Shortly after the Fellowship departed from Rivendell in The Lord of the Rings, Sam noticed an unusual flock of birds. In the chapter "The Ring Goes South" from The Fellowship of the Ring, J. R. R. Tolkien described these birds as large, black crows that were "flying at great speed... wheeling and circling, and traversing all the land as if they were searching for something." Aragorn recognized them as "crebain out of Fangorn and Dunland" and urged Sam to stay out of their sight.
Source：https://www.cbr.com/lord-of-the-rings-crebain-explained/
