This individual was found yesterday by William Legge at Rodeo Lagoon (37.8319843,-122.5287520). I visited the site 18 September 2018 at about 2:30pm. A group of birders were viewing the bird from the side of the road. I spent the next hour or so viewing the bird off-and-on and attempting to photograph it. It was slightly larger than adjacent Song Sparrows, but with a decidedly longer tail. The upperparts were an unstreaked brownish-gray color with a bold white supercilium contrasting with a dark lore stripe. The face was gray but the throat and underparts were a pale off-white except for some brownish-gray smudging on the side of the breast. The bird showed two obvious whitish wing-bars formed by pale tips to the greater and median coverts. Tertials were very long extending beyond the folded primaries and had obvious pale-buff fringing on the outer webs. The tail was fairly long, black with white trim along the sides. I judged the tail to be about the same length as the folded wing. Tail was often cocked as the bird leapt in the air chasing files. Lets seemed brownish. Bill, slim and dark with a slight bit of yellow near the base of the lower mandible.
Yellow Wagtails seen in California have been assumed to be Eastern Yellow Wagtail (called "Green-headed Wagtail" by Christides et al.) now split from the similar Western Yellow Wagtail (Motacilla flava). Eastern Yellow Wagtail currently has four accepted subspecies in three groups. Birds breeding in Alaska are the nominate race M. t. tschutschensis. The race M. t. simillima breeding in the Bearing Sea is now merged with nominate; thus only M. t. tschutschensis has been recorded in North America. California has 18 previously accepted records but none in the last ten years. Prior records were all in September except for two in late August.
Identification of immatures (formative plumage) such as this has been problematic. Analysis by Bot et al. (2014) suggest that first fall Eastern Yellow Wagtails usually have a monochrome cold gray and white plumage. Such coloration is rare in the western species which is usually more brown and yellow. A long hind claw is also an indicator favoring Eastern Yellow Wagtail. The rear claw is clearly visible in this photo taken by David Nelson and included by permission on Kris Dunlap's eBird checklist. Vocalizations differ with Eastern having a more rasping flight call. Calls need to be recorded and analyzed to confirm. Fortunately a voice recording of this individual was obtained by Mark Stephenson and included in his 17 September eBird report. I briefly heard the bird call one time today.
Digiscoped with Panasonic LX5 | Nikon FS III | 30X fixed | no adapter.
References:
Alström, P. & Mild, K. (2003) Pipits & Wagtails of Europe, Asia and North America. Identification and Systematics. Christopher Helm, London.
Badyaev, A. V., B. Kessel, and D. D. Gibson (1998). Eastern Yellow Wagtail (Motacilla tschutschensis), version 2.0. In The Birds of North America (A. F. Poole and F. B. Gill, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.382
Bot, Sander & Groenendijk, Dick & Van Oosten, Herman. (2014). Eastern yellow wagtails in Europe: identification and vocalisations. Dutch Birding. 36. 295-311. [PDF]
California Bird Records Committee [CBRC] (R. A. Hamilton, M. A. Patten, and R. A. Erickson, eds.). 2007. Eastern Yellow Wagtail (Motacilla tschutschensis) in: Rare Birds of California, Western Field Ornithologists, Camarillo, CA: (retrieved from Rare Birds of California Online: http://www.wfopublications.org/Rare_Birds/Eastern_Yellow_Wagtail/Eastern_Yellow_Wagtail.html on 18 Setpember 2018).
Christidis et al. 2014. The Howard and Moore Complete Checklist of the Birds of the World, version 4.1 (Downloadable checklist). Accessed 18 September 2018 from https://www.howardandmoore.org/
Clements, J. F., T. S. Schulenberg, M. J. Iliff, D. Roberson, T. A. Fredericks, B. L. Sullivan, and C. L. Wood. 2018. The eBird/Clements checklist of birds of the world: v2018. Downloaded from http://www.birds.cornell.edu/clementschecklist/download/
del Hoyo, J. & Collar, N. (2018). Eastern Yellow Wagtail (Motacilla tschutschensis). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/1344120 on 19 September 2018).
Gill, F & D Donsker (Eds). 2018. IOC World Bird List (v8.2). doi : 10.14344/IOC.ML.8.2. Available at http://www.worldbirdnames.org/
Harris RB, P Alström, A Ödeen & AD Leaché. 2018. Discordance between genomic divergence and phenotypic variation in a rapidly evolving avian genus (Motacilla). Molec. Phy. Evol. 120 183-195.