I have both rouens and mallards and I never had an issue with my mallards taking off. They will fly but, mine only fly around the farm and always come back. My mallard follows the rouens around the yard, I also have pekins. They are one big happy flock and they dont like to be seperated...But my aunt has had a few of hers in the past fly off, they no longer get mallards because of that reason. I guess i got lucky, my mallard is on a nest about to hatch this week. Im soooo excited!
Goodluck with your ducklings!