Duckling beak hole/deformed
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When he was in the egg he may have stuck to the side a little bit and got deformed or he's just unlucky. If he's walking and eating ok then I wouldn't worry too much about it. Keep an eye on him though, this may cause him problems later on. Also make sure you put brewers yeast or niacin supplements in their food to prevent any other leg problems.
Please keep us updated week to week. They grow quickly, and I think a number of us would like to help you keep an eye on this precious little cupcake!