All good stuff above but one old bit of wisdom I received was that crows can't count! If you put the off where they're feeding they go away but one or two will sit up a tree half a mile away, watch you build the best hide and lay out all your decoys perfectly, get into your hide and then warn off every other corvid that comes near for the day..... But, if two of you turn up, set all the kit out, and go into the hide and one of you then walks away...... Back they come! Thus proving the theory they can't count.
Probably a load of old tosh but it worked sometimes!