Scientists have figured out how male hummingbirds serenade the ladies. And it doesn't necessarily involve humming a tune.

In this week's issue of the journal Science, Yale University biologist Chris Clark and his colleagues reveal that the birds can make their tails flutter at set frequencies as the dive toward the females for an aerial display. Different species of hummers make their own signature sounds, which are dependent on how the tail feathers interact with each other. Other factors, such as the size, shape, weight and stiffness of the feathers, contribute to the variations in tone.

"The sounds that hummingbird feathers can make are more varied than I expected," Clark said in a National Science Foundation feature article on the research.

The high-speed courtship dives give the males a chance to show off their shimmery feathers and their aerobatic prowess. But what's the purpose behind the tail-flapping? Clark suggests that the loudness of the buzz may serve as a proxy for a male's fitness, and therefore his suitability as a mate.

Some hummingbirds flutter their tail features to make a sound almost identical to its vocal courtship song. In fact, Clark theorizes that the vocal sound may have evolved from the tail sound.

Clark measured the flutter of the feathers using a scanning laser doppler vibrometer, and analyzed high-speed videos of the tail feathers of hummingbirds in a wind tunnel.

"This work is an excellent example of the use of physical approaches to understand the function of biological structures, and it reveals aerodynamic — rather than vocalized — signaling during courtship," said the NSF's William Zamer. "It is significant that the diversity of feather structures in these hummingbirds may result from sexual selection."

But is there any practical application to this work? Well, maybe so. Clark notes that airplane wings can also flutter as air flows over them, and if they're not engineered just right, they could even break due to all that fluttering. In contrast, hummingbird feathers are built to bend rather than break. Engineers just might be able to learn a thing or two from dive-bombing birds.

If you want to learn more, check out the NSF online feature, this posting on the Dot Earth blog, this one from Not Exactly Rocket Science, this ScienceNOW report ... and, of course, the tuneful video above.

More songs of the animal kingdom:

• Gibbons sing with different accents
• Ultrasonic love songs drive female mice wild
• In a noisy world, birds are changing their tune
• Tiny insect makes loud noise ... with its genitals

In addition to Clark, authors of the Science paper, "Aeroelastic Flutter Produces Hummingbird Feather Songs," include Damian Elias and Richard Prum.

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