Bird song properties and auditory sensitivity

Hello. I’m Dr Alejandro Vélez and this video is about a paper I just published in Functional Ecology with Dr Megan Gall, an assistant professor at Vassar College, Jianing Fu, undergraduate student at Purdue University and Dr Jeffrey Lucas a professor at Purdue University. In this study we address a central question in sensory ecology and animal communication which is: what are the factors that shaped evolution of signal processing mechanisms. We specifically asked whether auditory sensitivity to different sound frequencies depends on song frequency content, song structure or the predominant habitat of nine species of songbird. We start with a simple question. Do birds with high frequency songs tend to have particular good high frequency hearing? This pattern has been described several times. We studied nine species of New World Sparrows, Three of them predominantly live in forests, three in scrub-like habitats and three in open habitats. Habitat is important
here because birds that live in open habitat tend to have high frequency songs than those that live in forests. This is in part because high frequency sounds do not propagate through forests as efficiently as they do through open habitats. Within each habitat we selected one species that produces
songs that are simple and tonal like the white-throated sparrow one species that produces trilled songs
like the Chipping Sparrow and one species that produces complex songs that include tones, trills and buzzes like the song sparrow. Our selection of species allowed us to investigate differences in auditory sensitivity that might reflect habitat dependent selection on songs and variation in song structure. We analysed several properties of the songs and just as we expected we found that songs of species that live in forests are more tonal and have overall lower
frequencies than songs of species that live in open habitats. In the second part of our study we asked whether hearing sensitivity correlates with song and habitat characteristics. Based on the results of our song analysis we predicted that open habitat species would have higher sensitivity to high frequency sounds than forest species. We used a technique known as Auditory Brainstem Response or ABR to measure auditory sensitivity. ABRs are similar to EKGs which measure electrical activity in the
heart but for the ear. ABRs are widely used in clinical trials with humans and are often used to determine whether the auditory system of a newborn baby is healthy. We used ABRs to determine the minimum loudness of a given tone of a given frequency that was needed to generate a neural response in the auditory brainstem In effect, these data tell us how sensitive the auditory system is to different tones.
We expected to find ower hearing thresholds, and thus greater hearing sensitivity at high frequencies in open-adapted species than in forest species. But what we found was surprising.
Contrary to what we expected, species from different habitats had very similar hearing. Interestingly, high frequency hearing sensitivity differed between species with different song types. Species that produced complex songs were more sensitive to high-frequency sounds then species that produced tonal or trilled songs irrespective of the habitat. So the constraints of habitat and song replictation effect the frequency properties of the song, but there is no apparent habitat constraint on the type of song used. Moreover, our results show that the type of song used, not just the frequency of the song, dictates evolution of high frequency hearing ability. It is not yet clear why auditory sensitivity correlates with song structure. One possibility is that birds with complex songs use a broader range of frequencies in order to decode all of the note types. So it is possible that the amount of information encoded in songs correlates with auditory sensitivity. Our results then highlight the importance of considering the different dimensions of communication signals when we think about the evolution of sensory mechanisms that allow for signal processing. Thank you very much for watching and we hope you enjoy our article.

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