When to separate different size ducklings

[MandBducks]

Winnie is now considerably smaller than her siblings. They are about 1.75 pounds and she is 7.5 ounces. Today one of her siblings nipped her and she screamed. It looks like a patch of fuzz is missing now. This behavior seems to be rare and only when there is a high value treat. Just wondering if I should be separating them.

 

[TLWR]

Is Winnie able to eat easily? With the upturned top bill, she may have issues eating which could account for her much smaller size

 

[MandBducks]

Is Winnie able to eat easily? With the upturned top bill, she may have issues eating which could account for her much smaller size

She doesn't eat as easily and her bill, I'm sure, is the reason she is smaller. Thanks to everyone's advice here I have been giving her mashed hard boiled egg and Nutridrench so she has been growing and gaining weight significantly, she just can't keep up with the others weight wise.