Bonnie Stout and all,

I have heard a suggestion that synchronized diving is a predator-defense
mechanism, in that nobody is left alone on the surface and everyone resurfaces
(relatively) synchronously. The recent study I have in mind was on scoters, I
believe, although it might have been goldeneyes. It was done by videotaping
foraging flocks and hitting the rewind button a lot (I suspect).

More than once I have seen double-crested cormorants diving synchonously, with
many among the flock resurfacing with fish in the beak. Flock sizes in these
cases have been 70 or more.

I haven't observed synchronous diving in grebes. Most of my grebe watching has
been small groups or loafing westerns.

FWIW
--
Scott Richardson
Olympia, Washington
salix at halcyon.com