For a couple days now my mallard hen hasn't been feeling well. She isn't standing much, and when she does walk her legs are shaky. She didn't have much appetite yesterday, but she is eating more today. We've given her apple cider vinegar and she has been drinking plenty of that. I felt her underside and vent area and I can't feel a egg, so I doubt she is eggbound. Occasionally she stands up for some food and water, flaps her wings and preens a tiny bit, but otherwise she is very lethargic. What could be wrong with her, and what does she need?