How many feathers does a duckling need to be outside with its two sisters?

I have two ducklings and one chick two of them seem to be fine outside by themselves but one of them is smaller than the two and I'm worried that she might die in the cold they are sheltered from the wind the other duckling that is coping much better with the cold is also a mallard so I think that might be one of the reasons the other duckling is a Pekin which I believe is more domesticated they don't have a mother.