A Black-headed Gull was a nice surprise at Tiscornia for Tim, Rhoda, Warren Whaley, and myself this morning. Tim and I had more expected Black-headed to appear on one of those days when Bonaparte's are moving in flocks or, better yet, foraging in large numbers along the pier. This morning however, Tim saw one Bonaparte's. I saw one Bonaparte's. And then this bird.
From above the bird has a Bonaparte's wing pattern with a leading white triangle. The black under-primaries, however, are unique.
Here's what the bird looked like as it appeared in my scope flying towards me.
I could pick up some slight reddish at the bill base, and the black ear spot seemed finer than a Bonaparte's would be, but flying head-on I couldn't convince myself of the black primaries. In hindsight, I could have called it out immediately; adult Bonaparte's don't have reddish at the bill base (I'm gonna say I was thinking of Franklin's since they can show a reddish bill base this time of year).
It flew in with a Ring-billed (which briefly harassed it). The only other Black-headed that I've seen was an over-wintering bird along the Chicago lakefront. That bird, also an adult, was hanging out with Ring-billed's, so maybe they're not as tightly associated with Bonaparte's as we thought. They're certainly closer in size than one might expect.