The frequency at which birds sing depends mainly on body size, but is also influenced by sexual selection, according to a new analysis from the Max Planck Institute for Ornithology. The research suggests that habitat characteristics do not affect birdsong frequency, which contradicts a long-standing theory.
Many animals use acoustic signals for communication while finding a mate or avoiding predators. One of the fundamental characteristics of acoustic signals is the frequency of the sound.
In the forest, acoustic signals become attenuated because of sound absorption and scattering from foliage, which makes it difficult to communicate using high-frequency sounds. With this in mind, a theory emerged in the 1970s which predicts that animals living in habitats with dense vegetation emit lower-frequency sounds compared to those living in open areas.
To investigate, the researchers analyzed the variation in song frequency of more than 5.000 bird species, including 85 percent of all passerines. The team collected song recordings from xeno-canto, a citizen science repository of bird vocalizations, and from the Macaulay Library of the Cornell Lab of Ornithology.
The study revealed that the peak birdsong frequency does not depend on habitat type. In fact, the findings suggest that species living in densely vegetated habitats sing at lower frequencies, which is the opposite of what was predicted by the theory.
The researchers found that birdsong frequency strongly correlated with body size and shared ancestry. “Both limit the range of sound frequencies an animal can produce,” said study first author Peter Mikula.
According to the experts, heavier species sing at lower frequencies due to the larger vibratory structures of the vocal apparatus.
The study also showed that species in which males are larger than females produce songs with lower frequencies than what is expected based on their size. “This supports the hypothesis that the frequency of acoustic signals is affected by competition for access to mates,” said study co-lead author Bart Kempenaers.
Birdsong frequency may act as an indicator of an individual’s size, as well as its dominance or fighting abilities. With this in mind, song frequency could influence reproductive success through competition with other males or by attracting more females.
“Our results suggest that the global variation in passerine song frequency is mostly driven by natural and sexual selection causing evolutionary shifts in body size rather than by habitat-related selection on sound propagation,” concluded study co-author Tomáš Albrecht.
The research is published in the journal Ecology Letters.