Birds Soaring Against a Contrary Wind
Why birds fly low into a head wind and gain height by tacking against the wind's wedge
Continuing the debate 'On flying creatures', Leonardo argues that a bird moving obliquely in the wind's course is struck from below and so keeps rising. He observes that birds fly low when the wind opposes their path, which shows that wind is stronger up high than down low; the adversary counters that wind striking the earth gains density and force. Driven by the wind, a bird descends obliquely, then reverses with its acquired impetus, spending it against the wind that forms a wedge and lifts it higher than before; repeating this, it gains height until it reaches its desired place. A central band of the sheet shows five birds in flight along the wind lines.
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Birds fly low against a contrary wind
Birds always fly low when the wind's course is contrary to their path, which teaches that the wind is more powerful up high than down low. The adversary adds that wind striking the earth gains density, becoming more powerful and heavier.
Gaining height by tacking into the wind's wedge
Driven by the wind, the bird declines by oblique motion; to regain its height it turns back with the velocity of its acquired impetus, which is spent against the wind that forms a wedge for it and raises it higher than the height already left. Repeating this, it climbs at last to its desired place.
A bird struck from below continually rises
The adversary holds that if the bird's motion is oblique in the wind's course, made in the region of equilibrium, the bird is struck by the wind from underneath. And a bird struck from below continually lifts.