Black-necked Stilt, Merced National Wildlife Refuge.
This is a very common bird in the bay area wetlands year-round, nesting here in the Spring but easily found any at any time. it's a small, fun bird that I've come to nickname the "Marsh Poodle", because it's the wetland's early warning alarm system. They are typically the first bird to strike up warning calls when things move or predators arrive, and they call insistently when flying -- and the call is a loud, raspy one that's hard to ignore. A flock of stilts and an arriving Peregrine is a fascinating situation, but one that leaves you considering ear plugs….
As you can see in this image, these look to be basic black and white birds, but in the right light, they glow with this fascinating sheen….