I had my lovely assistant (wife) run up and stomp. The first 2 times they just turned and swam a bit faster. I had her wave her arms the third time. You shold have seen the looks she got from the other people watching birds :)
i'll bet it was really funny - she must have made some really big/crazy gestrues to get them all to fly like that! tell her she did a great job, as you did capturing them. :-)