Is it a bird? Is it a plane? Actually it's a bird that flies faster than a plane, relatively at least.
The dramatic courtship dive of a small hummingbird has been found to be the quickest aerial manoeuvre in the natural world for an animal compared to its size. It even outpaces the movements of a jet fighter and the Space Shuttle on re-entry into the Earth's atmosphere.
Anna's hummingbird lives in the American south-west and the courtship display of the male is renowned for its death-defying dive that ends abruptly with a dramatic upturn with outstretched wings and tail feathers that stop the bird from crashing into the ground.
Scientists calculated that the 50mph speed of the hummingbird at the fastest point in its descent is equivalent to it moving 383 times its body length each second. The G-force as it turns out of its dive is nearly nine times the force of gravity – the same as the maximum G-forces experienced by fighter pilots. But Christopher Clark, of the University of California, Berkeley, estimates that the G-forces created as the bird comes out of its dive would make many trained fighter pilots black out as a result of the rush of blood away from the brain.
"During their courtship dive, male Anna's hummingbirds reach speeds and accelerations that exceed the previous performance records for vertebrates undergoing a voluntarily aerial manoeuvre," said Dr Clark.
"After powering the initial stage of the dive by flapping, males folded their wings by their sides, at which point they reached an average maximum velocity of 383 body lengths per second. This is the highest known length-specific velocity attained by any vertebrate," Dr Clark said.
Aerial diving is seen in the courtship displays of many other birds, such as nighthawks and snipes, and it is a common feature of many bird species that attack their prey from the air – such as kingfishers, seabirds and falcons – but none come close to matching the speed and acceleration of the hummingbird, he said.
Anna's hummingbird dives at nearly twice the speed relative to its body size than the peregrine falcon, which flies at a maximum velocity of about 200 body lengths per second. The hummingbird is also faster than the swallow, which dives from high-altitude migratory flights at a speed of about 350 body lengths per second.
Dr Clark conducted his measurements using high-speed digital cameras that were able to take images of the entire dive from start to finish and provide the accurate data that allowed him to estimate the acceleration and speed from the time taken for the bird to cover different points in the dive. The study, published in the journal Proceedings of the Royal Society B, was a follow-up to early research proving that Anna's hummingbird was able to "sing" through its tail.
As they come out of their dive, they emit a short "chirping" sound which was recently shown by Dr Clark to come from its tail feathers rather than a "song box" in its throat. It does this by exploiting the noise generated as air rushed through the vanes of its tail feathers at the end of a dive, similar to the sound produced by air moving over the vibrating reed of a clarinet.
"This is a new mechanism for sound production in birds," Dr Clark said.
Male hummingbirds make their display dives on their territories as part of their courtship ritual, which acts to ward off other males as well as to attract potential females.
Like other hummingbirds, Anna's hummingbird, which grows up to four inches long, drinks high-energy nectar while hovering over flowers. It also eats small insects to supplement its diet. Males are adorned with a brilliant red crown and their rounded tails have white tips on the outer feathers.
Why does Anna's hummingbird nosedive?
Diving from a great height like a fighter jet and then pulling out just at the right moment is hardly a safe activity, so why does Anna's hummingbird do it? A clue is that only the males do it, and then as a way of attracting females. Charles Darwin was the first to seriously study the acquisition of sexually selected traits that appeared to serve no useful purpose other than to attract mates. It usually means that the male of a species is burdened with a handicap of some sort, such as the peacock's lethally long tail, or a death-defying behaviour, like Anna's hummingbird. One idea is that the lethal nature of a sexually selected trait is a way of testing the "fitness" of the male, or more specifically his genes. Only a supremely fit and parasite-free male hummingbird could possible perform his dance of death and live to tell the tale.
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