Sick mallard duckling

[Eggaholic]

I have eleven ducklings in an outside pen and two days ago I noticed one of the them was sitin on the ground with his eyes half closed so I opened the pen is they could all get out and they all went out except for him which is really strange because they always stick tOgether.. also his feathers are really ruffled all he does is sit on the ground except when I get to close to him he tries to run but he's really wobbly