Agricultural food resources and the foraging ecologies of American black ducks (Anas rubripes) and mallards (Anas platyrhynchos) at the northern limits of their winter ranges

Atlantic Canada is the northern limit of the American black duck (Anas rubripes) wintering range, and more recently, the mallard (A. platyrhynchos) wintering range. Atlantic Canada has experienced considerable urban and agricultural development over the last century, and wintering black ducks and mallards appear to be using these habitats, in addition to traditional coastal habitat to survive the winters. This paper combines digestive tract content and stable isotope analysis to provide a comprehensive analysis of winter black duck and mallard diet in Atlantic Canada, and compares mallard and black duck winter diet in agricultural and coastal areas. Coastal black ducks wintering in Atlantic Canada had a diet consisting mainly of marine invertebrates and wild plant matter, while coastal mallards appeared to rely less on invertebrates and more on plant matter through winter. As winter progressed, the agricultural black ducks relied more on the feed provided by the farmer at our study site, indicated by a significant decline in their δ¹⁵N ratios. Mallards at the agricultural site maintained a low and steady δ¹⁵N ratio through winter, indicating a less diverse winter diet than sympatrically-wintering black ducks. Urban black ducks had a diet consisting entirely of vegetation and anthropogenic food supplies, which appeared to meet their nutritional requirements for winter survival. Our research highlights the importance of agricultural and urban habitats for wintering black ducks and mallards in Atlantic Canada.