I would guess it’s about genetic variety. If a female duck had complete control, genetic variety would likely decrease quickly as fewer, genetically similar ma

I would guess it’s about genetic variety. If a female duck had complete control, genetic variety would likely decrease quickly as fewer, genetically similar males fathered all the children. This group of ducks would be more susceptible to disease, genetic defects, etc. It could also be the case that if male ducks can’t be certain who the father is, they are more likely to help protect/not kill the young.