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Feathers and flight, section 3

This is a slightly edited version of a section from "History of Life"
by Richard Cowen, published by Blackwell Science, 1994.
Copyright Richard Cowen.

The Origin of Powered Flight in Birds

Most people feel sure that protofeathers must already have been evolved 
before birds attempted flight. The question of the origin of flight is 
thus independent of the origin of feathers, because flight evolved in 
bats and pterosaurs without feathers. There have been three important 
hypotheses for the origin of bird flight, and I shall add a fourth.

The Arboreal Hypothesis

The arboreal hypothesis suggests that a ground-running biped first 
became adapted to life in trees, where it took to leaping from branch 
to branch, then parachuting. Later it developed flapping flight. 
Feathers became aerodynamically important at the jumping stage and 
evolved directly into flight feathers. The arboreal theory is the most 
favored at the moment, but it does have some difficulties. 

Like all little theropods, Archaeopteryx was bipedal, with legs and 
feet that were well adapted for ground running. Bipedality is a rather 
poor preadaptation for living in trees, and Archaeopteryx had long, 
erect hind limbs that were particularly ill-suited to climbing tree 
trunks (some arboreal theorists suggest that it climbed sloping 
branches instead!). Archaeopteryx, with long, erect limbs, a 
comparatively short trunk, and bipedal locomotion, was exactly the 
opposite in body plan of all living mammals and reptiles that jump 
and glide from tree to tree.

The claws on the hands of Archaeopteryx were long, thin, and sharp. 
They look like very effective tearing and slicing weapons, but were 
far too sharp and pointed to have been useful for climbing either trees 
or rocks. The claws on its feet have been compared with the claws on 
the feet of perching birds, but they were also very like the talons 
of an eagle or a theropod dinosaur, which shows only that they were 
equally well adapted for clutching branches or prey, or both, and we 
cannot tell which. Certainly bigger theropod dinosaurs with perching 
claws did not climb trees.

Altogether, the arboreal hypothesis is not unreasonable, but it does 
require a lot of special conditions. It looks vulnerable to a better 
suggestion that would explain more of the evidence.

The Cursorial Hypothesis

Perhaps some adaptations in a ground-dwelling protobird could provide 
some of the anatomy and behavior necessary for flight, such as lengthening 
the forearms, especially the hands, placing long, strong feathers in 
those areas, and evolving powerful arm movements. An early version of 
the cursorial hypothesis suggested that a fast-running reptile might 
evolve long scales on the arms. In this theory, the scales generated 
lift as the arms were actively flapped on the run. The animal could 
now take long leaps, perhaps encouraging the scales to evolve into 
feathers and the leaps to evolve into powered flapping flight.

We know now that feathers did not evolve from scales, and in any case 
this idea does not work mechanically. Any lift generated by a flapping 
arm detracts from the ground traction given by the feet, and acceleration 
is lost. A racing car is held down on the track by its airfoils for 
good traction, and an aircraft cannot be driven through its wheels on 
the takeoff run. A protobird that flapped its arms on the run would 
increase drag: the faster the running, the greater the drag. Only a 
very small amount of thrust would have been generated by the arms in 
the early stages of the process. Running takes a lot of energy, and 
it is not clear why leaping would have benefited the animal.

Both the arboreal and cursorial hypotheses must face the problem 
of changing from parachuting flight (from a jump off a branch or a 
leap into the air from the ground) to true powered, flapping flight. 
Flapping arms or protowings - in fact, any feathers at all on wings or 
tail - increase drag. Aerodynamically, the transition from gliding to 
flapping is difficult: there is only a narrow theoretical window through 
which the transition could have been made. This transition would have 
been very difficult for Archaeopteryx because it had such a long, bony 
tail with long feathers on it. Such a tail (an obvious display structure 
in my opinion) adds much more drag than it adds lift. Therefore the 
arboreal and cursorial hypotheses are not impossible, but they invite 
a better idea. 

The Running Raptor

More sophisticated recent versions of the cursorial hypothesis are 
much better: they are mechanically sounder and include behavior that 
involved strong, synchronized arm strokes and the evolution of strong 
pectoral muscles.

John Ostrom suggested that the protobird was a fast-running hunter, 
perhaps using its arms to strike down insects that it disturbed. 
Such an action could encourage the evolution of the muscles and the 
joint movements that would approximate a wing-stroke. No living bird 
catches insects this way, however, probably because a feathered wing 
generates an air blast, while a well-designed fly-swatter has holes 
in it to avoid blowing away the prey. Some egrets scare fish into 
motion by wing flapping, but not at a run, and the wings are not used 
in strikes.

Gerald Caple and his colleagues at Northern Arizona University 
suggested instead that a protobird hunted by running fast and leaping 
after flying or jumping insects it disturbed. To catch an elusive dodging 
prey while its feet were off the ground, a protobird would have to be 
able to adjust its body attitude in the air and then regain a stable 
position for landing. Such adjustments could be made aerodynamically 
by generating a small amount of lift or drag at appropriate points on 
the body surface. Calculations suggest that a small amount of lift at 
the tips of the arms would have a large effect on the body as a whole, 
and if the right arm movements were added, the effect would be greater 
still. The protobird would now be well on the way to flapping takeoff, 
and the flights would be gradually prolonged until complete control 
had been reached.

But this proposed activity would consume a lot of energy. No predator 
today, bird or otherwise, runs at high speed to flush out insects it 
can leap after. Furthermore, effective attitude control for a leaping 
animal is not possible below a critical airspeed that is highest in 
the earliest stages of the process, when the protowings are just 
beginning to generate lift. The required speed might have been 10 
meters per second, over 25 mph, far too much for any reasonable 
protobird. The idea cannot explain the evolution of flight on its 
own, but it may have components that could apply to later stages in 
the evolution of flapping flight.  

A new and powerful argument for the evolution of flight among 
fast-running protobirds is related to CarrierUs Constraint. Powered 
flapping flight demands a sustained high-energy output, so animals 
that operate it have to have excellent respiratory and circulatory 
systems. Flying insects pump air in and out of their spiracles in 
synchrony with their wingbeats. In flying vertebrates, the muscles 
that flap the wings are anchored on the rib cage, and expand and 
contract the chest cavity with each wingbeat. Fruit bats, vampire 
bats, and pigeons take exactly one breath per wingbeat, big geese 
take one breath every three wingbeats, and pheasants and ducks take 
one breath every five wingbeats. Perhaps, then, a bipedal reptile 
running rapidly on the ground with erect limbs would already have 
its breathing synchronized with its running, and it would have a high 
metabolic rate and the capacity for sustained power output. Such a 
preadaptation for powered flight would be more likely to occur in a 
fast, bipedal runner than in a jumping, quadrupedal tree-dweller.

To be continued......