In early spring, people walking through the deserts of California might be able to hear a high-pitched whistle. That noise comes from a male Costa’s hummingbird, but not from his throat—it’s all in his tail.
Males woo females in a number of ways. They sing. They spread the iridescent feathers of their throats, transforming their heads into shiny, violet octopuses. And they fly up to tall perches to plummet into acrobatic dives, careening downward before pulling up at the last second. When they hit a critical speed, the back edges of their outermost tail feathers start to flutter. That’s the source of the whistle.
Christopher Clark likens that whistle to the Stuka dive bombers that Germany deployed in World War II. As these planes dove, propeller-driven sirens on their wings produced a wailing sound that will be instantly familiar to anyone who’s watched old war movies. The Stuka’s wail was intended to weaken morale and intimidate the enemy. The Costa’s hummingbird’s whistle, by contrast, is intended to attract a mate.
By placing isolated feathers from the hummingbirds in a wind tunnel, Emily Mistick, a graduate student who worked with Clark at the University of California at Riverside, showed that the frequency of the whistle they produce is directly related to the airspeed around them. In other words, the faster a male dives, the higher the pitch of his whistle. For a female, that’s a potentially useful piece of information. By listening to the sound as it passes overhead, she could conceivably judge how fast the male is traveling, and how fit and athletic he is. The whistle, then, seems like an honest signal, which accurately reflects the male’s quality, in a way that weaker individuals simply cannot fake.
But not quite.
By filming the males, Mistick and Clark found that they dive in a way that dilutes some of the information that could be gleaned from their whistles. “At first, I thought it would be easy to take a recording of a bird in the wild and work out how fast it was going based on the sound it was making,” says Clark. “But I gave this problem to an undergraduate and they couldn’t do it. I gave it to Emily and she didn’t figure it out either. And I realized that if this is hard for a biologist to measure, it might be hard for a bird to measure.”
As the male …read more
Source:: The Atlantic – Science