If the corpses are left where they fall, the fox will return to collect the rest of the kill when it's hungry again and when it's safe to do so. People who want to trap foxes often leave a body out to attract the fox back again. Unless disturbed, a fox will usually make off with one bird, maybe two if they're small birds like ducks or bantams, just taking what he can carry in his mouth. They're not worried about the waste if they kill more than they can carry, they just get into gear of hunting and kill as many birds as they can, before leaving with what they can take. As you say, doing what comes naturally. And if the fox didn't carry off any of your birds, it may have been a young one, or have been disturbed before he managed to get away with his kill. Obviously, a fox has to look out for its own safety - better hungry that night, than dead!
If your duck didn't seem to be injured, it's possible she died from the shock of the attack, or maybe the fox had her in his mouth and dropped her, having caused her some kind of internal injury from which she later died.
A fox will always return to where he has made a successful kill, and keep an eye on what's happening there, in the hope that the run will have been re-stocked with more birds. (As yours has been!) Then, when hungry enough, or tame enough to risk it, he will have another go, especially in the depths of winter, or in Spring when they have cubs to feed as well as themselves as parents. A successful attack means that fox, or pair of foxes, will defend that place as part of their territory, and will drive away any other foxes that try to hunt there. If they are killed, then others will move in, if the poultry keeper hasn't taken adequate steps to protect his flock.