) two-syllable nonsense words. Once participants learned the task, we presented both novel nonsense words, and familiar nonsense words that had certain cues removed (e.g., pitch, duration, loudness, or vowel quality) to determine which cues were most important in stress perception. Members of both species learned the task and were then able to generalize to novel exemplars, showing categorical learning rather than rote memorization. Tests using reduced stimuli showed that humans could identify stress patterns with amplitude and pitch alone, but not with only duration or vowel quality. Budgerigars required more than one cue to be present and had trouble if vowel quality or amplitude were missing as cues. The results suggest that stress patterns in human speech can be decoded by other species. Further comparative stress-perception research with more species could help to determine what species characteristics predict this ability. In addition, tests with a variety of stimuli could help to determine how much this ability depends on general pattern learning processes versus vocalization-specific cues.     

The precedence effect in three species of birds (Melopsittacus undulatus, Serinus canaria, and Taeniopygia guttata)

The perceived locations of paired auditory images, simulating direct sounds and their echoes, have been recently studied in budgerigars (Melopsittacus undulatus; M. L. Dent & R. J. Dooling, 2003a, 2003b). In this article, the authors extend those experiments to include measurements of the precedence effect using a discrimination paradigm in two additional bird species: canaries (Serinus canaria) and zebra finches (Taeniopygia guttata). Although time courses of summing localization, localization dominance, and echo thresholds were similar across all species, budgerigars had slightly higher overall levels of discrimination. The results from these experiments add further support that the precedence effect in birds is similar to that found in other animals and that the ability to suppress echoes that might degrade localization and auditory object perception may be a general property of the vertebrate auditory system.