TINY male fairy-wrens channel crooner Barry White when trying to woo a mate with their song, researchers have found.
The lower a male fairy-wren can sing the more appealing he is to females, as a deep tweet tells prospective mates that he is a large, desirable bird. A low pitch also sends a message to rivals to steer clear.
It's the first time scientists have shown song pitch is an indicator of body size in song birds.
''We essentially asked how low can they go,'' said Michelle Hall from Melbourne University's zoology department. ''And we found that that depends on their body size.''
Size is important in the animal world, as larger males often come out on top in fights and bigger males are more attractive to females. But male purple-crowned fairy-wrens rarely fight and instead defend their territory, which can stretch up to 300 metres, with song.
To establish the role, a low pitch is played in song. Dr Hall studied more than 400 songs recorded from 45 adult male purple-crowned fairy-wrens at the Mornington Wildlife Sanctuary in the Kimberley.
''I focused on the lower end of the frequency range to see how low each individual could go because the theory is that low-pitch sounds are constrained by body size,'' she said.
The pitch was then compared with the length of each bird's tarsus, the leg bone between the knee and foot which is considered an indicator of skeletal size.
''There was a correlation between the length of that leg bone and the pitch of the low songs,'' she said.
Dr Hall said studies had shown that frogs did a similar thing, with their croak being an indicator of body size. However this is the first time the relationship between pitch and body size has been shown in birds.
''This is also relevant to people as well,'' she said. ''Researchers have shown that men will deepen the pitch of their voice when they are speaking to a woman they find attractive. But it's that lowest pitch that tells you about body size because you can't bluff that.''
Listed as nationally vulnerable due to habitat loss, purple-crowned fairy-wrens are creek-dwelling birds from northern Australia. The research has been published in the journal Plos One.