well, for once he didn't bring up the french...
hey Matt,
i've seen this many times. most impressive though was in Sweden on the big rivers where i could watch the gulls do this over 200-300-400 meter 'drifts', fly back up, get on the right hatch lane and do it all over many times until there's no more bugs.
i used them to find the bigger hatches
(not so obvious otherwise when some stretches of rivers are several hundred meters wide)
cheers,
marc