For one thing, you'll need a larger than normal oven. The largest turkeys at the neighborhood turkey farm are about 30 lbs., the size of a small swan, and, while you might be able to physically stuff a bird that big into a newer oven that has maximal cooking volume, it won't cook very well. Turkeys are lean, but if you tried to roast a swan, your roasting pan might overflow with fat and if that catches fire it will burn like a gallon of liquefied paraffin. That's a pretty good way to burn down the house.