mixing my mad chicken

[kellykings]

I recently put two 2 week old ducklings in with my three 2 1/2 month old chickens the all got on fine until 1 day we had a bad storm i checked my girls after & 1 of my chickens had pecked a hole in the top of the smaller ducklings head why did this happen my duckling died if i get ducks the same size as the chickens will they do the same the other duckling seems fine