Eagles are the emissaries of Manwė who live their
own life freely on Middle-earth. They speak, keep watch, report to Manwė, and intervene at his request.
In the Silmarillion, "Of Aulė and Yavanna", Manwė tells Yavanna Kementįri what Eru showed him in a vision of the creation of the eagles:
"But dost thou not remember, Kementįri, that thy thought sang not always alone? Did not thy thought and mine meet also, so that we took wing together like great birds that soar above the clouds? That also shall come to be by the heed of Iluvatar, and before the Children are awake there shall go forth with wings like the wind the Eagles of the Lords of the West."Yavanna, thrilled with this, offers her trees to house Manwe's eagles, but Manwė tells her that only the "trees" of Aulė, the mountains, will be tall enough, and of their mission.
"In the mountains the eagles shall house, and hear the voices of those who call upon us."Reference: "Of Aulė and Yavanna", Silmarillion.