[Mystery bird] Ross's Gull, Rhodostethia rosea, photographed near the Tuttle Creek Reservoir outflow tubes in Kansas. [I will identify this bird for you tomorrow].
Please name at least one field mark that supports your identification.
In alternate plumage, the pinkish cast to the plumage and black ring around the head are diagnostic. In basic plumage, the evenly gray wings on both the upper and under surfaces and the wedge-shaped tail (which isn't easily seen in the above image) are also diagnostic. In first-winter plumage the wedge-shaped tail is diagnostic as is the small bill.
I'm pretty sure that's a Ross's Gull, a bird I've never seen, but always wanted to see. I'm basing that on the faint pink underparts (mainly), but checking Sibley I also note the "broad white trailing edge" on the wing, "gray underwing", and "very pale gray above; unmarked wings".
When the occasional report comes through of a Ross's Gull being seen somewhere in the western US, I always feel a tug. I consciously rejected the "chasing after rarities" thing after some bad experiences with people I viewed to be over-zealous, overly competitive birders when I was a teenager, and since coming back to birding later in life I've made it a point to slow down and find my enjoyment in things other than driving long distances in pursuit of checkmarks on a list. But if there was a bird that could make me forget that, just for the joy of seeing something so beautiful, the Ross's Gull would be it.