There is chirping, beeping and humming – in very different pitches: what characterizes the song frequencies of the many songbird species in the world, researchers have now clarified through comparative analyzes. They were able to refute the previous assumption that the pitch is determined by the characteristics of the respective habitat of the species. On the other hand, it depends on body size, ancestry history and also on sexual selection, according to the results. The males of certain species therefore tend to show off: their deep voice makes them appear larger than they are.
From the nightingale to the blackbird to the colorful exotic species of the tropics – many species from the large group of passerine birds (passeriformes) make themselves loudly noticeable all over the world: They emit tones or even complex tone sequences that we call singing interpret. These various acoustic signals are used when looking for a partner, marking areas or warning against predators. There is a great variety of these sounds in the bird world, which is also characterized by great variations in the frequency range of the voices. Researchers from Germany and the Czech Republic have now jointly investigated the extent to which the species-typical pitches in the representatives of the passerine birds are associated with basic characteristics.
To do this, they analyzed the song frequencies of more than 5,000 species, which represent 85 percent of all passerine birds. Since it is the largest group of birds, this selection also corresponded to half of all known species. An important data source for the study was a civic science project: the vocal samples were largely from xeno-canto, a citizen science database that committed bird lovers continuously enrich with new recordings.
Differently than thought
The researchers’ data analyzes showed that a previous assumption did not apply: the lower and upper peak frequencies of songbirds apparently do not depend on the characteristics of their habitat, as a theory from the 1970s suggested. It had been suggested that animals in a habitat with dense vegetation were more likely to emit sounds with lower frequencies. In a wooded environment, acoustic signals are attenuated due to sound absorption and scattering by leaves, and this particularly weakens high-frequency sounds. However, contrary to the prediction of this theory, the study showed that there is no tendency for species in densely vegetated habitats around the world to lower their vocal frequencies.
The researchers report, however, that the pattern of a basic physical principle emerged: small sounds high, big sounds rather deep. The researchers found a strong correlation between the song frequency and the body size of the respective bird species. Apparently, heavier species also develop larger vibrational structures in the vocal apparatus, which lead to the lower vocal frequencies. The researchers report that the importance of the species belonging to certain lineages becomes clear. “These aspects shape the range of audio frequencies that an animal can produce,” summarizes first author Peter Mikula at Charles University in Prague.
Acoustic boasting is emerging
However, there is another interesting detail aspect when it comes to size and voice, the scientists report: “The frequency of the acoustic signals in some species is probably influenced by the competition for access to the partners,” says co-author Bart Kempenaers from Max Planck -Institute for Ornithology in Seewiesen. The analysis results showed that the males of species with comparatively small females sing with lower frequencies than would be expected based on their size. In other words: the gentlemen evidently make themselves acoustically bigger with the extra deep voice. So they are likely to appear particularly attractive to women and more dominant than their competitors. The researchers explain that the vocal frequency could influence reproductive success in these cases.
“Our results indicate that it is not the type of habitat and the associated sound propagation that is decisive for the global differences in song frequencies of songbirds. Rather, they are determined by the physique and the competition for the greatest reproductive success, ”is how co-author Tomáš Albrecht from Charles University in Prague summarizes the results of the study.
Source: Max Planck Institute for Ornithology in Seewiesen, specialist article: Ecology Letters, doi: 10.1111 / ele.13662