On an early map of San Pablo Bay, made in 1775, the Spanish explorer Jose Canizares wrote this phrase: “forests of the red duck.” The “forests” were the North Bay marshes, and the “red duck” was Aythya valisneria, the canvasback. The male has an off-white back, chestnut head, and—in breeding season—startling red eyes; the female is more subdued in brown and gray. Also distinctive in the species is the steeply sloping, blackish bill.
The canvasback is among the largest of the North American ducks, one of the fastest flying, and one of the deepest diving, able to go down as far as 50 feet underwater. Numbering about half a million, it is also the least numerous among widely distributed duck species, and wildlife conservationists keep a wary eye on its population.
The canvasback breeds in the Arctic and the northern plains and spends the months between November and February in warmer climates. San Pablo Bay has long been one of its most important wintering grounds. It feeds here mostly on clams and other invertebrates, especially in underwater beds of eelgrass and widgeon grass. An alarming population sag in the 1960s was one of the factors that led to establishment of the San Pablo Bay National Wildlife Refuge in the shallows and adjoining marshes along Highway 37. No less important than the formal refuge were the adjoining Cargill salt ponds, where the vast majority of the region’s wintering canvasbacks could sometimes be found.
For decades now the population of canvasbacks in the Bay Area has been declining. The birds have been progressively shifting to the San Joaquin Valley, perhaps because more good pond habitat has become available there. Their total numbers overall, however, appear to be holding their own.
Though the awesome throngs of canvasback once found along San Pablo Bay seem to be a thing of the past, many of these beautiful ducks can still be seen there. The best month is November, and perhaps the best place to see them remains Pond 2 (access by water) or the relatively deep northern edge of Pond 1.