Traditionally John is supposed to have been exiled to the island of Patmos where he wrote the Book of Revelation. As one of the four evangelists, his symbol is an eagle (for far-sightedness, or vision). He is sometimes called 'John the Divine'.
The eagle symbolizes John because it is an animal which is considered to be wise and clairvoyant, which when it flies it looks straight at the sun, and the evangelist of John is more abstract and theological than the other Gospel writers.
Eagles represent honesty, truth, majesty, strength, courage, wisdom, power and freedom. As they roam the sky, they are believed to have a special connection to God.