Low-pitched song indicates fairy-wren size

"This is the first time we have been able to show that song pitch indicates body size in song birds," said Dr Hall from the University's Department of Zoology.

The study, which began when Dr Hall was at the Max Planck Institute for Ornithology in Germany, has been published February 20 in the journal PLOS ONE.

Reliable communication about body size between animals is particularly important when communicating with mates or rivals. For example the bigger the rival is, the more likely it is to win in a fight so a song pitch indicating a large size may deter rivals.

"Surprisingly, there is very little evidence that the pitch of calls indicates body size differences within species, except in frogs," she said.

"In birds in particular, there has been no evidence that the pitch of songs indicated the size of the singer until now."

The study involved measuring the leg length (a good indicator of overall body size) of 45 adult male purple-crowned fairy-wrens. It found there was a correlation between the lowest song pitches and male size.

"We found the bigger males sang certain song types at a lower pitch than smaller males," she said.

Purple-crowned fairy-wrens are creek-dwelling birds from northern Australia and, like their close relatives the blue wrens, males sing trill songs after the calls of certain predators, a context that seems to attract the attention of females.

Males have a repertoire of trill song variants, and it is the low-pitched variants that indicate the size of the singer.

Dr Hall showed that it may be the complexity of birdsong that has obscured the relationship between body size and song frequency in the past.

"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," she said.

"Focusing on the lowest pitches that males were able to sing was the key to finding the correlation with body size."