I found 3 ducklings by the roadside. I researched and found them to be whistling ducks. They are 3 weeks old now. Two of them weigh 27grams but the other one weighs only 21grams. They've been eating and drinking well but i notice the lighter one isn't gaining weight as quickly as others. I noticed it has trouble eating or swallowing. Whenever it pecks on the veges it will shake its head and keep on scratching the bottom of it beak and opening its mouth like something is stuck. I don't know what to do. I've never taken care of ducklings.