During my usual weekend morning jaunt along Del Monte Beach in Monterey (California), I noticed for the first time the Plovers, who normally seem to forage nonstop, actually taking a communal break. About a dozen or so gathered together in a circle, and all but one or two bent their heads backwards and buried their faces in their "shoulder feathers."
They seemed to be just catnapping (the eyes slits, but not completely closed).
The one or two "sentinels" seemed to keep on eye on other critters getting too close (humans and dogs) and also on the surf. If something got too close, these sentinels somehow indicated this to their brethren. But how? If it was a sound, it may have been subsonic (at least, I didn't hear anything).
How do they say, "Wake up, peeps, and move over a bit!" And all moving together in the same direction at that?
What is their communication "secret"?