I watched a video about the birds a while back, the researchers said the birds don't seem to have any awareness of the overall shape they make, each bird is just paying attention to the nearest (maybe) 5-7 birds and doing what it can to not crash into them.. this makes it a near constant change in direction to wherever they'll fit and not crash.. I'd imagine it's similar with fish (though I think I remember Nova showing sometimes they group up to appear larger to predators, no mention of 'how')