Here are some details on today's Yellow-browed Warbler along San Vicente Ck at Ember Ridge, Moss Beach, San Mateo County. First off, it was seen more or less continuously (never absent for more than about 15 minutes) today from 11:15ish, when I found it, through around 5 pm. As far as I know, it was not seen after 5 pm. It calls fairly regularly, sometimes in flight, and is often located by call. It is extremely active and stays at or above eye level. It was occasionally with loose flocks of gleaners, but not tightly tied to them. Fortunately, it remained quite faithful to the willows around (37.5245821, -122.5039195); I think the furthest it ranged was about (37.52411, -122.50385). The weather overnight looks reasonably promising for the bird to remain, for whatever that's worth; the Alpine bird stayed two days (10/24–25/2019) and the BC bird stayed seven (10/18–24/2019).
Birders are permitted to walk around the entirety of the Ember Ridge stables area. However, we are NOT permitted to park there. *Please park along Etheldore St* (e.g., around 37.52247, -122.50647) and walk about 5 minutes up the dirt road to the stakeout. Whatever you do, do NOT park in the stable's parking lot, where the bird was seen. Ember Ridge is a working stable and there are people walking horses around and so forth. They have all been okay with the presence of birders so far, even at times when there were more than 50 people there today, but please stay out of the way as much as possible and be super nice to them and all that.
Identification in this genus is of course challenging. In a North American context, the only Yellow-browed-type Phylloscopus recorded in Alaska are Yellow-browed (of course) and Pallas's Leaf, and Pallas's is readily eliminated by the San Mateo bird's lack of a bold central crown stripe, lack of a contrasting pale rump patch, less compact structure, etc. Other potential vagrants to North America, as yet unrecorded, include Two-barred and Hume's warblers. Two-barred is longer-billed, longer-tailed, and lacks contrasting pale tips/edges to the tertials, which the San Mateo bird shows; Hume's is supposed to lack a pale base to the lower mandible, which the San Mateo bird has. And, more importantly, the San Mateo bird has been giving "suweet" calls, kind of like the ascending call ("male position note") of a Western Flycatcher but more symmetrical, i.e., with more downsweep before the upsweep, and without any break in it. This is unlike the calls of Two-barred, Hume's, and I believe also rules out a bunch of really implausible things like Brook's Leaf Warbler, Emei Leaf Warbler, etc.
Photographs and audio are on eBird (e.g., https://ebird.org/checklist/S404567035), and further written details will be added as well. This is the second for California after a bird found by Todd Easterla in Markleeville, Alpine Co., in 2019. Consider submitting documentation to the California Bird Records Committee (https://www.californiabirds.org/report_sighting.html) if you see it.
Spending all day with this bird and so many awesome birders from around the Bay Area was the birding experience of a lifetime! Good luck to all chasers!!!
