Four bags a month? Are you sure chickens are all you're feeding? I can't imagine any flock of 15 eating four bags a month. I just don't think they'd be physically capable. They must be wasting quite a bit, or other animals have found a way to eat it. What breeds do you have? Have you tried weighing any of your birds? Have you wormed your birds recently? (Tapeworm infestation can cause a HUGE jump in feed consumption).
A sack or two of corn a month? That's very bad. Corn is nothing but fat and contains practically no real nutrition. I bet if you checked their fat pads (on the belly beneath the fluff) you'd find they're inches thick! That's the chicken equivalent of morbid obesity.
The average chicken eats 1.5 pounds of food a week. I would guess the average sized duck eats about the same. 1.5 x 15 = 22.5 pounds of feed a week; let's say 25 to include a normal amount of waste. So 25 pounds a week. That's two bags a month!
Get a feeder that prevents waste (I recommend one similar to this: http://m.horse.com/item/little-giant-11-lb-hanging-poultry-feeder/E007931/
), cut back on the corn significantly (Actually, I'd suggest forgoing the corn at all for a month or two!). Restrict scrap feeding to 10% of the diet and greens/veggies to 20%! And try switching your flock to a flock raiser feed to accomodate the ducks. You should probably start feeding limited rations for a bit - try only filling a feeder about the size of the one I just linked to halfway up. If it's completely empty at the end of the day and there is very little or no feed on the ground, add more the next day. If there is feed all over the ground and the feeder still has quite a bit of feed left, add less the next day. If there is a very small amount of feed on the ground and a layer of semi-dusty feed about 1/2 inch deep left in the bottom of the feed, you've struck a perfect balance. Keep them on this restricted diet for a while until they seem to be eating a proper amount.