The white bellbird, which lives in the mountains of the northern Amazon rainforest, is the loudest bird ever recorded, according to a study published on Monday in Current Biology.
Male white bellbirds belt out songs that are at least nine decibels louder than the previous frontrunner, the aptly named screaming piha, which also lives in the Amazon. These otherworldly calls are so ear-splitting that scientists were left baffled about “why females willingly endure these songs at such close range,” according to the study.
“The songs are beyond loud,” said study co-author Jeff Podos, at biologist University of Massachusetts Amherst, in a phone call. “We don’t know even if the females can evaluate them because they are so loud that if it doesn’t actually damage their hearing systems in a medical sense, it might just incapacitate them and their ability, at least temporarily, to evaluate sounds.”
The incredible blaring sound made by the male birds was noticed by Mario Cohn-Haft, a biologist at the Instituto Nacional de Pesquisas da Amazônia in Brazil, who co-authored the new study with Podos. This species lives high in the inaccessible treetops of rainforest mountains, but Cohn-Haft was able to obtain a bellbird specimen for dissection.
“When he saw what this bird looked like on the inside, he was really surprised,” Podos said of Cohn-Haft’s discovery. “He noticed, especially, that it has this well-developed abdominal musculature and thick ribs—much more so than other birds. That was pretty curious.”
Intrigued by Cohn-Haft’s finding, Podos joined his colleague for another expedition to observe the bellbirds during their mating season, when the males belt out these noisy songs. The team brought sophisticated recording equipment with them to ensure that they could adequately capture the sound pressure of the calls, measured in pascals, which is a more objective measure of loudness than decibels.
The recordings revealed that male bellbird calls emit three times the sound pressure of screaming pihas, which is a record-breaking and “ridiculous” acoustic amplitude, Podos said. If you were standing next to a male bellbird when it made this call, you might experience hearing damage because the sound is so intense.
The bird’s unique anatomy, which developed as a result of its fruit-based diet, may be the key to its deafening calls. The white bellbird swallows large fruits whole and then just kind of sits there chewing until it regurgitates the seeds. This feeding strategy requires a large beak and tough digestive muscles, which are traits that may also lend themselves to producing the strange calls.
“Their beaks are really wide and they open their beaks at tremendous angles,” Podos said. “It probably first evolved for fruit-eating, but then it turns out to be a secondary adaptation that helps them, I think, be very loud, because if you can open up your beak widely you get this acoustic effect that you wouldn’t be able to get otherwise.”
The male bellbird is adorned with a braid-like thread, called a rosette, that hangs off its head, which is a fairly normal example of a male trait that is intended to attract females. The loud calls presumably are also meant to show off the fitness of males as mates, but it seems to frighten the females rather than entice them.
The sexual strategy behind such a loud call, assuming there is one, is much harder to explain, however. Podos and Cohn-Haft were not able to observe whether males with the loudest calls had more success mating with the females, though they hope to investigate this further in future expeditions.
To that point, Podos and Cohn-Haft hope to learn more about the bizarre calls by returning to the mountain site and setting up automated recorders. This approach could help resolve some of the new questions raised by their study, including whether males communicate with each other using these noisy calls, or if they are only for the benefit of females.
“There’s a lot of unknowns in Amazon birds,” Podos said. “I would love to go back as soon as we can.”