And, perhaps the bruised tissue is swollen, or he has some inflammation from his system being out of balance due to trauma. Maybe some probiotics? You are right, keep an eye on him.
Are we talking about the same thing? Airways are the tubes we breathe through. Air sacs are waterfowl parts that help keep them afloat.
http://www.ncbi.nlm.nih.gov/pubmed/1897730
http://s113.photobucket.com/user/crowinghillsfarm/media/Air_sacs.jpg.html
There is a Where Am I Where Are You forum, you can probably find a thread for your area, and a Free Rehoming forum where you can post.
Again, sad for your loss.
Losing a duck is heart-wrenching for me. So far, we have not lost any to predators, but I know they are around, watching, waiting for me to make a mistake.
At the same time, I feel that the fox is simply trying to make ends meet, to feed the kits. But I tell them, and the hawks and the...
Very sorry for Boots' death. Predators return. Please secure your other duck, and yes, I would be looking for a buddy for him. Rather than worry, come up with a strategy. You have more resources than a predator does, though at times it may not feel like that.
You did the right thing getting all three together. Now, everyone else is safe from the fox? Make sure everyone is secure.
If you have some poultry electrolytes, I would put that in the water to help - I would expect he'd be in shock.
Some ducks get over that quickly, some take a day or...