All in a flap: Have we been clucking up the wrong tree about how the first birds took to the skies? Image shows chukar partridge developmental stages toward flight.
Credit: UM Flight Lab/Robert Petty.
PARIS: Experts have fought long and sometimes bitterly about how birds evolved – but a new study suggests that their intellectual battle may have been in vain.
Rival schools of thought squabble as to how the first avians took to flight some 150 million years ago, when Archaeopteryx, the oldest identified bird appears.
In one corner are those who argue that birds, believed to be descendants of two-legged theropod dinosaurs, first scuttled along the ground and learnt that by flapping their proto-feathered wings, they could shelter in trees.
In the other are those who believe that birds first learnt how to glide, leaping from trees or cliffs, and thereafter extended this trick to powered flight by flapping their wings.
Different strokes
What fuels the flappers-vs-gliders debate is the belief – commonly shared by the public too – that birds have a wide and complex range of different wing strokes to do all the things they do.
But now, U.S. researchers, reporting in the British journal Nature, say they have discovered that birds have a much narrower and rather simpler range of wing strokes, and this calls for a rethink of the whole debate.
University of Montana scientist Kenneth Dial and his team used four synchronised high-speed digital video cameras to record a North American bird, the chukar partridge (Alectoris chukar).
The birds were recorded every two days, starting the day after they hatched and continuing through to adulthood. Ten points on the birds' bodies and wings were then digitised and modelled on a computer to analyse wing shape, body angle, angle of attack, wing beat and other variables.
The investigators were surprised to find that the angles that the birds' wings made, relative to the ground, all lay within a very narrow range.
Angle of attack
The range remained constant, regardless of the activity and age of the bird: whether the bird was a fledgling that was running and flapping its wings; whether it was a juvenile, learning how to take off at near-vertical inclines; or whether it was an adult, able to glide and dive with ease.
The birds' bodies shifted position to accomplish their aerobatics but their wings were always pitched at roughly the same angle in relation to the ground. The angle of wing stroke fell within a narrow arc of only 19°.
The authors draw an analogy with the helicopter. Choppers can ascend and dive vertically. But this agility does not depend on the angle of attack of the rotor blades, which in fact move within a very restricted arc. Instead, it comes mainly by modifying engine power and by shifting the body around by using the tail rotor.
Dial said that the relatively fixed angle of wing stroke among that partridges has been confirmed by his lab in video recordings of more than 20 other bird species.
He argues that it is a basic feature shared by all flying birds, and there are critical changes in fossilised shoulder bones, among late theropod dinosaurs and early avians, to suggest how this change came about. "The fundamental wing stroke we describe is plesiomorphic" – meaning, an ancestral characteristic – "and elementary to understanding critical elements of avian locomotion and perhaps its evolution," he said.
Agility boosting
The arc of wing stroke provides several advantages, he said. One is that it directs aerodynamic force at about 40° above the horizontal, providing an airflow under the wings that boosts the bird's agility at lower energy cost. This would be very important for early birds with poorly developed feathers.
And it also provides an air-brake for fledgling birds. If a young bird falls from a tree, for instance, it can flap its wings and this will slow its fall, thus preventing injury.
Dial said that in the flappers-vs-gliders debate, both theories rest on the assumption that birds with proto-feathers could at first glide, at least a little way, until they eventually gained the ability to beat their wings for locomotion.
But all the evidence among today's birds and on non-avian gliders suggests this: the gliding came later. And it came only after birds were able to flap their wings the right way, a process that was probably much simpler than thought.
