What is "canvasbacks"

Usage examples of "canvasbacks".

Ducks… mallards and canvasbacks and big black ducks, ducks beyond all reason, more varieties than he could name.

Real turtle, we understand, and salmon, tautog, canvasbacks, pig, English mutton, good roast beef, or dainties of that serious kind, fit for substantial country gentlemen, as these honorable persons mostly are.

I asked about that, and found that they also sold quail, grouse, partridges, tame squabs, wild ducks including canvasbacks, redheads, and mallards, and hares and rabbits.