How a bird tacks against the wind
Wheeling right and left into the wind, as sailors tack against the winds
Leonardo notes that birds wishing to press into an oncoming wind wheel from right to left, just as sailors tack against the course of the winds. A second note explains that the bird does this to avoid descending too much, since driving straight into the wind would force it to give so much of its weight along that line as to overcome the wind's resistance. A zig-zag line and a small bird sketch on the sheet illustrate the tacking path.
On this page
Wheeling into the wind like a tacking sailor
Birds that want to penetrate into an oncoming wind are accustomed to wheel from right and from left. Leonardo draws the parallel directly to sailors, who work against the course of the winds by tacking from side to side.
Why the bird does not head straight into the wind
The bird tacks in order not to make too much descent. If it disregarded descending, it would drive itself straight against the oncoming wind, entering under the wind with the slope of its length so as to throw so much of its weight along that line that it would overcome the wind's resistance.