Canvasbacks are the largest diving duck, and arguably the tastiest. These used to be the prize duck for restaurant hunters. Canvasbacks frequent large open water, sitting in large rafts on bays or oceans -- they're certainly not the most commonly encountered at the refuge, but they mix into the bag now and again.
Canvasbacks have a brilliant red eye and a characteristically triangular black bill. Coupled with their brilliant copper head set against a black and white body, canvasbacks are easily recognized on the water but, like many divers, harder to pick out in the air -- not least because we don't see them all too often. Look for the large size, triangular head, and continuous fast wing-beats.