Species and sex effects on auditory processing in brown-headed cowbirds and red-winged blackbirds

Brown-headed cowbirds, Molothrus ater, and red-winged blackbirds, Agelaius phoeniceus, are frequently used as model systems for communication and vocal learning; however, relatively little is known about their auditory processing. Although auditory space and acoustic signal space are assumed to coevolve, it is less clear when, or even if, sex differences are expected in auditory processing. Here we explored frequency-specific auditory responses using auditory brainstem responses (ABRs). We generated sex-specific audiograms using two methods: visual detection and cross-correlation. We also measured the amplitude and latency of ABRs to stimuli over a range of frequencies (0.5–6 kHz) and intensities (8–72 dB sound pressure level). Thresholds generated by both methods were good predictors of behavioural thresholds in the best frequency range (2–4 kHz). The visual detection method resulted in a slightly better correspondence with behavioural thresholds than the cross-correlation method above and below best frequencies. We found no significant differences between red-winged blackbird and brown-headed cowbird ABR thresholds; however, overall, females had lower thresholds than males. Sex differences were larger than species differences for ABR amplitude, while latency differences were greater between species than between sexes. Taken together our results suggest that despite high-frequency vocal elements, brown-headed cowbirds do not have enhanced high-frequency auditory sensitivity when measured with auditory evoked potentials. Moreover, we show that females have greater auditory sensitivity than males, which is generally not seen in other passerine species.