It looks like feathers are an excellent defense against stings, especially with the layer of down on a duck. However, I got to wondering about stings UNDER the wings, where the ducks would have tried to hide their heads. I thawed one of the carcasses to take a look.
I found a few stingers in the under-wing down where the bees could not quite reach the skin. There were just a few more stings along the wing bones where the feathers are short and the down is shallow. I also noticed a few stingers among the feathers, falling out as I spread the wings. So, a lot of honeybees died without adding to the injuries. And, a few blows were delivered while the birds tried to protect their heads.