Scientists have observed a tropical hummingbird species, the black jacobin hummingbird, making an unusual cricket-like sound. According to new research, the high-frequency pitch is unrecognizable by other birds.

Researchers first heard the chirping will studying hummingbirds in the rainforests of eastern Brazil.

"We heard prominent high-pitch sounds that sounded perhaps like a cricket or a tree frog," Claudio Mello, researcher at Oregon Health and Science University, said in a news release. "But then we noticed that the sounds were actually coming from these black hummingbirds."

Scientists knew what they were hearing was high-pitched but they didn't have the technology to measure the strange birdcalls. The next year, researchers returned with the proper equipment.

As part of the new study, scientists recorded the high-frequency pitch with sound sensors designed to record bat calls. Analysis of the hummingbird chirps proved portions of the call include frequencies in the ultrasonic range. The research -- published in the journal Current Biology -- also showed the sounds are surprisingly complex.

Just as humans might used a specific CB radio channel to avoid vocal traffic, the hummingbirds may use higher-frequency calls as a more private line of communication. Some 40 hummingbird species can be found in the forests of eastern Brazil, making the airwaves quite crowded.

That's the theory, anyway. While it's unlikely other birds can hear the high-pitched calls. Scientists can't yet be sure whether black jacobins themselves can pickup the sounds they make.

"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.

If scientists want to confirm that the species can hear at unusually high frequencies, they'll need to get some black jacobin hummingbirds into the lab.

It's likely the hummingbirds utilize a specialized vocal organ, or syrinx, to produce the high-pitched chirps, the researchers say. If so, it would make sense for the species to have adopted special hearing, as well.

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