The author uses the literary device of imagery to completely build the character of his unusual angel. Instead of basing his descriptions on the typical ones of grandeur, beauty, perfection, and grace, the author makes the angel much more human. He does not create the obvious contrast between angels and humans as has been developed over the centuries. The angel is more like a dirty, senile old man than a majestic angel with a glowing golden halo surrounding him.
It rather reminds me of the movie Michael with John Travolta playing the angel of that name. He is not the perfect, graceful, awe-inspiring figure we expect that Mary saw when she was told she would have a baby and it would be the Lord's child. Quite the contrary, Travolta is very much like the old man in Garcia Marquez's story--human with the qualities of a bird. Unclean, unkept, sloppy, old, perhaps senile and forgetful, but definitely not angelic or magical.