A bird is seen from a distance, and the poem follows the speaker's mind as it keeps revising what the bird is. At first it looks like a crow, then a hawk as it rises above the trees, and finally, once its wings are clearly visible, a vulture. The title, "Evolution," plays on this: the bird seems to change form, though really it's the speaker's understanding that shifts.
The mood is quiet and watchful, with a faint chill at the end. The trees are "bare-boned," chopping the light into pieces, which gives the scene a wintry, skeletal feel. Each new identification feels a little darker than the last, moving from a common crow to a predator to a scavenger of the dead. The wings "splayed like hands / open to the ground" are the most striking image, almost like a gesture of offering or reaching down. The speaker doesn't say how they feel outright, but the slow, careful looking suggests fascination mixed with unease.