I have had ducks inside the house for about 14 months now. Mostly only babies until they are grown enough to go outside- but I also care for some disabled ducks. Ollie is currently molting, so a have in a room with a lino floor for ease of cleaning at the moment. He lives in a babies portacot and goes outside on the grass in good weather and has a swim in the bath tub most nights. Being disabled his mobility is poor, so its not like he is wandering around the house. He has a soft dogs bed in his cot with food and water within easy reach.
A few years ago I also had Harvey inside for 5 months while he underwent treatment to save him from a usually fatal infected reoccuring abcsess in his sinus tract. He had three operations and was on antibioics for 5 months. He was already about 6- 7 weks old when he was rescued, so by the time he was cured he was well and truely grown.
Keeping the smell away is just a matter of cleaning regularly. I currently have 5 brooders and Ollie in the house- so changing the shavings andwashong towels every day is time consuming.
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