Once a body is off the ground, a pair of wings is a good way of keeping it there. But what about getting aloft in the first place? Aeroplanes gather speed until the flow of air over their wings generates enough lift for take-off. But Kathleen Earls of Brown University, Rhode Island, has found that most birds are more like jump-jets -- they power take-off with their back legs.
She studied two species: the starling (
Earls measured the forces produced in the birds' legs by having them fly from a metal plate similar to a set of bathroom scales. By videoing their first few moments of flight, she then calculated their total velocity and acceleration, and compared them with the readings from the plate. A large gap between the force measured by the plate and the total generated in take-off, for example, would show that the wings were making a large contribution.
As she reports in the
Starlings jump "much like we would," she says, "crouching down first, and then extending the legs and leaving the ground. The wings start the downstroke as the feet leave the ground." Quail, on the other hand, "start in a crouched position, and begin the downstroke with their wings while they are still pushing with the hindlimbs".
Slow flight requires more power than fast. This has left evolutionary biologists puzzling over how the ancestors of modern birds could have got off the ground: their rudimentary wings would have had a tough time producing enough lift and thrust. Any newly evolved aspect of anatomy or behaviour must give an individual an immediate advantage for natural selection to favour that aspect. But the original wing will have been crude and inefficient -- certainly no good for powered flight -- so how did the triumph of sophisticated biological engineering we see today ever get started?
"If we find out what it is that living birds do to get off the ground, we may get a better idea of what extinct animals did," Earls says. One theory has it that birds' ancestors ran at a speed where their 'proto-wings' were useful. But she believes that this is "unnecessarily complex". Today, most birds begin flying with a jump, leading Earls to propose that this is a better explanation for the origin of flight.
Fossil evidence suggests that birds evolved from chicken-sized dinosaurs that ran around on their hind legs and had some feathers, maybe for insulation or sexual display. These animals would probably have been good jumpers, perhaps adding height or distance with a fortuitous flap or two of the forelimbs.
Earls says, "If living birds primarily use the hindlimbs to get off the ground, then it's possible [that their] early avian relatives did as well". Hopefully more work on the mechanics of flight in living birds will help tip the argument one way or the other.