Song complexity determines high frequency hearing in birds
Several factors appear to have affected the evolution of bird song. In particular, birds that live in wooded habitats seem to use relatively low-frequency, tonal songs that are not overly degraded by reverberations off of trees and leaves. In contrast, birds that live in open habitats tend to use quite complex songs with rapid trills, high frequencies and rapid frequency sweeps. The implication is that birds that occupy different habitats use different vocal cues to carry information. In turn, properties of the auditory system of birds seem to co-evolve with properties of the songs: woodland species tend to be quite good at processing tonal information and open-adapted species seem to be quite good at processing rapid changes in the properties of sound. Similarly, species that use high frequencies in their songs tend to have very good high-frequency hearing. At least this is what the literature suggests. We tested these ideas using 9 species of sparrows that occupy a diversity of habitats and that use a diversity of song types. Moreover, we tested 4 pairs of closely related species to see if phylogenetic similarity affected either hearing properties or song properties.
Surprisingly, our initial results suggested that sparrows with high-frequency songs have poor high-frequency hearing! Several potential explanations for this result do not hold up: there is no effect of taxonomic similarity or an effect of habitat on the relationship between song frequency and hearing sensitivity. The solution to this conundrum appears to lie in the complexity of a sparrows' vocal repertoire. Sparrows with complex songs (that include rapidly changing tones, rapid trills and buzzes) have excellent high frequency hearing, and those that use simple trills or other simple elements tend to have poor high-frequency hearing. In addition, trills tend to encompass a broad range of frequency, which is the basis for the conundrum: trills typically have a broad range of frequencies, but birds that use trills do not have sensitive hearing at high frequencies. The implication of this study is that the higher frequencies of trills do not carry much information because they are not well processed, suggesting that not all components of a vocal signal are likely to carry information.
Velez A, MD Gall, J Fu, JR Lucas. 2015. Song structure, not high-frequency song content, determines high-frequency auditory sensitivity in nine species of New World sparrows (Passeriformes: Emberizidae). Functional Ecology doi: 10.1111/1365-2435.12352
Article and photo provided by Dr. Jeff Lucas, Professor and Associate Head of Biological Sciences.