scaup (skôp) (aka bluebill)

Average length: male 17 inches, female 16.5 inches

Average weight: male 1.8 pounds, female 1.6 pounds

Radically fast bird, quick on the wing, a true rocket of the north. In size, the male mostly closely resembles the ring-necked duck. Differentiating the two is possible on the wing but not easy -- particularly in early season, when the birds are not yet fully plumed, and especially in the half-light of predawn. The key is seeing the speculum, which is a band of feathers visible on the top of the wing. On bluebills -- male and female -- the speculum is white. On ringnecks, the speculum is a light, dusty gray. In the hand, meanwhile, look for a "blue'' bill. Also, males have glossy black heads with a purple or green cast, depending on the angle of light. A female bluebill has a brownish head, neck and chest and a white oval band on the face, at the base of the bill. Hen ringnecks don't have this band but do have a smattering of lightness on the face, at the base of their bills.

ring-necked duck (ring´nek´)

Average length: male 17 inches, female 16.6 inches

Average weight: male 1.6 pounds, female 1.5 pounds

In the hand, and when fully plumed, ring-necked ducks have more tufted heads than bluebills. Most noticeably, ringnecks have gray speculums (on the tops of their wings), not white, like bluebills. Still, on dark, cloudy mornings, don't expect easy identification between the two. And don't look for a ring around their necks, at least not in early fall -- it's generally only visible much later, when these ducks are fully plumed. Instead, and ironically, check for a ring around the bills of these birds. Yet in some instances even these rings are not fully developed in early fall, particularly on the hen ringneck, which is most difficult to distinguish not from hen bluebills but from hen redheads. Experienced waterfowlers can, however, tell one of these from the other by size (redheads are bigger) and wingbeats (redheads are slower) among other characteristics.