I thought I add a few comments on Philip's email, hoping to explain why the alula is important.
The alula is in fact a rather special development and believed to be one of the key features why birds fly as well as they do. This is so because it allows birds to increase the angle of attack of the wing and the delay the stalling caused by turbulence on the top of the wing, allowing increased lift and thus flight at a lower speed and increased control. It occurs by creating a slot at the leading edge of the wing, which causes a high-speed jet of air to be driven across the top of the wing and the pressure to be decreased (lift is caused by the phenomena of a wing caused high pressure underneath and low pressure above which pushes it upwards). It also helps birds in take-off and landing.
Bats, in fact, can achieve the same effect by "lowering membrane parts anterior to the arm and third digit by lowering the thumb and by pronation of the manus" (Lindhe Norberg 2002).
In fact, it is widely believed that the evolution of the alula is what allowed birds to begin flying, as opposed to gliding. The oldest species known to have an alula is named after it: Eoalulavis hoyasi and some birds have since evolved to discard the alula; most notably the Hummingbirds, as they flap their wings on both downbeat and upbeat, effectively making the alula redundant.
There are plenty of references explaining this better than I can, here's some for a start:
Lindhe Norburg, U.M. (2002) Structure, Form, and Function of Flight in Engineering and the Living World, Journal of Morphology 252:52-81.
Savile, D.B.O. (1956) Adaptive Evolution in the Avian Wing, Evolution 11:212-224.
From:
To: ;
Date: Wed, 10 Oct 2007 16:33:35 +1000
Subject: [canberrabirds] About the alula
This time I don't especially agree with Ian
Fraser's avian whimsy in GG. Although it is interesting as are most. I am not
convinced that the alula is anything to consider as unusual or even particularly
notable. Sadly Ian suggests the alula is at the "elbow" although he does
get around to correcting that. It certainly isn't there.
The shoulders, elbows, wrists and fingers of birds are all features
easily observed from any plucked chicken, whether cooked or not. They are in the
same position as in humans. It is well worth considering this arrangement for a
really big long-winged bird like an albatross as to how the wing is folded down.
When an albatross folds its wings, its elbow sits over its rump.
The alula is the group of feathers attached to the
second finger. So it is not the elbow, if anything it could be called near the
wrist. As the third and fourth fingers of the hand hold the primary feathers, it
is hardly surprising that the second finger (which looks more like the thumb)
should also have feathers attached to it. Indeed it would be odd if they didn't.
As for the development of these feathers, this is not that odd or difficult to
understand. Within the last few hundred years, domesticated forms of both
pigeons and chickens have been bred that have feathers on their toes. This is
more remarkable than that the second finger should have feathers attached to it.
I don't know much about the aerodynamic features of the alula, presumably they
do help. I note though that all bats fly well, without any convergent
feature. The reason that Archaeopteryx did not have this feature presumably
relates to that it still used its fingers with claws on, for climbing.
Philip
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