Duck I incubated this spring doesn't eat as much as she would need to. Each time I check on them, she has empty crop and only eats grass and greens outside. She does eat grains, corn or feed, but only a few bites, then she start grazing grass around while other 3 ducks ferociously attack the feed. Sometimes she even spits out the feed or grain and rather starts plucking grass instead. All the ducks are indian runners. She is healthy, but quite a bit thinner. How can I tell her to eat more, at least now during the winter?