Why Migrating Birds Fly Against the Wind
Windward flight is more lasting and less tiring; wing-spread and slow flight
Leonardo explains why migrating birds fly against the wind: windward flight is not faster but more lasting and less tiring, the bird wedging obliquely into the wind and being lifted like a wedge before descending again to regain impetus. He then argues why birds rarely fly with the wind, which ruffles and chills their feathers and, worst of all, prevents them from rebounding upwind to the height they had left. A closing note relates wing-spread to slow flight, citing the 'seventh of the Elements' that a body of greater width becomes lighter. Small bird sketches and marginal line-diagrams, carrying further untranscribed labels, run down the right margin.
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Windward flight is more lasting and less tiring
Migrating birds fly against the wind, Leonardo says, not because it is faster but because it is more durable and less fatiguing. With a small wingbeat they penetrate the wind obliquely; the wind then enters beneath their oblique body like a wedge and lifts them until the acquired impetus is spent, after which they descend downwind, gather speed, and repeat the upwind rebound to regain lost height.
Why birds rarely fly with the wind
Flying with the wind's course is rare, Leonardo notes, because it entangles and disarranges the feathers on the bird's back and chills its bared flesh. The greatest harm is that after descending obliquely the bird cannot enter upwind and rebound to the height it left, unless it turns back—which would defeat the purpose of covering distance.
Wing-spread and lightness
The more a bird spreads the feathers of its wings, the slower the motion of its flight. Leonardo grounds this in the seventh of the Elements, which holds that a body acquiring greater width becomes lighter.