An analysis of the songs of most of the world’s passage birds reveals that the frequency of birds singing mainly depends on body size, but sexual selection also influences it. A new study by Institute of Ornithology researchers Max Planck and colleagues suggests that habitat characteristics do not affect song frequency, thus refuting a long-held theory.
Many animals use sound signals to communicate. These signals have evolved to maximize the efficiency of transmitting and receiving sounds, as this helps in finding a partner or avoiding predation. One of the basic characteristics of sound signals is the frequency of sound. In wooded habitats, sound signals are attenuated due to the absorption and scattering of sound from leaves, which is especially problematic for high-frequency sounds. Thus, the theory from the 1970s predicts that animals living in habitats with dense vegetation emit sounds of lower frequency compared to those living in open areas.
A team of researchers led by Bart Kempenaers of the Max Planck Institute for Ornithology in Seewiesen and Tomáš Albrecht of Charles University in Prague and the Czech Academy of Sciences analyzed variations in the frequency of songs of more than 5,000 passerine birds, covering 85% of all animals and half of all birds. Doctoral student Peter Mikula collected recordings of the songs primarily from the xeno-kant, the city’s bird vocalization science repository, and from the Macaulay Library of the Cornell Ornithology Laboratory.
Relationship between song frequency and body size.
Contrary to theory, the study reveals that the peak frequency of a passerine song does not depend on habitat type. If nothing else, the data suggest that species living in densely vegetated habitats sing at lower frequencies, which is contrary to prediction. As expected from basic physical principles, the researchers found a strong link between song frequency and body size and the common ancestor effect. “Both limit the range of sound frequencies that animals can produce,” says first author Peter Mikula. Heavier types sing at lower frequencies only because of the larger vibrational structures of the vocal apparatus.
The study further reveals that species in which males are larger than females produce songs with lower frequencies than expected from their size. “This supports the hypothesis that the frequency of audio signals is affected by competition for access to spouses,” says Bart Kempenaers. The frequency of a song can act as an indicator of an individual’s size and thus his dominance or fighting abilities. Therefore, song frequency can affect reproductive success through competition with other men or even because it affects men’s attractiveness to women.
“Our results suggest that global variations in passerine song frequency are mainly caused by natural and sexual selection that causes evolutionary changes in body size, rather than habitat-related selection in sound propagation,” sums up Tomáš Albrecht.
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