I know some people who mix them, but in a flock that large your asking for disaster. Your duck would be much happier living with its own species. Ducks are also too mesy to be kept with chickens in my opinion. They spill water everywhere and make a muddy mess - making it dangerous for the chickens' health.
I have a question I was given 2 ducks they were a cpl days old there mom was killed they grew up and each night they would fly away but come back and 1 day they didn't come back will they ever come back again
Where they mallards, and how long did you have them before they took off? Mallards most often will return but maybe not until next spring. Getting to be migratory season soon, have seen the Canadian geese gathering in the corn fields, and Farmers Almanac calls for cold to set in early this year in northeast, if that is where you are. You can tell by the patterns of the migratory birds. All I can say is maybe set out their favorite food, but not enough to attract rodents and such, and see if they come back. But if they can fly, they will migrate, can only hope they might survive winter and return, but don't get your hopes too high. If you had a male and female, they will stick together. It is a bit early, maybe they saw a nice recently cropped cornfield and are hanging out there.
Sorry to hear that but I bet they went looking for mates. and maybe next spring they will come home to mate and nest. let us know if they do.
Awww. I'm betting they found the wild mallards at your local waterways and decided they wanted to be with them. They may still even be there, though they would be hard to tell them apart from the others.