On SouthBayBirds (a Northern California Yahoo! group), Mike Danzenbaker posted links to these photos, along with the following notes:
The photos and description all look like classic Gull-billed to me. However, experienced birders are puzzled by this bird and are wondering if this is a hybrid, perhaps, ArcticxForsters or an Arctic in tail moult. Some of the reasons include:This may not appear to be a textbook example of Gull-billed Tern. I've shown these to a few good birders but will withhold the contents of the ensuing discussion so as not to create any preconceptions.
A couple of notes that aren't conveyed by photos:
size: perhaps a bit larger than the surrounding Forster's Terns, but this is hard to judge given the difference in build and the shorter tail.
behavior: slower, more relaxed wingbeat than Forster's. Did not engage in diving for fish as the Forster's were doing. Did make a few short swoops downwards after unseen prey (not sure if over water or not), I'm guessing chasing insects.
no vocalization, unfortunately.
* bill shape too slim (and is there a hint of a reddish base?)
* dark gray trailing edge to the primaries on the upperside of the wing not expected in Gull-billed
Do you find the structure and plumage of this bird to be at odds with Gull-billed?