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How deep is your cheep? Why songbirds sing their size

The melodious beauty and elaborate complexity of birdsong has long inspired poets, writers, and musicians – as well as behavioural ecologists! But besides appreciating the aesthetics of birdsong, we are…

Bigger male purple-crowned fairy-wrens can sing their ‘trill song’ at a lower pitch than smaller males. Michelle Hall

The melodious beauty and elaborate complexity of birdsong has long inspired poets, writers, and musicians – as well as behavioural ecologists!

But besides appreciating the aesthetics of birdsong, we are interested in why birds sing, and what they “say” with their songs.

Our research was conducted by myself and my colleagues Dr Anne Peters (Monash University) and Dr Sjouke Kingma (University of East Anglia, UK), and published today in PLOS ONE.

We found that, although the pitch of the lowest frequencies male purple-crowned fairy-wrens could produce depended on their size, the average pitch of all a male’s different song types did not depend on his body size.

So, what are some of the important questions we attempted to answer by conducting this study?

Fighting and flirting

The most common reason birds sing is to compete with rivals and attract mates. Much of the complexity of song seems designed to intimidate or appeal.

This often happens by way of revealing something about the singer to the listener. The quality of a bird’s singing can reveal its age to listeners because birds have to learn their songs, and their performance often improves with practice. Youngsters that haven’t yet perfected their art are not as intimidating to rivals, or as attractive to prospective mates.

There are sound theoretical reasons for believing that song can also reveal the size of the singer.

Deep voices, honest signals

The physical properties of sound (low-pitched sounds have long wavelengths) mean that only big bodies can produce low-pitched sounds (think of the deep rumbles of elephants versus the high-pitched squeaks of mice).

Male purple-crowned fairy-wrens sing trill songs in response to predator calls. Michelle Hall

In principle, this makes a deep voice an unbluffable sign of a big body. This kind of honest signal is known as an “index” signal because it is intrinsically linked to some property of the signaller, and can’t be faked.

Low-pitched calls are a textbook example of an index signal of body size, and there is good evidence from frogs that the calls of bigger males are lower-pitched than those of smaller males.

In the case of frogs, listening males know to steer clear of calls that are lower-pitched than their own, while females might find the lower-pitched calls more attractive.

So what about birds?

Bigger birds sing lower pitched songs

Surprisingly, until our recent study, there was no evidence that the pitch of birdsong revealed the singer’s size to rivals and potential mates.

Our work demonstrates that bigger male fairy-wrens sing certain song types in their repertoire at a lower pitch than smaller males. Such songs therefore provide listeners with reliable information about the size of the singer.

(Example of a trill song. Audio credit to Michelle Hall, University of Melbourne)

Why is this the first time a correlation between pitch and size has been shown in a songbird, when it has been shown in so many frog species?

Frog calls are simple, and it is the very complexity of birdsong that has caused the relationship between size and song frequency predicted by theory to be overlooked. Birds can have large repertoires of song types that span a wide frequency range, and potentially communicate many messages.

Some birds may not use song to signal their size, while others may signal their size along with many other messages in their songs.

The key to our study was that we focused on the low end of the full frequency range, where body size is expected to impose a constraint.

Needless to say, what we discovered was music to our ears.