When you're reading an anthology of poetry and you come across a title like "To a Waterfowl," you might think to yourself, "I bet this is one of those poems about nature." You wouldn't be wrong to think that because, true to its title, the poem is in fact about a waterfowl. More specifically, it's about a waterfowl that is flying around in the sky, one that appears to have the ability to fly and fly and fly for a really long time, even when it's cold out and night is approaching.
But the poem isn't just about a waterfowl with impressive stamina, and in this regard the title is just a wee-bit misleading. At the end of the day, the waterfowl is just the occasion of the poem. The speaker notices the waterfowl flying, starts thinking about it, and then has a revelation about the spiritual order of the universe. In other words, it's not really a nature poem, or rather it's a poem about the spiritual nature of the world (the speaker ends up claiming that the whole world is guided and maintained by a mysterious "Power"). If you wanted to give the poem a shiny new title, you could call it: "Thanks, waterfowl, for helping me see things right." That's not as catchy, though.
There is one other little matter to talk about with respect to the title. "To a Waterfowl" is by no means the only poem in the English language to break out a bird dedication in the title. For example, there is John Keats's "Ode to a Nightingale", and Percy Shelley's "To a Skylark". Even though we can't be sure if Bryant knew of these poems when he wrote "To a Waterfowl," most of these types of poems tend to have certain similarities that a title like "To a [Name of Bird]" should bring to mind (like a direct address to a bird, a discussion of bird, and then a shift towards larger spiritual questions). In that way, whether he knew it or not, Bryant was taking his place in a well-established tradition of poetry, reflected in his poem's title.