Low-Pitch Songs Indicate Body Size of Male Fairy-Wren Birds

A new study finds that the pitch of a song indicates body size in song birds.

For the first time, a team of researchers led by Dr. Michelle Hall from University of Melbourne, Australia, has shown that larger the size of a male fairy-wren, the lower the pitch of his song.

Body size is one of the significant sexually selected traits in many animal species. When animals communicate with their mates or rivals, it is important that they indicate their body size. For instance, the bigger the rival, the more likely it will win in a fight. Using a song pitch to indicate their body size may discourage their rivals.

There has been very little evidence linking pitch of vocalizations to body size. It is known that the pitch of calls in frogs indicates the body size differences within the species. Until now, no such evidence has been found in birds.

This new study shows a correlation between pitch of the songs and the size of the male fairy-wren birds. For their study, the research team measured the leg length (a good indicator of overall body size) of 45 adult male purple-crowned fairy-wrens, which sing trill songs to grab the attention of the females.

These male birds have a stock of trill song variants. Researchers found that the low-pitched variants of the birds indicate the size of the singer. The bigger males sang certain song types at a lower pitch than smaller males. 

"Birds can have large repertoires of song types spanning a wide frequency range, and some birds even shift the pitch of their songs down in aggressive contexts," Hall said in a statement. "Focusing on the lowest pitches that males were able to sing was the key to finding the correlation with body size."

The findings of the study, "Male Songbird Indicates Body Size with Low-Pitched Advertising Songs", are published in the journal PLOS ONE.