The male has a glossy dark head and gray back; the female is marked by a white patch at the base of the bill.
The male has a black head with a green sheen and a gray back. The female is brown, with a white patch at the base of the bill.
Long lines of greater scaup on the water can occur above mussel beds. In the Polish Odra River Estuary, observers recorded elongated flocks hundreds of metres offshore and dense groups farther out. Large flocks occurred where zebra mussels covered extensive areas of the bottom, while scaup were scarce where mussels were scarce.
A 2015 study divided the estuary into 22 subareas and compared bird counts from 2002–2014 with bottom samples. Analyses used 22 censuses covering the whole area within a short period under favourable weather and ice conditions. Birds drowned in fishing nets during 2008–2013 provided a separate dietary sample: 32 digestive tracts contained food. Zebra mussels made up about 97% of the recovered food biomass. Chironomid larvae, snails and other small bottom-dwelling animals were also present, contributing to the food used by migrating and wintering scaup.
An extinction-risk assessment has not been added yet.
The Odra estuary dietary study used greater scaup drowned in fishing nets, documenting bycatch as an actual local hazard. The authors also proposed testing whether trace elements accumulated in zebra mussel tissues and shells affect scaup reproduction. Understanding both the mussels’ value as food and the substances they contain can inform conservation of this migration and wintering feeding area.