How Birds End Their Flight: Oblique Descent and Landing
Straight and curved obliquity of descent, and a note on the membranous wings of bats
Under the heading 'Birds', Leonardo analyses how birds conclude their flight, distinguishing straight-oblique from curved-oblique descent and noting that a straight-oblique glide must be almost flat, as marked m n. He describes the moment of landing after an upward flight, when the bird spreads its wings and tail, beats rapidly to spend its impetus, then settles with the least shock to its feet. A final passage explains why bats need fully membranous wings to twist after their erratic nocturnal prey, a feat impossible with perforated feathered wings, illustrated by two bats drawn with open wings.
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The purpose of a bird's flight, ended in oblique motion
Leonardo states that the end of birds' flight is in some species made with straight and oblique motion, and by others with curved-oblique motion. A straight-oblique ending must be of very great obliquity, that is almost flat, as shown by the line m n. If the descent is steeply declining, it becomes mixed with many rebounds, especially toward the end.
Settling to rest at the end of an upward flight
When a flight made from below upward nears its end, the bird flattens its obliquity and opens its wings and tail very wide. The wings then follow with frequent, minute beats that consume the impetus, so that on closing them the bird hangs briefly above the spot and settles with the least percussion of its feet.
Bats' membranous wings for erratic nocturnal prey
The bat must have wholly membranous wings because the nocturnal insects it feeds on escape through confused, winding twists. To follow them the bat must sometimes seize its prey upside down, sometimes obliquely, and in other varied ways, which it could not do without ruin using wings of perforated feathers.
Straight versus curved obliquity of descent
A marginal query asks what straight oblique motion is. Two small figures answer it: a bird on a straight line is captioned 'straight obliquity', a bird on a curved line 'curved obliquity'. These distinguish the geometry of descent paths that govern how a bird loses height.