In three short lines, the speaker notices something small: the geese overhead sound different. They "must've changed" their calls, and there's "no hint of Canada" in them anymore. The geese are local now, not travelers from somewhere far off.
The poem leans on a quiet joke and a quiet observation. Canada geese are named for a faraway place, yet these birds seem to have settled down and lost whatever sound once connected them to it. The word "must've" shows the speaker guessing. They can't be sure the calls have changed. It may be their own hearing, or memory, that has shifted.
The mood is gentle, curious, and a little wistful. There's amusement in imagining geese losing an accent, but also a soft sense of something wild and migratory becoming ordinary. The poem lets us feel how even familiar sounds can change without our noticing exactly when.