Three short lines, and the first thing that happens is a dash. "swans over the house—" gives us a sight and a season. The title, "In migration time," has already told us when we are, and now a flock passes overhead. Then the sentence swerves. What follows is not more birds but a body: "his great body in pain / refusing to go." The poem lives in the gap between those two pictures.
Its central idea is a contrast between departure as nature intends it and departure as a person experiences it. The swans know when to leave and simply do. The body below does not. The poem never says whose body it is, what the pain is, or where "go" would take him, and that reticence is the point. "Go" can mean to leave the house with the birds, to get up and move, or to die. The word "house" puts the body, judging by the poem's own staging, indoors and beneath the flight, which suggests someone confined and perhaps ill. But the text only suggests this. It is also possible to read "his" as a swan, a wounded bird left behind while the others pass. The word "great," which fits a swan as well as a man, keeps both readings alive.
The poem has no rhyme and no regular meter, but it is carefully measured. Counting syllables gives five, six, and five. The shape recalls haiku, but the middle line overshoots by a beat. "his great body in pain" has three heavy stresses, on "great," "bod-," and "pain." Two of those words share the same long vowel, so the line sounds slow and weighted. The extra syllable makes the line heavier than its neighbours, which suits a line about weight. The last line is smoother, "re-FUS-ing to GO," but it ends on one flat syllable, the very verb the body will not carry out. The long o of "over" in the first line returns in that final "go." The movement of the birds is already in the poem's sound, and the ending sets a refusal against it.
The word choices are small but exact. "Body" is a colder word than "man" or "father" or even "he." Pain has reduced a person to the thing that hurts, and by making the body the one that refuses, the poem separates flesh from will. We cannot tell whether the man is resisting or the body is simply failing to obey. "Great" pulls the other way. It gives the sufferer size and dignity, and it quietly matches him to the swans, so that he becomes a large creature grounded among creatures that fly. "Refusing" is a present participle, an action still in progress. The poem also ends without punctuation, so the refusal seems to go on after the last word. The dash after "house" works differently. It is a hinge, like the cut that haiku often makes between two images, and it leaves the reader to build the connection. Nothing says the swans cause the pain or that the pain answers them. The poem simply places them side by side.
"House" matters too. It is a domestic, enclosed word set against the open sky, and the swans pass over it without stopping. They are indifferent in the way nature always is, neither cruel nor kind, just moving on. The poem does not complain about this. It lets the indifference stand.
What makes the poem worth reading is how much it holds in so little. In seventeen syllables or so it sets the great cycles of the natural world against one stubborn individual, and it avoids both sentimentality and explanation. It does not tell us how to feel. The refusal is pitiful, since the body is hurting and cannot join the birds, but it is also fierce, since "refusing" is something a living thing does. Many poems about death lean on large words. This one leans on a gap, a dash, and a verb that will not move, and it trusts the reader to supply the missing sentence. For a student, it shows how a poem can work by leaving things out.