The odd couple

This enormous black duck with a huge bulbous chest  and the mallard were definitely an item wandering round the pond  together ignoring all the other ducks. I have not been able to identify him.  Any idea what he is? 

BTW my blip was going to be the extras the pond with the great lumps of ice , that I thought were plastic bottles which is why I stopped to take photos. 

PS Thanks to Paola for explaining that the duck has a problem with his "crop", ie his digestive system. Perhaps too much bread. Poor duck , it looked very heavy and uncomfortable.