Why is the eagle referred to as "he" in Tennyson's "The Eagle"?

The eagle is personified in Tennyson's poem. Personification occurs when human qualities are given to an object or an animal. The eagle "clasps," has "hands" and "stands" as he surveys the blue sea below. It is only natural that "he" should be referred to as a man.

Romantic poets of the nineteenth century often personified nature (see Wordsworth's famous poem "I Wandered Lonely as a Cloud" where flowers dance or Keats's "On the Grasshopper and the Cricket" where an insect is also referred to as "he"). In stanza two, Tennyson portrays the eagle as a powerful force in the world, as strong as a thunderbolt, echoing Victorian confidence. In the latter half of the century, while Tennyson was poet laureate, England was considered the strongest nation on earth so it is only fitting that Tennyson's eagle is the epitome of that vigor.

In order to understand the impact of the choice to refer to the eagle as "he" instead of "it," consider the first line of "The Eagle":

He clasps the crag with crooked hands

This is an example of personification; we typically think of eagles as having talons, not hands. This particular description makes the eagle seem quite human; to match this sense of personhood, the author has chosen to refer to the eagle with the pronouns "he," "him," and "his."

The eagle sits on the crag, which is a steep and rocky cliff, in a way that makes him seem like royalty. No human can reach this space, yet the eagle claims it as his own; these are "his mountain walls." (emphasis added). He is in a position of power, which enables him to keep a watch on the world around him, much like a king watching over his kingdom. In the final line, the eagle is compared to a "thunderbolt," alluding perhaps to Thor, the god of thunder.

These characterizations of the eagle transcend the typical associations with the animal world, which usually places man at the top of all life. Instead, the use of "he" helps to develop the great power of the eagle as he watches over the land in ways that humans cannot fathom or achieve.

Why does the poet use "He" to describe the bird in "The Eagle"?

Tennyson does not say anywhere in this very short poem who or what he is actually describing, except in the title. This is interesting because, without the title, the poem could easily be misconstrued as being about a king or some other ruler in a high castle, watching the world below "from his mountain walls." This effect is largely due to the considered use of the "he" pronoun, which has a humanizing effect. The eagle in the poem seems majestic and commanding, something that could not easily be achieved if it were described using the pronoun "it." Note also the use of other words which have a personifying effect: the eagle is described as having "hands," which of course an eagle does not really have, and he also "stands," which seems a curiously human choice of verb for a bird. Birds might more usually be said to "perch," rather than "stand."

The "he" pronoun, then, helps Tennyson convey his desired effect of the eagle as a powerful and commanding being, the master of all he surveys.

I think Tennyson is trying to do one of two things by using the pronoun "he" to describe the bird.  He could be trying to elevate and equate the bird with humans, or Tennyson could be trying to illustrate to his readers that mankind and birds (animals) are equivalent.  By using "he," Tennyson makes mankind and the bird equivalent, because the pronoun is now being used for both species (instead of "it" for the bird).  

The pronoun usage makes sense when you think about Tennyson being a Victorian author.  Victorian authors are writing their stuff at the same time that Charles Darwin published his Origin of Species.  The book was a breakthrough piece and extremely controversial, because it showed humans as completely interrelated with other animals.  The book showed that we are descended from a common ancestral species.  Tennyson's poem gives the bird the same quality as humans, because he uses a human pronoun to describe the bird.