Duffy's poem reimagines the story of the ancient Greek myth of Thetis: a sea nymph/water goddess with the power of metamorphosis (that is, she can change form at will). After learning of a prophecy that Thetis's son will be more powerful than his father, Zeus (the king of the gods) and Poseidon (Zeus's brother and god of the sea) decide to marry Thetis off to a mortal man named Peleus (i.e., to limit her son's potential). Peleus binds Thetis in her sleep in an attempt to prevent her from shape-shifting and subdue her into consent.
The poem retells this tale from Thetis's perspective, beginning in the middle of Peleus's pursuit. First, Thetis describes becoming a little bird singing a "Sweet, sweet" song. She's not violently, explicitly fighting back here, but rather trying a subtler tactic: presenting herself as innocent and docile, likely in order to placate her suitor/captor.
The phrase "the size of bird in the hand" also alludes to the proverbial saying "a bird in the hand is worth two in the bush," which means that it's better to hold onto something you already possess than to risk losing it by trying for something better. Here, the idiom concludes with "of a man," highlighting how Peleus arrogantly assumes that Thetis belongs to him.
The enjambment of lines 2-3 also seems to enact the shrinking Thetis describes, as line 3 is much shorter than line 2:
to the size of bird in the hand
of a man.
And by describing her own size in terms relative to the size of a man's hand (presumably that of Peleus), Thetis emphasizes the gender dynamics at play in this scene: Thetis has made herself physically vulnerable to a man.
Perhaps that's why she takes such care to display docile, pleasant behavior. The sibilant alliteration of lines 4-5 mirrors the softness and gentleness of Thetis's song, which is perhaps intended to entertain and soothe the man who holds her:
Sweet, sweet, was the small song
that I sang,
However, Thetis's submissive behavior fails to protect her from Peleus's power: "I felt the squeeze of his fist," she claims. Symbolically, this scene reflects the unequal power dynamic between women and men in a patriarchal society: being agreeable and "sweet" is one way in which women may try to protect themselves from men's anger and control, but it doesn't work for Thetis here.
The sounds of this stanza create an almost light-hearted music that contrasts the poem's dark content. For example, "hand" rhymes with "man," and. Just as Thetis takes on various forms to subtly escape Peleus's control, the poem itself takes on the form of a chipper song to subtly critique patriarchal violence.