This shot was pure luck, as the bird was swimming and occasionally rolling around in the water a little bit. I happened to take a picture when the wing was spread showing the light and dark pattern of the outer primaries that is the mark of the Thayer's. The inner webs are light and outer webs dark, creating a distinctive pattern from above. The lighting here is creating more of a window effect on the inner primaries than there actually was. From below Thayer's also lacks the dark outer primaries of Herring, appearing entirely silvery on the flight feathers, with just a narrow dark line on the trailing edge of the primaries. This is largely invisible here, but a hint of silver is seen on the underside of the left wing that is folded back over the body. The secondaries are also 'striped' with a light and dark pattern, but this isn't visible from this wing angle.