This Black Oystercatcher showed the same body dipping habit that the Spotted Sandpiper is famous for. The Willet does the same thing. The sandpiper probably does it more often, but the basic behavior is the same.
Why do they do it? There’s another bird, the American Dipper, that does it so constantly that the behavior stuck as its common name. They’re found on rocks in rapid streams, not here. But scientists haven’t come to agreement why they do it. There’s three theories:
So why do dippers dip? Let’s consider three theories: One suggests the dipper’s repetitive bobbing against a background of turbulent water helps conceal the bird’s image from predators. A second asserts that dipping helps it sight prey beneath the surface of the water. A third theory holds the most promise. Dipping – as well as the rhythmic flicking of those flashy white eyelids – may be a mode of visual communication among American dippers in their very noisy environment. That dippers make exaggerated dipping movements during courtship and also to threaten aggressors lends support to this theory.https://www.audubon.org/news/why-dippers-dip
How well can those theories transfer to the dipping behavior of our local shorebirds? Well, the concealment theory doesn’t work here because the water isn’t usually turbulent. On the contrary, a moving shape is more obvious than a still one, which is why hunters like egrets and herons often hold more still than Marcel Marceau. The sighting-prey-underwater theory runs against the fact that our shorebirds dip and dip again even when just traveling among rocks and not looking for prey under water. And the communications theory looks good when you watch oystercatchers in a group, as here; but they also do it when alone. So there’s room for a fourth or even fifth hypothesis. Anyone? Could this behavior help their digestion? Could it help focus their eyesight? Does it correct an imbalance?