only small breeds can fly. Calls, black east indies, mallards, snowy mallards, mini appleyards, ect. can fly because they are small and lightweight. i believe silkies are the only "small" duck that can't fly, due to the weird feathers.
Runners occassionally can fly depending on how small they are. I have two runners, one is 3 1/2lbs the other is closer to 5lbs, neither can fly.