The functional significance of auditory variation in songbirds

Kenneth S Henry, Purdue University

Abstract

Natural selection for efficient communication generally favors a close match between signal design and sensory capabilities. In songbirds, the acoustic properties of vocal signals vary extensively across species, but species differences in auditory capabilities remain less explored. Furthermore, despite considerable variation in vocal production between seasons in many species, seasonal differences in auditory processing are relatively unexplored. I used auditory evoked potentials to assess auditory function within the cochlea and brainstem in six songbird species: the Carolina chickadee (Poecile carolinensis), tufted titmouse (Baeolophus bicolor), white-breasted nuthatch (Sitta carolinensis), house sparrow (Passer domesticus), white-crowned sparrow (Zonotrichia leucophrys), and dark-eyed junco (Junco hyemalis). Auditory function of house sparrows was assessed in multiple seasons. I assessed (1) auditory sensitivity using auditory brainstem responses (ABRs) to tone bursts, (2) frequency resolution using ABRs to tone bursts in notched masking noise, and (3) temporal resolution using ABRs to paired click stimuli and phase-locked responses to amplitude modulation. Auditory differences between species indicate adaptations for processing species-specific vocal communication signals, but also auditory constraints. In support of adaptive evolution, species with greater maximum vocal frequencies were more sensitive to frequencies above 4 kHz, species with tonal vocal signals tended to have greater frequency resolution, and species with rapidly modulated vocal signals tended to have greater temporal resolution. In support of constraints, most species were not particularly sensitive to the maximum frequency present in species-specific vocalizations, and species with greater frequency resolution had lower temporal resolution (and vice versa). Sensitivity to high frequencies in birds may be constrained by phylogenetic history, whereas the negative relationship between frequency resolution and temporal resolution probably reflects physical constraints imposed by spectral decomposition of sound within the cochlea. Finally, ABR amplitude of house sparrows was greater in spring than summer and fall, consistent with adaptive seasonal plasticity of the auditory system. Auditory upregulation during the early breeding season may enhance processing of mate attraction signals, whereas down-regulation in summer and fall may conserve metabolic costs.

Degree

Ph.D.

Advisors

Lucas, Purdue University.

Subject Area

Ecology|Physiology

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