I came into the year with 15 ducks, 4 geese and 17 chickens (I think), most a year or two although a pair of ducks was definitely older. The pair of ducks both died and I think that was just old age although supposedly they were only about four years old. I also lost one young cockerel of unknown causes, one goose from egg peritonitis and one hen from heat exhaustion whilst brooding (still feeling awful about that). The rest remain fit and healthy - or a few have been sold on. So about 14% loss but that was from mostly young birds and not all 'natural causes'.
Once I'm over the hurdle of hatching, the youngsters seem pretty resilient too, touch wood. One humane dispatch of a gosling with a malformed foot and a duckling got eaten by the cat - that was from fifty odd birds hatched. Oh, and the fox took three ducks (blame husband for that). Mind you some will get eaten by us too because I have ended up with a lot of cockerels and drakes. Surprisingly I'm finding it slow to clear the hens too but have had lots of phone calls about ducks.
So it depends how you add it up but I think that's fairly low if you exclude the predation of cats and foxes and my stupidity re. heat.