Picky duck - wants to be vegan :)

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?

 

Haha, I am growing superworms just to bribe my ducks. She's not vegan when I bring them the worms. Far from it.
The thing is, they all used to eat their feed, but this one doesn't want to eat it and only started eating corn kernels I had for backup when I didn't have anything at hands to give them. I remember my ducks last year, they would not touch the greens at all, but this one grazes grass and other stuff rather than eat the feed. The thing is,.. It was below freezing whole day and she ate frozen grass while other ducks just laid around. Low temp is also an issue, because by wetting the feed it will become rock solid quite fast. They don't eat much at this point, looks like they all started liking grass. I will try making them warm soupy feed tomorrow just to see how they react. I guess they are far from being hungry. Perhaps just picky and maybe she needs sone vitamins from the greens or perhaps she is just mor into vegetables. She is not thin or malnurtured, so it's ok.