The Windhover Resources
The Victorian Web is a handy resource for students studying the 19th century. This is a link to their page on Gerard Manley Hopkins, in case you want to focus your sleuthing.
PoetryFoundation.org has a solid biography of our poet. Turns out his family life helped turn him into the crazy-good poet he later became.
Not to be outdone, poets.org has got the goods, too.
Here's some information about windhovers, a.k.a. kestrels. With pictures, of course.
Here's a video of a real-life windhover (another name for the kestrel, which is a kind of falcon) in flight. Check it out—it might make Hopkins's imagery make more sense.
This site has both the text of the poem and a link to hear it read out loud. Listen up for that sprung rhythm.
Here's a photo of our poet looking very pensive. Or maybe just bored.
Wonder what Hopkins's bird would look like? Here you go.
This awesome book about poetic meter, The Rise and Fall of Meter: Poetry and English National Culture, discusses Hopkins at some length.
Take a gander at this collection of the major poems and some journals, letters, etc. by Hopkins, with notes and an introduction by an English professor. Check it out from your local library, or look through excerpts on Google Books.