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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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