Thetis was one of the Nereids, the fifty daughters of Nereus, who lived in the sea. Her father was a shape-shifter, an ability that is passed on to his daughter in this poem. She rejected the advances of the god Zeus, who then learned that she was destined to bear a son who would be greater than his father. To avoid disaster among the gods, Zeus married her to a mortal, Peleus, to whom she bore a son, Achilles. When he was a baby, she dipped him into the river Styx to give him immortality, but the heel by which she held him was left vulnerable and he was later killed in the Trojan wars.
In the poem, Thetis is fated to be brutalised by men, whatever shape she takes. Stanza 1 presents her as a song bird being crushed by a male fist – perhaps an association with the story of Philomel who was ***** by her brother-in-law, Tereus, and had her tongue cut out, so she could not accuse him. The gods turned her into a nightingale, forever singing her sad song. Duffy plays on words by substituting "sweet sweet" for the usual ‘tweet tweet’. The use of the saying ‘a bird in the hand is worth two in the bush’ emphasises the man’s role as a hunter.
The emphasis here is on "shrank, a bird in the hand, sweet … small song" and the stanza abruptly stops with "squeeze of his fist." The use of half-rhymes "shrank / hand / man / sang and sweet / squeeze" highlights the contrast between the fragile bird and the male fist.
In stanza 2 she becomes an albatross, phrased as "shouldered the cross" which has Christ-like connotations, especially when combined with up the hill a reference to Calvary. This is the albatross of Coleridge’s poem, ‘The Ancient Mariner’, which follows a ship, bringing good weather, until it is shot with a crossbow, and the ship is doomed to disaster. The rhyme scheme in this stanza of "cross / albatross / crossbow" and "sky / why / eye" links Christ with the bird and with death and also links the sky or heaven…