Go to your nearest farm store and by the fly traps that are have a yellow base and a clear globe thing over it. You add water and stuff that comes inside the unit in a packet...it smells sort of dead or rotten...and it draws in about a million flys and once in - they can't get out. Hang up a couple around your pen/farm and prepare to be amazed. They cost any where from $5.99 to $6.99 here in Nebraska.
Don't spill the packet contents on yourself - it stinks really bad but that's what makes it work as well as it does. Once it is hung up - your job is finished. Once it is full of flys you toss it and replace with another. These things really work! I hang our traps about 4 feet off the ground...a short one way trip for the flys.
It also helps to let your duck area dry out - move the water container to another part of the pen. You can also use "Stall Dry" or sweet barn lime in your pen and that will help too.
Good luck & happy trapping!
Joni