Why Small Birds Fly Low and Large Birds Fly High
Climbing against the wind without wing-beats; the cold of the thin upper air
Folio 43r continues the study of soaring flight, showing along the top a bird gliding through streaming currents of wind. Leonardo argues that when the bird's impetus meets the wind head-on the two collide, and the resulting rebound carries the bird higher than where it began, so it can climb against the wind without a single wing-beat. A marginal note answers his own question of why small birds keep low and large ones fly high: small, thinly feathered birds cannot bear the cold of the upper air and need the thick lower air to support their weak wings, whereas well-feathered eagles and vultures endure the heights.
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Climbing against the wind without beating the wings
The bird moves against the wind without beating its wings, doing so under the wind by its declination and then being reflected above the wind until it consumes the impetus already acquired. It is necessary that the descent be so much faster than the wind that the death of the acquired impetus at the end of the reflected motion equals the wind's speed. The end of that reflected motion stands much higher than where the incident motion began, and so nature does not break its law.
Colliding impetus: two motions meeting strike harder
When two impetuous motions meet, the percussion is more powerful than if they were without collision. Therefore the bird's impetus, meeting the wind's impetus, increases its simple impetus, and the reflection is greater and higher.
Why small birds keep to the thick lower air
Small birds, being without enough plumage, cannot withstand the immense cold of the great height of the air, where the well-feathered vultures and eagles endure. Moreover the small birds, with weak single wings, sustain themselves in this low, thick air, and would not sustain themselves in the thin air, which resists little.
Birds gliding through streaming wind-currents
Across the top of the sheet Leonardo draws long horizontal streams of moving air, with small bird figures set into the current at the corners and along its course. The drawing illustrates the 'evolutions' of birds carried and rebounded within the flowing wind.