The Corbies have a major strength that is also a weakness. Check your "Legends and Lore of Quendor".
They are able to tell colors even in this gray wasteland.
Have you found anything that can drive away the Corbies?
Reread the descriptions of the various Scarecrows in the Fields.
One Scarecrow seems to be working, while the rest are not: the Corbies are afraid of one Scarecrow.