Duckling's beak is starting to deform

[R3M1X]

It's not much but the top beak is growing slower than the bottom beak. I read that this could be a vitamin d deficiency? I am feeding them chick starter, but due to the heater breaking in their outdoor brooder I had to bring them inside. This is only happening to this one duckling and I considered it to be a birth defect, but from the looks from when it was first hatched the beak looks normal. This duckling is also smaller than the rest, but seems like it is not affected by this issue. It has about the same energy level as the other babies and doesnt seem to be acting weird, the only thing is its beak growing abnormally.

Here is a photo of the beak when it first hatched:

Here is its beak now:

 

[VarietyChickensRule]

I was just wondering the same thing. I had a favorelle chick that I brought in with my bantams because it was doing poorly out side with the hen several weeks ago. He is developing fine now but his beak is the same as your duckling.

 

[R3M1X]

I was just wondering the same thing. I had a favorelle chick that I brought in with my bantams because it was doing poorly out side with the hen several weeks ago. He is developing fine now but his beak is the same as your duckling.

I have asked in several places with no one giving answers or suggestions . I started adding niacin, vitamin D, and the packs of chick vitamin and probiotics. It's only been a day, but I am hoping it doesn't get worse...
Apparently the beak is made of keratin just like finger nails so I am wondering if something similar to biotin would also help. So a mix of egg whites and brewer's yeast . Just to be safe instead of using human biotin supplements.