Alliteration is a key poetic device throughout "Stormcock in Elder." Though the poem is already highly regular, with a consistent form and structure and a rarely-broken rhyme scheme, the alliteration also adds another layer of cohesion through sound. This is the case both within individual stanzas, where alliteration links the poem's imagery together, as well as across stanzas, with some alliterative sounds repeating from one stanza to the next.
For example, the first stanza relies on /w/, /s/, and /gr/ alliteration, which creates a harmonious effect in that stanza alone by linking both "world's," "sight" and "sound," and "ground" and "groped." However, it also introduces some sounds that resurfaces as alliteration later in the poem. The /br/ sound in "bread," for instance, is echoed in the use of "broken" in the second, fourth, and seventh stanzas, as well as in words like "braggart," "brindled," "bright" and "breast."
A similar effect occurs with the /th/ sound, both in its voiced and unvoiced varieties. The poem frequently uses "the," "there," "that," and "through," which creates unity, but the repetition really pays off when, in line 15, the poem refers to "The throbbing throat that made the cry." The line feels propulsive in its distinct diction, but truly harmonious (even euphonious) as a result of the groundwork laid by all the other /th/ sounds.
Ultimately, the impact of all this alliteration is a strong sense of cohesion and beautiful unity—fitting for a poem about a bird in which every part has its place.