The fact that there is a connection between a singer's voice and his body volume is ultimately a physical effect.
Scientists from the Max Planck Institute for Ornithology in Seewiesen have now demonstrated in a worldwide study that the chirping of birds is also largely dependent on their body size.
Together with colleagues from the University of Prague and the Czech Academy of Sciences, the researchers analyzed the singing of more than 5,000 species of passerine birds.
There are around 5700 species of passerine birds in total.
In the journal "Ecology Letters" the researchers present a clear connection: the larger a sparrow, the lower the frequency of its song.
This is not surprising and can simply be explained by larger vibrating structures in the birds' throats.
But physics is not everything here.
There is also a biological aspect.
Birds, like humans, can vary the pitch of their voice within certain limits.
Here, the researchers were able to refute a 50-year-old theory that birds that live in areas with dense vegetation such as forests chirp at lower frequencies than their fellow species in more open landscapes.
The basis for the thesis was the physical fact that sound waves are scattered on the foliage and ultimately weakened.
And this effect is greater, the higher the frequency of the tones.
Since the song of birds is not an end in itself, but serves communication and courtship between the animals, it was previously assumed that they chirp at lower frequencies in forests than in open terrain.
But the research team around Bart Kempenaers could not confirm that.
There is very little, if any, correlation.
The singing frequency of passerine birds depends primarily on their height.
Nonetheless, when analyzing the large amounts of data, the scientists came across another, previously unknown connection.
Sparrow bird species in which males are larger than females show a peculiarity.
Here the males sing with an even lower frequency than one would expect based on their size.
This supports the thesis that the frequency of birdsong plays a role in the competition between males for sexual partners, says Max Planck researcher Kempenaers.
With lower tones, males could make themselves more attractive to females.
After all, the females can expect a male with very low chirping frequencies to be particularly large.
And the size of a bird in turn correlates with its dominance and flight abilities - characteristics that play a role in the choice of mate with regard to reproductive success.
The scientist Tomáš Albrecht from the University of Prague, who was involved in the study, sums up the new findings as follows: “Our research results suggest that the different high frequencies of passerine song we observed are primarily due to sexual selection and the associated evolutionary trend towards larger bodies are to be explained, and not by habitat-related effects of the sound propagation. "