The Crebain were known to fly in large flocks resembling a huge black cloud and a big shadow cast below them. They made a horrible croaking, squawking noise when in flight which makes their presence known to all near them.
Like crows and ravens, the birds seemed to possess at least some level of intelligence, although they have never been known to speak any language apart from their own. They were often used as spies by various evil powers. During the time of the War of the Ring, flocks of Crebain were used by Saruman as spies to search for Frodo Baggins and The Fellowship since he wanted the One Ring.
Crebain "crows" would be the regular plural of Sindarin *craban "crow," a word which (while unattested) seems to have been adopted by Tolkien from Indo-European languages, particularly a pre-Germanic form *krabn-, whence the proto-Germanic *hrabnaz, from which descended both Old High German hraban and English raven.