You already have the perfect answer but just add from someone in Florida where our weather gets extremely hot, I have a Muscovy hen that bathes two or three times in a seven day week. You can easily let the water go that long in a kiddie pool before you have to refill it again. Also you don't have to fill the kiddie pool completely up, just enough water for it to reach the underside and they will flap their wings to bring the water above them and dip their head down in a swan like motion to get the water over their backs. Muscovies are not waterfowl so they don't get in the water just to float around like my Mallard derived ducks do. My Muscovy hen gets in, does her business and gets out. I have 2 x 125 gallons and 2x kiddie pools so it's not because of the lack of options LOL
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