Hummingbirds in the Brazilian forest are singing to one another at a higher frequency than the known hearing range of birds – even of the owl family. A new study, published in the journal Current Biology, has recorded the Black Jacobin hummingbird (Florisuga fusca) calling at a frequency of above 10 kilohertz.
By comparison, the Australian magpie’s familiar carolling and warbling is mostly around three kilohertz.
Claudio Mello from Oregon Health and Science University, in the US, and his team discovered the ultrasonic songsters quite by accident, whilst studying the many hummingbird species in the mountainous forests of eastern Brazil.
“We heard prominent high-pitch sounds that sounded perhaps like a cricket or a tree frog,” Mello says. “But then we noticed that the sounds were actually coming from these black hummingbirds.”
The researchers returned to the field site with specialised recording devices usually used to analyse the very high frequency calls of bats. The sonograms revealed that the tiny black and white hummingbirds appear to have a calling and hearing ability not previously observed in any other bird.
The researchers aren’t even completely sure whether the birds can hear their own calls. Research on the hearing abilities of the Blue-throated hummingbirds (Lampornis clemenciae) found that the birds cannot hear above the seven kilohertz range.
But Black Jacobins vocalise almost exclusively above 10. “It seems more reasonable to assume they do hear the sounds they make, but we have not yet examined whether this is true,” Mello says.
The Black Jacobins would not only have to have specialised hearing apparatus, but also a unique voice-box, or syrinx, in order to create such incredibly high frequency sounds.
“They would need to vibrate very quickly and likely have a special composition, which may be different from other birds,” Mello explains.
He wonders whether the birds might rely on the unusually high-pitched vocalisations as a private channel of communication. Black Jacobins are part of a little-studied group that diversified from other hummingbird groups some 22 million years ago – and they share the Brazilian forest with some more than 40 subspecies all competing for the same air-space.