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This immature, and probably female, Bay-breasted Warbler was initially reported on 7 October 2017 by Maggie Smith and photographed here by Curtis A. Marantz on 12 October 2017 at Oso Flaco Lake, Pismo Dunes State Recreation Area, San Luis Obispo County, California.
This was a relatively large warbler with a conspicuously plump shape and a relatively short tail. The bill was slim, but not as obviously so as those of many warblers, and it appeared to taper from a base that was deeper than those of some warblers to a pointed tip along a culmen that appeared to be slightly decurved. I further thought the bill would have extended backward on the face to a point just behind the eye. The forehead was weakly sloping and the crown gently rounded on a head that was unremarkable in size. The neck was short and inconspicuous, but the body was conspicuously plump and full-chested on a bird the posture of which was closer to horizontal than diagonal, and with the tail held straight back in the plane of the body. I had a difficult time seeing the primary projection on a bird that I typically saw foraging above me. The undertail coverts appeared to reach about halfway out a tail that was about as long as the body without the head and neck, slightly flared, and weakly notched at the tip. The legs were unremarkable in their length and mass for a warbler of this size. This bird had relatively plumage coloration yet the patterns were muted apart from that one the wings. The crown, nape, back, and scapulars were a relatively bright, olive-green in color and I further noted some fine, dark streaking one the back. I was less sure that the forehead was green, though I think it was, or if the crown was streaked. Demarcating the lower edge of the green cap was a yellowish-buff supercilium that was short and relatively inconspicuous as it extended through the supraloral region, over the eye, and back only a short distance beyond before tapering to a point. I further noted that the lores were indistinctly dusky, but this bird did not have an obvious eyeline, and my impression was that the auriculars were yellowish-olive without any obvious pattern. Moreover, the more greenish colors of the face blended into the buffy-yellow color that extended across the lower part of the face and the sides of the neck and breast. The throat and the center of the belly were off-white, though tending toward buff, and they blended into both the buff suffusion across the breast and the pale, chestnut suffusion that extended along the flanks. The faint chestnut color of the flanks blended not only into the more whitish belly and the buffier sides, but also with a diffuse region of buff that extended across the vent region and the seemingly more whitish to light buff color of the undertail coverts. The underside of the tail was white internally, apparently representing the inner webs of the outer feathers, but with darker frames that represented the outer webs of these feathers Contrasting conspicuously with the green back and chestnut to buff underparts, the wings were slate-gray to blackish apart from bold fringes that most conspicuously represented broad, white tips to both the greater and median coverts that produced two conspicuous wingbars. I also noted narrow edges of greenish-olive to the greater coverts that resulted in a series of fine stripes between the wingbars. The remiges were narrowly yet crisply edged with what appeared to be either light gray of olive to produce a finely striped pattern to the rear part of the wing, and one in which the primaries and secondaries had fringes that were similar in both color and character. I did think the edges to the innermost secondaries were broader, more conspicuous, and seemingly more gray and less olive. I also noted fringes on the primaries that were just broad enough for me to note the various feathers in the wingtip. The tail did not appear to contrast strongly with the remiges. The bill was generally dark, yet I noted what appeared to be a moderate amount of medium-gray on the lower mandible. The eyes were dark and the legs were slate-gray.