Sex differences in auditory filters of brown-headed cowbirds (Molothrus ater).

Receiver sensory abilities can be influenced by a number of factors, including habitat, phylogeny and the selective pressure to acquire information about conspecifics or heterospecifics. It has been hypothesized that brood-parasitic brown-headed cowbird (Molothrus ater) females may locate or determine the quality of potential hosts by eavesdropping on their sexual signals. This is expected to produce different sex-specific pressures on the auditory system to detect conspecific and heterospecific acoustic signals. Here, we examined auditory filter shape and efficiency, which influence the ability to resolve spectral and temporal information, in males and females at center frequencies of 2, 3 and 4 kHz. We found that overall, cowbirds had relatively wide filters (lsmean +/- SE: 619.8 +/- 41.6 Hz). Moreover, females had narrower filters (females: 491.4 +/- 66.8, males: 713.8 +/- 67.3 Hz) and greater filter efficiency (females: 59.0 +/- 2.0, males: 69.8 +/- 1.9 dB) than males. Our results suggest that the filters of female cowbirds may allow them to extract spectral information from heterospecific vocalizations. The broader auditory filters of males may reflect limited spectral energy in conspecific vocalizations in this frequency range, and hence, weaker selection for high resolution of frequency in the range of 2-4 kHz.