There are only a few species in these categories but they can be confusing. Immatures often look like adults of other species and from a distance in the air or on the water, size is a difficult thing to judge.
Accidentals are always possible.
Body shape and how the bird sits on the water or flies are important clues. Song can be relied on but only if singled out for the bird in the scope. Usually where these species are seen, the waters are open with wind and waves making matters worse. The Snow Goose is all white and shows pronounced black wing tips when flying. It is an uncommon spring migrant but can be very abundant in the fall.