Sitting low in the water, close to the river bank, a scaly-sided merganser rests, half-hidden by the shade of an overhanging tree and by a line of jagged black rocks. Waking, the bird moves away from us quietly, and is soon joined by another and then another. Seen well, the details of each of these birds is exquisite: jagged crests, bright fire-red bills and flanks with black and white fish scales that mirror perfectly the ripples and reflections on the water.