The most notable figure of speech in Ted Hughes's poem "Hawk Roosting" is personification. Personification is when something non-human is given human qualities; it is personification to say that the wind "sighs," for instance. Normally, we project something human onto the non-human, but Hughes takes a somewhat particular approach. Rather than standing apart from the hawk and describing it in human terms, the speaker of the poem projects himself into the hawk in order to describe the experience of being a hawk in language that a human might use.
In the first line, Hughes writes, "I sit in the top of the wood, my eyes closed." The hawk is perched on top of a tree, but in order to describe it, Hughes employs layers of personification. Consider if he had written the line differently: "It sits in the top of the wood, its eyes closed." Using the verb "sit" instead of "perch" would already involve some personification, though the speaker's position would remain external. To convey the full experience of hawk-ness, Hughes goes further and actually unites speaker and hawk into one.
What would it be like to be a hawk? What would a hawk think about if it were endowed with the human capacity for thought? These are the basic questions of the poem, and only the unusually deep personification that we see here can really answer them. Where Hughes arrives is a casually worn omnipotence. The third stanza reads,
My feet are locked upon the rough bark.
It took the whole of Creation
To produce my foot, my each feather:
Now I hold Creation in my foot.
Here the speaker gives voice to the hawk's unwavering certainty in its own power. Evolution has made it a keen predator. Through Hughes's filter of personification, the hawk knows its own skill and voices that knowledge.