Nesting in North America appears to commence in early June. The nest is placed in a natural depression or in an old nest on the ground, often near water or on small islands. A typical clutch consists of 7 eggs, although they vary from 2 to 11. Incubation takes about 26 days, with a range of 24 to 29. Young mature very rapidly and are ready for flight in only about 35 days.
Food consists mainly of crustaceans, although fish, mollusks and aquatic insects are also taken.
They are incredible divers and individuals on Lake Michigan were recorded to dive to 156 feet, while some have claimed dives as much as 240 feet. Oldsquaw begin leaving their wintering grounds for the north by late March to early April. Because of the enormous size of their winter range, they are still on migration into late May in some locations. Some individuals remain through the summer on the wintering grounds, or between the wintering and breeding grounds. They begin arriving on their breeding grounds by late April and continue into mid-June.