Buffleheads (Bucephala albeola) and the North American population of Barrow's Goldeneyes (Bucephala islandica) typically breed in boreal and montane regions where food is less available relative to most habitats used by temperate nesting ducks. We investigated diets and digestive organ morphology of sympatrically breeding female Buffleheads and Barrow's Goldeneyes in central British Columbia. Because those congeners exhibit interspecific aggression in defense of breeding territories, we predicted that competition for food could be a principal factor in evolution of that behavior. We also hypothesized that breeding Buffleheads would show greater variation in digestive organ morphology than Barrow's Goldeneyes because of their smaller body size and consequently greater reliance on their diet to meet nutritional requirements for egg production. Both species fed predominantly on aquatic insects during all reproductive periods, particularly larvae of damselflies (Zygoptera), midges (Chironomidae), and phantom midges (Chaoborinae). Plant foods, primarily seeds of submergent and emergent vegetation, generally represented <15% of diets in both species. Diets of breeding female Buffleheads and Barrow's Goldeneyes were similar throughout reproduction, hence competition for food has likely been a factor in evolution of territorial aggression between those species. The digestive tract morphology of Buffleheads was more variable than that of Barrow's Goldeneyes suggesting that the former relied more on dietary nutrients during reproduction than did their larger congener.