What is the theme of the poem ''To a waterfowl?''The reason I ask is because I was asked myself to explain how archaic words support the theme of...

Bryant could have a couple different themes in this poem.
I think the strongest theme that you could support with archaic words would be in
reference to religion. The first mention of God or a "higher being" would be in these
lines:

"There
is a Power whose care Teaches thy way along that pathless coast,-- The desert and
illimitable air,-- Lone wandering, but not
lost."

Like the bird, he too
is alone in the world trying to find his way. He needs guidance as well--just as the
bird needs help finding his way along the coast to wherever he is heading. But the key
in this stanza is the last line. He and the bird are both alone, wandering, but they are
not lost. They do have a Power that is guiding them safely through their lives. Bryant
ends with assurance that he will have support when he needs it, just as the bird had
guidance. This ending gives both the reader and the author/speaker confidence that all
will be fine if he follows his guidance.

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"He, who, from zone to zone, Guides through the
boundless sky thy certain flight, In the long way that I must tread alone, Will lead my
steps aright"