You already know that hummingbirds are amazing animals — they can hover in place for minutes at a time, fly backwards at will, and flap their buzzing wings up to 70 times per second.
But how much do you know about their bizarre courtship rituals?
The details vary among species, but the principles are basically the same: The male usually begins by hovering in front of the female, singing to her, and flexing special feathers on his throat to form a bright, flamboyant mask.
In Costa’s hummingbird, which is local to the deserts of Southern California, this display resembles a brilliant purple six-armed octopus.
However, this feather flexing is just the beginning of the seduction dance.
After gaining a female’s attention, the would-be suitor flies as high as 100 feet into the sky, tucks his legs into his body, and dives head first toward the ground at speeds of 60 miles per hour before looping back up, usually to do it again.
Depending on the species and the situation, he might repeat this dive bomb between three and 20 times.
It would seem that these maneuvers are at least partly a visual display of speed. From the perspective of the female hummingbird, the would-be suitor would look like a small comet hurtling through the sky with sunlight glinting off his iridescent head feathers.
But hummingbird researchers have revealed that there is an audio component to the diving ritual as well.
It turns out that male hummingbirds have evolved special elongated tail feathers that make noise by vibrating during the high-speed descent. The shape of the feathers, and in turn, the sounds they make, differ by species.
“The physics of how the feathers are making a sound is like the strings of an instrument,” said Chris Clark, a professor of biology at UC Riverside. “Thick, heavy feathers make low sounds, and thinner, lighter strings make higher-pitched sounds.”
Clark has been studying hummingbird courtship displays for more than a decade with a particular emphasis on how the shape of their tail feathers affects the sounds they make during their dives.
His early work was on Anna’s hummingbird, which can be found up and down the West Coast of America, but in the last few years he turned his attention to Costa’s hummingbird, in part because it can be found on the UC Riverside campus.
And, as he details in a paper published Thursday in Current Biology, Costa’s hummingbirds perform their mating ritual with a twist — literally.
The work began about two years ago when Clark and his lab tech Emily Mistick began sticking the tail feathers of Costa’s hummingbird in a newly built wind tunnel in his lab.
From these experiments they learned that the sound the tail feathers make during a descent has what’s called “directionality.” In the case of the Costa’s hummingbird, the song produced by a male’s tail feathers would sound 11 decibels louder if they were pointed directly at you than if they were pointed anywhere else.
Next, Clark asked Mistick to go into the field to determine how fast the birds were flying during their dives. He thought this would be easy to calculate based on the sound produced during their fall — higher pitch should equal a faster flight. However, it turned out to be more complicated.
Using high-speed video and an acoustic camera that tracks where in space a sound originates, Clark and Mistick discovered that Costa’s hummingbirds have evolved a unique diving strategy that effectively masks the speed of their descent.
Instead of diving close to the female like most other species, Costa’s hummingbirds hurtle off to her side and then twist their bodies 90 degrees mid-dive to aim their tail feathers in her direction.
This complicated maneuver allows them to mitigate the Doppler effect on their dives, which in turn keeps them from revealing to the female how fast they are going based on pitch.
So, why is it beneficial for these hummingbirds to obscure the speed of their dives? Clark is not entirely sure.
One possibility is that females have a preference for fast males, and have somehow been tricked by this choreographed dive into thinking the males are going faster than they are.
Another possibility is that this type of dive simply sounds better to females. Clark notes that the vocal song of the Costa hummingbird sounds remarkably similar to the sounds its tail feathers make during its courtship display. Perhaps its distinctive dive evolved to mimic the male’s vocal song.
To find out, Clark hopes to learn more about just what it is about the males’ courtship dance that appeals to the ladies.
“So far we’ve just measured what males are doing, but we don’t have any data on actual female preference,” he said. “That’s what’s necessary to test this hypothesis.”
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