That is really odd, I have investigated further, when you said there was blood on the legs, did you see if this came from the vent, only if there was any possibility of them ingesting any insecticide or Rodenticide, this could kill them and there would be blood.
http://www.duckhealth.com/duckhlth.htmlAs you know, I have had Geese,Ducks and chickens- and they all roamed free range together, even though Gandalf was a bit on the aggressive side, he only used to tell the Drakes off and me, he did peck and the hens, (and one in particular he was very fond of, have you ever seen a gander trying to mount a chicken, sorry, but it is hilarious in a sort of way.)
I just felt for you finding them dead like that
, though it is worth looking at other possibilities.
xx