The form of the poem closely resembles the form of a Petrarchan sonnet. I say close because it isn't exact. It's 14 lines, just like a sonnet should be. It's also written with an opening octet followed by a sestet. The rhyme scheme is ABBAABBA in the octet, which is what a Petrarchan sonnet would have. The sestet also follows the traditional rules, CDCDCD. Hopkins messes around with the form a bit, though, which is noticed in the opening sestet. The rhyme is almost but not quite a full AAAAAAAA. The other major change is that the poem is not written in full iambic pentameter. It's written in sprung meter. It has an equal number of stressed syllables per line, but the unstressed number may change.
Hopkins probably chose to take a traditional form and mix it up to cause his reader to look at something traditional in a new way. The hovering bird of prey is not rare, but Hopkins is calling renewed special attention to it through this poem, just like he is calling new attention to an old sonnet form.
The poem has a few literary devices. Line one contains a metaphor. Hopkins calls the bird a minion, which means servant, but a bird most certainly is not a servant. That same line also has alliteration with the repetition of the "m" sound. Line 2 also makes use of alliteration with the "d" sound. The function of those devices, and many other literary devices, is to call special attention to parts of the poem, paint vivid pictures for the reader, and affect the flow of the poem.