Sometimes you can throw 20+ dozen out and it won't make any difference. Ducks go where they want to go. We hunted a small pocket this morning and it took all of 4 dozen decoys just to get the ducks to look at us. It worked out pretty good though.
It sounds like you are hunting big open water where the birds raft up in the middle. The best thing you can do when that happens is to be in a layout boat with 300+ decoys out.
ducks are smart, they work the middle of big impoundments and there's not much you can do..., best thing you can do is hope for some windy days and they will come in for a closer look. You can run a string of dekes in the middle of your spread out a lot further than you normally would but leave good landing holes close to you on the left and right sides. Sometimes they'll fly by and hit that long string of dekes and bank in and hit the closer landing holes on either side.
I saw this all the time where I hunted, so I ended up buying a few dozen Flambeau mallards and painted all of the hens into coots. This was before Duck Dynasty so I got them for $20 a dozen. Before I get the "Did the ducks tell you that?" guy on here, I have a raft of decoys, mostly coots, and I get a lot to come in. I saw that the divers would mostly land in the coots, so my diver spread for this area is mostly coots.
But, in one case clear out in Ogden Bay, this does not work. People shoot at everything that moves out there, so all of the ducks usually go way into the middle and ONLY to the middle. Your best bet would be to either get a massive spread, or get some binoculars and see where else they are landing around the edges of the pond or lake you're talking about.
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