Motion helps. Try rigging a few dekes through the eyes heavy weights and running a line to the boat for creating a jerk rig to imitiate diving birds. Also, to get the birds' attention from a distance, use flapping wings, alternating black and white. This can be achieved using a Mojo duck, or if you are looking to be a little more economical, try using children's pinwheels. Paint the blades in an alternating black and white pattern, drill two holes in the back of a few dekes, fill the dekes with insulation foam, and then attach the new flapping wings to the dekes. It takes a little effort, but you can create a whole flock of fluttering dekes for the price of one Mojo duck.
As for the spread, that varies with the location. Large open water, like open lakes or rivers require large spreads (6 dozen +). We use 10 dozen on the river to get the birds to turn. Marsh habitats vary with time. Early in the season, we use a large spread (6 dozen) to compete with the hundreds of other hunters. Later, once the birds wise up and the hunters fade out, we scale back to about 3 dozen birds. This keeps the birds from getting suspicious.
Best of luck.
Anyone can hold the helm when the sea is calm.
- Publilius Syrus (~100 BC)