Lesser scaup breed and raise their young may be tied to some specific ecological conditions—such as abundance of key food items—which shifted winterwards due to global warming, without the ducks being able to adapt. In this regard, it is alternatively or additionally possible that greater scaup, which may be increasing in numbers, is putting the lesser scaup under increasingly severe competition. However, it seems that greater scaup eats larger food items on average, and the species are sympatric in part of their range and presumably have been for millennium without any problems due to competition. The experience of the past as well
Greater scaup molluscs, aquatic plants, and aquatic insects. During the summer months, the greater scaup will eat small aquatic crustaceans. There is a report of four greater scaups in April near Chicago swallowing hibernating leopard frogs (with body length about ), which they dredged out of a roadside freshwater pond. In freshwater ecosystems, the greater scaup will eat seeds, leaves, stems and roots, along with sedges, pondweeds, muskgrass, and wild celery. Owing to the greater scaup's webbed feet and weight, it can dive up to and stay submerged for up to a minute, allowing it to reach food sources that are unobtainable