Whither, 'midst falling dew, While glow the heavens with the last steps of day, (1-2)
Right off the bat, death is in the air. The heavens still "glow," but the action of the poem takes place in the "last steps of day." Dusk, twilight, the end of a day—these are all potential metaphors for death that tell us that this poem may be a little darker than we thought.
Vainly the fowler's eye Might mark thy distant flight to do thee wrong, (5-6)
Like the first stanza, the second also foreshadows the speaker's more obvious discussions of death later in the poem. A fowler is a bird hunter, which makes this a particularly violent little passage.
As, darkly painted on the crimson sky, Thy figure floats along. (7-8)
The waterfowl seems kind of lifeless here. It is just floating along, as if it were dead. We know it's not dead, and in fact it only appears to be floating. Perhaps things look "dead" to the speaker because he hasn't yet had his spiritual awakening.
All day thy wings have fanned, At that far height, the cold, thin atmosphere, Yet stoop not, weary, to the welcome land, Though the dark night is near. (17-20)
Okay, this is where things start to take a much darker turn. Images of death are everywhere: the "cold, thin atmosphere" hardly seems like a place that could support life (it kind of reminds us of Pluto), while the whole "dark night is near" business sounds very, well, ominous. Sure, it's dusk, but also seems like a reference to the eternal night of death.
And soon that toil shall end; Soon shalt thou find a summer home, and rest, And scream among thy fellows; reeds shall bend, Soon, o'er thy sheltered nest. (21-24)
Soon, soon, soon, the speaker tells us, the waterfowl will find a home, and rest, and have a nest. Okay, on one level, this is meant to be literal (the waterfowl will build a nest and settle down). At the same time, it's also about death, only not a dark and scary death, but a peaceful, restful, sheltered death.
Thou'rt gone, the abyss of heaven Hath swallowed up thy form (25-28)
Okay, the speaker obviously means that he can no longer see the waterfowl, but the violence of the language here ("swallowed up," "abyss") really makes us think of death, which the speaker starts to get obsessed with in the second half of the poem.