Alliteration is a subtle but importance presence in "Barn Owl." In the first three stanzas, fleeting moments of alliteration add sonic interest to the poem and suggest a relationship between certain words. The shared /f/ of "fiend" and "father," for example, reflects the close relationship between the speaker and parent in the poem—a relationship also reflected later by the alliteration of "obedient" and "old nay-sayer." Later, the plosive /p/ of "power" and "prize" reflects the speaker's belief that shooting the owl—that "prize"—will be a reflection of the speaker's own power over the natural world.
In the fourth stanza, when the speaker shoots the titular barn owl, the alliteration really takes off. The /s/ of "struck" and "swayed" draws attention to the way the speaker's actions affect the innocent bird, while the fricative /f/ repeated in "fallen" and "final" captures the speaker's shock and horror in response to the owl's wound (this sound is also repeated in the stressed syllable of "afraid," which can thus be considered alliterative as well). The repetition of the /w/ sound in "wing" and "watched" underscores the child's inability to look away from what they have done.
In the fifth stanza, alliteration also dominates, creating a kind of sonic claustrophobia that captures the speaker's sense of being overwhelmed and frozen with horror. Hard /b/ sounds throughout suggest the speaker's thumping heartbeat, while the repetition of /dr/ sounds in "dropped" and "dribbled" sonically resembles the dripping guts that the speaker describes falling out of the owl. The double /m/ sound in the last line emphasizes the speaker's reluctant but honest ownership of his cruelty:
bundle of stuff that dropped,
and dribbled through the loose straw
tangling in bowels, and hopped
blindly closer [...]
mirror my cruelty
When also considering consonance, these lines become even more claustrophobic, with /b/, /d/, and /l/ sounds tangled thickly throughout.
The following stanza similarly uses multiple instances of alliteration to draw attention the owl's wounds. The imagery throughout these three stanzas is vivid, almost painterly, but nevertheless horrific—and the alliteration makes it almost impossible to look away. Each repetition of the same sound hammers home the brutal picture of the wounded bird.