New duckling can't walk - HELP! duckling
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It sounds to me like he hurt that leg somehow. Was he fine when you bought him and you checked on them and he was like this? IF it's just one leg he probably sprained or broken something. This sounds like what recently happened to my duck, I came home and he was quacking and acting fine but he wouldn't put any pressure on his right leg. I would take the duckling to a vet just to make sure he's not seriously hurt.
That's how it was for mine when he sprained his leg, he acted fine, I could move it without him protesting, there wasn't blood or swelling and in the pic it's holding it's leg the same way mine did. I'm thinking the duckling may have just sprained it's leg playing or trying to jump out of the brooder.
I did get a paint pan for supervised swim time. They're so funny when they get excited when you drop feed in the water. I do have 2 other chicks in with them, their to small to be in with the older batch I have. My little Sebright Grumpy likes to sleep on their backs and follows the lighter duckling around.