Who satisfieth thy mouth with good things; so that thy youth is renewed as the eagle’s. Psalms 103:5
This fact in the life of the eagle is given as a promise to the righteous. The eagle lives to a very great age. As he grows old his beak becomes so long that he can no longer eat; then he flies away by himself to the top of a cliff and pecks and pecks on a rock until his bill falls off, after which a new bill grows in its place.
While without the use of his bill, the bird also loses his feathers because of fasting. After the new bill grows and he again takes food, new feathers start growing, so that he looks and appears like a young eagle, going forth in a new covering with youthful beauty and strength.
(Bowen, Barbara M., Strange Scriptures that Perplex the Western Mind, WM B. Eerdmans Publishing Co., Grand Rapids, Michigan, 1944, pg 22)