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The primary reason for raising ducks is simply for the sake of companionship. Ducks can be entertaining to watch, since they enjoy playing in the water and can form bonds to the humans that care for them. Duck eggs are richer and higher in protein, calcium, iron, and potassium as compared to chicken eggs. Many breeds of duck also lay more eggs on a yearly basis than chickens do.  Note that if you have an allergy to chicken eggs, you might be able to tolerate duck eggs. Confirm this with your doctor before consuming duck eggs, however. Duck eggs can be used for almost anything that a chicken egg would be used for, but be aware of the fact that they tend to be larger, so you will need to take this difference into account when adding them to recipes. If you end up raising a large number of ducks, you can consider using some of them for food. Duck meat contains vast quantities of protein, iron, zinc, and selenium.  Roasted duck leg contains 217 calories with 11 grams of fat, which is comparable to the amount of calories and fat in a broiler leg of chicken. Similarly, roasted duck breast has 140 calories with 2.5 grams of fat, while broiler chickens have 165 calories and 3.6 grams of fat. If you have live ducks, ducklings, and eggs to sell, you might be able to find a buyer at a nearby farm or in a rural area. When you only need to sell a few, you can usually do so without obtaining a license, but you should check with your local and state laws first before doing so.  If you plan to make a business out of selling your ducks, you will need to obtain the right licenses from the city, state, and federal levels of government. You will also need to follow strict health code regulations. If you plan to sell eggs or food or duck meat, you will also need to follow strict food and health regulations concerning the processing of meat.
Keep ducks as pets. Eat the duck eggs. Use the ducks for meat. Sell eggs, ducklings, and adult ducks.