Pitch of birdsong is determined by body size – Physics World

Physics

Non-learner: the white-tipped plantcutter. (Courtesy: Allan Drewitt/ CC BY 2.5)

The characteristics of the white-tipped plantcutter’s song are directly linked to its body size, a new study shows. A team of physicists and ornithologists in Argentina and Germany, including Gonzalo Uribarri at the University of Buenos Aires, discovered the relationship through a detailed analysis of recordings and museum specimens of the birds. Their findings could lead to new insights into the intriguing acoustics of birds that develop their songs independently.

Birds can convey a rich array of sounds through their song. Around half of species are “vocal learners” that develop their complex, specific songs by copying older members of their species. In contrast, “non-learners” develop their vocal characteristics by themselves. These birds have evolved a diverse variety of biomechanical mechanisms for enriching the sounds they make.

Uribarri’s team has explored these mechanisms in detail and have concluded that the physical characteristics of non-learners – particularly their body sizes – can impact their songs.

Rusty door hinge

The researchers tested this idea by analysing recordings of the white-tipped plantcutter: a non-learner native to South America, whose song resembles a long, rough creak like a rusty door hinge. To predict the sizes of the birds making the recordings, the team used museum specimens to show that the bird’s body size tends to increase with altitude, making them better adapted to colder environments. This then allowed the team to plot the frequencies of the recordings against the altitudes at which they were made – confirming that the larger the bird, the deeper their song.

Uribarri’s team also performed acoustic analysis on the recordings to create mathematical models of the bird’s call. This revealed that their songs begin with sudden, sharp vibrations, and then taper off exponentially. Therefore, the researchers deduced that rather than pushing air straight through its vocal folds, as is the case for most birds, the white-tipped plantcutter builds up air inside its vocal folds, which escapes in sudden, explosive energy pulses when the pressure becomes high enough.

After a burst, the sound resonates inside a cavity in the oesophagus of the bird. The cavity then dissipates these vibrations, producing the exponential decay in sound. Ultimately, the frequency of the resulting song depends on the fundamental frequency of this cavity, which directly depends on its size. Uribarri and colleagues now believe that this mechanism is likely to apply to other non-learner bird species. With future research, this could lead to a better understanding of the rich variety of sounds such birds produce.

The research is described in Physical Review Letters.

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