How a bird descends and turns in flight
Drawing in wings and tail to descend; extending one wing to bank into a turn
This page continues the treatise on bird flight with two passages and marginal sketches. The first explains that in an oblique descent the bird brings the shoulders of its wings toward its own shoulders and draws the wing tips and tail inward, so as to meet a smaller sum of air resisting its fall. The second describes turning: the bird extends the wing on the side toward which it wishes to turn, which meets more air and greater resistance and is thereby slowed, so that the difference between the two wings converts straight motion into a curve. A bird with outspread wings and a fainter sketch are drawn in the left margin.
On this page
Contracting wings and tail to lessen resistance in descent
When descending along any slanting path, the bird draws the shoulders of its wings toward its own shoulders and pulls the wing tips in toward the tail. The tail likewise contracts upon itself, so the bird finds less air to resist its descent.
Turning by extending the wing on the chosen side
To turn right or left the bird extends the wing on that side, which meets a greater sum of air and so more resistance and is slowed relative to the opposite, more contracted wing. Because one wing then moves more than the other, straight motion is transformed into circular motion.
Marginal sketch of a bird with outspread wings
A bird is drawn from the front with both wings fully outspread, beside a fainter sketch lower in the margin. The drawings accompany the notes on descent and turning.