WHAT IS IT?

It's an American Oyster Catcher (Haematopus palliatus); a.k.a. American Pied Oystercatcher.  Common along the east coast of the U.S. from New England to Florida...although this one lives at the zoo in Milwaukee.
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This one is clearly on the low end of the intelligence spectrum as it's searching for "oysters" in gravel.  No....oysters do not live in the gravel.  But it its defense, Milwaukee is 1000 miles from the sea and any oysters so it looks for "eats" where it can.
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BTW.....How hard can it be to catch an oyster? They don't move a lot. Well, here's the answer:

The American oystercatcher feeds almost exclusively on shellfish and other marine invertebrates.[8] Oysters are a staple of their diet, as their name suggests, but they also eat mussels, clams, limpets, sea urchins, starfish, crabs, and worms.[8] In general, they use their bills to catch shellfish. As they walk across a shellfish bed, they look for a mollusk with a partially opened shell. When they find one, they jab their bill into the shell and sever the muscle that causes the shell to clamp shut.[8] This can be dangerous, however, as they are sometimes drowned when they don't completely sever this muscle and the shell clamps down on their bill. A strong, tightly rooted mollusk can hold the bird in place until the tide comes in.[8] They also feed by carrying loose shellfish out of the water and hammering at the shell or by probing the sand for soft-shell or razor clams.[8] (Wiki)
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