Spatial structure of a diving duck (Aythya, Oxyura) guild: how does habitat structure and competition influence diving duck habitat use within northern prairie wetlands?

What circumstances allow the coexistence of similar species is a common but complex question in community ecology. It is often assumed that sympatric species within the same guild must employ some mechanism of niche diversification to coexist. Diving duck (Anatidae: Aythya Boie, 1822 and Oxyura Bonaparte, 1828) competition is poorly understood and current evidence of coexistence mechanisms is contradictory. In the spring and summer of 2001 and 2002, we tested whether diving ducks foraging within prairie potholes segregate by depth to avoid competition. We sought to explain any segregation by sampling sediment particle size, compactness, and organic content; submergent vegetation; and benthic invertebrate densities in the foraging locations of each species. Our study was conducted at the wetland scale in southwestern Manitoba, Canada. We found that Redheads ( Aythya americana (Eyton, 1838)), Ruddy Ducks ( Oxyura jamaicensis (J.F. Gmelin, 1789)), Canvasbacks ( Aythya valisineria (Wilson, 1814)), and Ring-necked Ducks ( Aythya collaris (Donovan, 1809)) foraged most often at shallow depths (50–100 cm), while Lesser Scaups ( Aythya affinis (Eyton, 1838)) foraged at shallow to intermediate depths (50–150 cm). Thus, most divers did not segregate by depth. These results conflict with an earlier study that reported diving ducks segregated by depth to avoid competition. Diving ducks likely forage where prey abundance is greatest relative to the cost of diving to obtain that prey.