Answer:

The combination of color pattern, long neck, and sharp, skinny bill rule out almost all other species of swimming birds, leaving us with just Western and Clark’s Grebes. This photo was taken in Prado Regional Park, which is a breeding location for both Western and Clark’s, and is where the two frequently hybridize. Western has extensive dark on the face, usually well below the eye, and a dull, sickly yellow-green bill. Clark’s has a very bright appearing red eye, accentuated by the white around it; the dark on the head does not reach the eye, and the bill is a bright orange-yellow. Here we see a combination of marks — the dark is extensive, and “hides” the red eye within it. The bill is also a vibrant orangish color, unlike the greenish of Western, leaving us with a Western x Clark’s hybrid grebe as the last option.

Congratulations to Adrian for figuring out Western x Clark’s hybrid! This was a difficult one.