Through years of open sea fishing in a quiet and solemn way, Santiago has learned to identify the sounds, scents and dynamics of all the nature that surrounds him. He has turned the fish into his only friends, and he respects the sea and its mysteries, the superstitions associated to it, and because it is a major source of life.
Since he is aware of nature and the changes that it undergoes, it is easy for Santiago to use his intuition as well as the vast knowledge that he has acquired after a lifetime living in nature to follow the behavioral trends of animals to make a connection to his own circumstances.
The Man-Of-War bird is flying around Santiago's boat looking for fish. It is mainly a water bird and, for this reason, Santiago knows that there may or may not be fish nearby. However, Santiago has such a familiarity with all things in nature that he even personifies them as if they were people.
Just then he saw a man-of-war bird with his long black wings circling in the sky ahead of him. He made a quick drop, slanting down on his back-swept wings, and then circled again.
“He’s got something,” the old man said aloud. “He’s not just looking.”
Shortly after the fish around Santiago start jumping around, which is an indication, in Santiago's expert opinion, that there may be a big fish near, perhaps a dolphin. From all of these instances it is safe to conclude that the Man-Of-War bird is one of the many guides that Santiago has learned to use to identify the different changes that occur in nature, particularly at sea. It shows a very close connection with nature that has developed from a combination of experience, exposure, and the love and respect that Santiago renders nature as the main source of life.