Why are your aylesbury ducks trying to kill female mallard duck?

If you have a lot of aylesbury ducks and have introduced a female mallard, they may just be trying to set the pecking order. Aylsbury's are large ducks, and they may not accept the duck that is a lot smaller than them. It is the pecking order.

I recommend putting the mallard into a small cage and put it into the duck run. This way, the other ducks cannot attack the mallard, and will get used to her presence. I do this when introducing any new ducks/chickens into my chicken/duck run. At the end of the day, try releasing her again, and see how that goes.

~BlackWolf1112.