Bellbirds have the loudest bird calls yet documented in the world, according to a study which found that their mating songs pack more decibels than the screams of howler monkeys, and the bellows of bisons.
According to the study, published in the journal Current Biology, the male white bellbird's mating call is about three times louder than screaming phias -- the previously loudest bird singer.
The researchers, including those from the University of Massachusetts in the US, said that the bellbird's calls were so loud that they wondered how the females of the species listened to them at close range without doing permanent damage to their hearing.
"While watching white bellbirds, we were lucky enough to see females join males on their display perches," said study co-author Jeff Podos of the University of Massachusetts.
The males, Podos said, sang only their loudest songs, and swivelled dramatically while singing so as to blast the song's final note directly at the females.
"We would love to know why females willingly stay so close to males as they sing so loudly," he said.
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The researchers said that the females could be trying to assess the males up close, though at the risk of some damage to their hearing systems.
Podos said that the well-studied howls of howler monkeys, and the bellows of bisons were both pretty loud, but were not as loud as the male bellbirds.
He said that this was especially impressive since the bellbird, weighing about a quarter of a kilogramme, is tiny in size compared to those mammals.
The birds had some intriguing anatomical features like unusually thick and developed abdominal muscles and ribs, which the researchers suspected might be related to their singing.
The loud singing ability also came with a trade-off, according to the researchers, who said that as the songs of bellbirds became louder, they also got shorter in duration.
Podos and his team suggested that this could be because of limitations in the ability of the birds' respiratory systems to control airflow and generate sound.
The discovery, the researchers said, offers another example of the consequences of sexual selection.
Sexual selection happens when males compete for mates, driving the evolution of truly bizarre and exaggerated traits such as the peacock's tail, and the now found loud singing abilities of the male bellbirds, the study noted.
The researchers plan to further explore "the physical and anatomical structures and behaviours that allow bellbirds to produce such loud sounds and to endure them without hearing damage.