Wiktionary
vb. (present participle of thin out English)
Usage examples of "thinning out".
The mud and water became shallower and the cattail growth began thinning out.
Ahead of them, the trees were thinning out, although the individual specimens seemed to be even larger than those on the slopes, reaching thirty to forty meters high.
Both sorts were thinning out, however, and disappearing wherever they disappear to.
Hendley let the distance between them narrow, in spite of the fact that the crowd was thinning out away from the main entertainment complex.
Roscoe imagined how unpleasant it could be to have a snake drop on one's neck--he took care to ride under as few limbs as possible and was glad to see the trees thinning out as they rode west.
A little further we found another piece, and looking ahead we could see that the vegetation seemed to be breaking up in the distance, as if the jungle was thinning out.