On Birds: The Alula, Flight-Feathers and Shoulder Rudders
Stiff bony wing members and the bending of long feathers in descent
Headed "On birds," this page studies the structural members of the wing, with two feathered wing studies drawn across the top and a winged creature with raised wings at lower right. Leonardo describes the alula or timone a as a very strong, barely flexible member of hard bone armed with powerful sinews and clothed with the strongest feathers, each supporting the next. He explains that the long, unsupported flight-feathers bend backward most in rapid oblique descent, citing the third, ninth, and eleventh propositions on things bent by the percussion and density of air, and concludes that the rudders set on the shoulders of the wings are essential for holding the bird suspended and steady against the wind.
On this page
The alula (timone a): a stiff, sinew-armored bony member
This rudder a is placed near where the wing-feathers bend, and being very strong it bends little or not at all, both for being sited in a very strong place and armed with powerful sinews. In itself it is of hard bone and clothed with feathers of the greatest strength, of which one serves as support and armor for the other.
Long unsupported flight-feathers bend most in rapid descent
The wing-tips bend much more in pressing the air than in gliding without beating; the simple part of the wing bends backward in rapid oblique descent. By the third, ninth, and eleventh propositions, the longest feathers bend because they are uncovered by those behind them and do not touch one another from middle to tip, and that one bends most which first opens the air or is struck by denser air.
Shoulder rudders hold the bird steady against the wind
The rudders placed on the shoulders of the wings are most necessary, since these members are the cause of keeping the bird suspended and steady in the air against the course of the winds.