Do we hear what birds hear in birdsong?

Abstract

Peter Marler's fascination with richness of birdsong included the notion that birds attended to some acoustic features of birdsong, likely in the time domain, which were inaccessible to human listeners. While a considerable amount is known about hearing and vocal communication in birds, how exactly birds perceive their auditory world still remains somewhat of a mystery. For sure, field and laboratory studies suggest that birds hear the spectral, gross temporal features (i.e. envelope) and perhaps syntax of birdsong much like we do. However, there is also ample anecdotal evidence that birds are consistently more sensitive than humans to at least some aspects of their song. Here we review several psychophysical studies supporting Marler's intuitions that birds have both an exquisite sensitivity to temporal fine structure and may be able to focus their auditory attention on critical acoustic details of their vocalizations. Zebra finches, Taeniopygia guttata, particularly, seem to be extremely sensitive to temporal fine structure in both synthetic stimuli and natural vocalizations. This finding, together with recent research highlighting the complexity of zebra finch vocalizations across contexts, raises interesting questions about what information zebra finches may be communicating in temporal fine structure. Together these findings show there is an acoustic richness in bird vocalizations that is available to birds but likely out of reach for human listeners. Depending on the universality of these findings, it raises questions about how we approach the study of birdsong and whether potentially significant information is routinely being encoded in the temporal fine structure of avian vocal signals.

Keywords: audio; budgerigar; call; canary; song; temporal fine structure; vocal communication; zebra finch.

Figure 1

Temporal waveforms of three pairs of negative and positive Schroeder-phase harmonic complexes. These harmonic sounds were generated using the Schroeder algorithm to minimize envelope cues (Schroeder, 1970). These Schroeder-phase waveforms have fundamental frequencies of 100 Hz, 200 Hz and 400 Hz corresponding to period durations of 10 ms, 5 ms and 2.5 ms, respectively (from Dooling & Lohr, 2006).

Figure 2

Results for zebra finches, canaries, budgerigars and humans tested on positive/negative Schroeder waveform discrimination at different fundamental frequencies ranging from 150 Hz to 1000 Hz (periods of 6.7 to 1.0 ms, respectively). Error bars represent standard errors between subjects. Human thresholds begin to fall to chance levels at fundamental frequencies around about 300 Hz (periods less than 3.3 ms), while zebra finch thresholds remain high at fundamental frequencies up to 1000 Hz (periods of 1.0 ms) (replotted from Dooling et al., 2002).

Figure 3

Example of natural motif with seven syllables (A–G) and where the last syllable has three parts (1–3). This stimulus was played as a background stimulus (top). Two targets are shown below. In one case the duration of the interval between syllables A and B is doubled (left) and in the other case the last syllable G is reversed (from Vernaleo & Dooling, 2011).

Figure 4

(a) Time waveform of a female zebra finch contact call showing regions of individual periods that were excised and concatenated to produce 200 ms synthetic calls consisting of repeated single periods of a natural call. (b) The fine structure of both forward and reversed versions of these calls is shown below each corresponding synthetic call. (c) Results for zebra finches and humans tested on forward/reversed synthetic call discrimination. Zebra finches performed at much higher levels in discriminating between forward and reversed versions of such calls compared with humans. Fundamental frequencies ranged from 690 Hz to 816 Hz corresponding to periods of 1.45 ms to 1.225 ms, respectively (from Dooling & Lohr, 2006).