One of my 4 new ducklings has been mouth breathing since they arrived (at I guess 3 days old, now is 6 days old) she is otherwise fine eats drinks is growing same as others I noticed her nares are not open, it looks as if there is skin covering them, (the form of them is there but no hole) I thought maybe they would open as she grows? But after watching her mouth breathe for 3 days I decided to try and help, so I took a warm wet Q-tip and very gently rubbed and one came open ! The other was taking longer to open so I put her back with her sisters to relax and see how she does, I can work on it again in the morning and see if the other one opens up. I'm just sharing this because I searched and found no information on here about it. I will update tomorrow . she is doing well right now is not mouth breathing bless her heart I guess one nare is better than none 
