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

Abstract

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.

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

“…Then, each warble recording was segmented into acoustic elements by a custom-written MATLAB program described in Tu et al (2011b). Briefly, this program computes root-mean-square amplitude values using a 0.83 ms window through each warble sequence and determines an intensity threshold based on the noise level of each recording session.…”

“…Compared to the songbird species typically used in syntactical investigations, this species has a much larger and more complicated vocal repertoires characterized by extreme variations in syllable structure, sequential structure, and duration (Tu et al 2011b; Farabaugh et al 1992). These very features tend to make the vocalizations of this species difficult to study, but they also offer several new parallels with the way humans communicate with speech as described below.…”

“…The long, rambling warble song of adult budgerigars is composed of multiple acoustic elements delivered at the rate of 3–4 elements per second in no discernible order (Fig. 1a; Tu et al 2011b; Farabaugh et al 1992; Brockway 1969). Warble is almost exclusively produced by males when courting females in close quarters, accompanied by various body, head, and eye movements and other intimate behaviors.…”

“…It also reinforces the pair bond between mates and promotes ovarian development and egg laying (Brockway 1964b, 1965, 1969; Farabaugh et al 1992). Recent work has shown that the complex acoustic elements making up the warble of male budgerigars can be reliably and objectively assigned to at least broad acoustic categories and, more importantly, are perceived in categories by the birds (Tu et al 2011b). Because the length and complexity of this song makes it experimentally intractable, there has to date been virtually no information on how budgerigars perceive complexity in warble songs.…”

Budgerigars and zebra finches differ in how they generalize in an artificial grammar learning experiment

“…Budgerigars are vocal learners with more vocal variation than zebra finches and an open-ended learning capacity. This parrot species produces long and flexible warble songs that vary both in the sequential structure as well as in the shape and duration of elements (35,36). Even with this strong variation, budgerigars are able to detect minor changes in familiar songs (37).…”