We can't discuss "The Windhover" without talking about the poet's inspiration for writing: the awe-inspiring strength and skill of the bird itself. This bird, commonly called a windhover because of its ability to hover on the wind, can actually fly in place in the air, even with high winds buffeting it around. (Don't believe us, or having trouble visualizing it? Go check out a video of a kestrel hovering in the air in the "Best of the Web" section.) No wonder Hopkins felt inspired.
The poet was clearly showboating as he wrote "The Windhover": his incredible skill at crafting a Petrarchan sonnet with a revolutionary new meter throws his own poetic skill in the reader's face.
The fact that the speaker hardly identifies himself at all, but focuses entirely on the bird, suggests that the motivation for writing the poem is purely admiration for the strength and skill of the windhover.