The Great Eagles were the messengers and spies of Manwë, and possessed the ability to see through all physical matter except for the blackness of Morgoth's evil pits. Morgoth first discovered the limits of their sight prior to the fall of the great stronghold of Utumno.
For a time the King of the Eagles, Thorondor, kept his eyries at the top of Thangorodrim, the three mighty peaks that Morgoth raised from the Iron Mountains above the gates of Angband. While they lived there, Thorondor helped Fingon rescue Maedhros. Thorondor's folk later removed their eyries to the Crissaegrim, part of the Echoriath about Gondolin. There they were friends of Turgon, and kept spies off the mountains.
