Hummingbirds are the extreme athletes of the avian world, living fast thanks to having the highest metabiolism of any bird or mammal. They're also known for the diversity of the sounds they make. Nevertheless, ornithologists didn't expect to come across a species that makes sounds so high pitched they are out of the known hearing range for birds. It's unclear at this point whether even other members of the black jacobin (Florisuga fusca) species can hear the sounds they produce, and if not, what purpose they might serve.
The black jacobin inhabits South America's Atlantic mountain forests. Reports of the jacobin making high-pitched sounds have been published before, but never investigated by Dr Claudio Mello of Oregon Health and Science University, who came across the phenomenon by chance when doing field work in the jacobin's habitat. "We heard prominent high-pitch sounds that sounded perhaps like a cricket or a tree frog," Mello said in a statement. "But then we noticed that the sounds were actually coming from these black hummingbirds."
Mello and colleagues returned with ultrasound microphones capable of collecting frequencies up to 250 kilohertz, far beyond the reach of the human ear. Far from being random cheeps, the recordings revealed complex collections of sounds, most commonly a triplet of “syllables” in the 10-14 kHz range, including rapid frequency modulations (trills). In Current Biology, the team also report regular patterns, with longer peak frequencies on the first and third syllable than the second, and a lower average fundamental frequency.
The hearing of many bird species has been tested previously, and none can hear sounds above 7 kHz, even owls, which rely heavily on their hearing to hunt at night.
Fascinating as it would be for an animal to make sounds other members of its species can't hear, Mello thinks it is more likely black jacobins are extreme in another way, with hearing well beyond the capacity of other birds. Whether any other hummingbirds share this super sense, and if so which ones, remains to be seen, but studies of the blue-throated hummingbird found its hearing consistent with other birds.
Mello explained that we lack methods for testing birds' hearing in the wild, and although black jacobins are not endangered, the team are reluctant to capture them for lab testing. Nevertheless, the observation that these high-pitched sounds are often emitted by birds competing around food suggests they are warnings to potential rivals. However, similar sounds, with quite minor differences, were also detected when birds were perched in bushes near food sources and even on high branches.
The authors suspect the black jacobin must have distinctive vocal organs, or syrninxes, that, along with their inner ears, deserve further study.