§ Of the Voyage of Eärendil and the War of Wrath The Silmarillion

On those journeys Elwing did not go, for she might not endure the cold and the pathless voids, and she loved rather the earth and the sweet winds that blow on sea and hill. Therefore there was built for her a white tower northward upon the borders of the Sundering Seas; and thither at times all the sea-birds of the earth repaired. And it is said that Elwing learned the tongues of birds, who herself had once worn their shape; and they taught her the craft of flight, and her wings were of white and silver-grey. And at times, when Eärendil returning drew near again to Arda, she would fly to meet him, even as she had flown long ago, when she was rescued from the sea. Then the far-sighted among the Elves that dwelt in the Lonely Isle would see her like a white bird, shining, rose-stained in the sunset, as she soared in joy to greet the coming of Vingilot to haven.