First time duck owner. Possible sick duckling

[suzuki39y]

Hi! I'm a first time owner of ducklings and learning as I go how to take care of my new little ducklings. From the first day I could tell that one duck was considerably smaller from the rest still ate and ran around just like the rest of the ducks. Now a few days into haveing my ducks this one has its chest puffed out real big always sits away from the rest and it's head droops down to the side and is a lot weaker when you grab it then the rest of the ducklings. Also doesent eat or drink near as often as the others. I haven't read anything that matches these symptoms but I know enough to tell something is not right with this duck. Is it just not settling in well or is there a health issue possibly? Any tips or advice would be great. Thank you!