Since there have been a few eBird checklists from Colorado recently that have endeavored to differentiate among the two subspecies of California Gull, I thought that I would post a cautionary note about such in this venue.
As most lariphiles are aware, there are two subspecies of California Gull, the smaller, darker nominate subspecies (californicus, obviously) and the larger, paler subspecies albertaensis. The former is the form that breeds in Wyoming and Colorado, the latter occupies the prairies portion of the breeding range, nearly all of which is in Canada. [Link to Jehl (1987) that described albertaensis.]
The paper reporting on the first known nesting of the species in Colorado was written by the venerable Ron Ryder.
Most birders are probably unaware of the well-established fall-migration pattern of the species, which is used by both subspecies -- heading west to the Pacific Ocean, then, eventually, south (from Winkler, David W. 1996. California Gull (Larus californicus), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.):
There is considerable scatter in orientation of some juveniles, with stragglers showing up in sites well east of breeding colonies (Behle 1958), but orientation of first-time migrants is generally westward. Birds from Canadian prairies apparently cross Rockies in s. British Columbia, since they are abundant there in autumn (Campbell et al. 1990a), and large numbers of California Gulls that follow Columbia River and its tributaries through Sierra-Cascade axis to Pacific Coast appear to be derived from colonies throughout n. Great Basin and Intermountain regions (Diem and Condon 1967, Littlefield 1990, Gilligan et al. 1994). Farther south, birds from large colony at Mono Lake, CA, cross Sierra directly (DWW). Adults and those subadult birds that returned to or near breeding areas migrate back to coast; most of these birds return to coast after most of the juveniles, which arrive in Jul.
Analyses of censuses up and down Pacific Coast (Shuford et al. 1989, L. B. Spear pers. comm.), together with analyses of banding returns (Diem and Condon 1967, Southern 1980, DWW and V. M. Norris unpubl. data; B. H. Pugesek, K. L. Diem, and C. L. Cordes unpubl. data), indicate that these gulls carry out slow but sustained migration up and down Pacific Coast throughout fall, winter, and early spring. Birds of all ages apparently move northward along coast briefly, perhaps in response to northward movement of front of upwelling along coast (Shuford et al. 1989 and references therein). Large numbers build up in early fall in southern coastal British Columbia (Campbell et al. 1990a), but these numbers drop off sharply by early winter. Birds of all ages appear to move south along coast during late fall and winter; younger birds travel farther, and birds from localities farther inland travel farther than those from nearer the coast (DWW and V. M. Norris unpubl. data; B. H. Pugesek, K. L. Diem, and C. L. Cordes unpubl. data).
The specifics of the fall migration of albertaensis is depicted well by Figure 1 in Houston (1977).
The upshot of all this information about fall-migrant California Gulls is that occurrence of the form in Colorado is probably unlikely, particularly in fall. Yes, lots of unlikely birds show up in Colorado in fall, but the point is that reports of such should be thoroughly documented. Considering that variation in size and mantle color is well-known in large, white-headed gulls, relying solely on those two features does not provide definitive proof of subspecific identification, with size being a particularly unreliable character. As proof of the unreliability of size as a differentiator, note the ranges of measurements of the two subspecies, as taken from Pyle (2008):
wing chord (distance from wing tip to "wrist"; a widely-used index of overall size):
californicus 353-413 mm (female - 353-390 mm, male - 368-413 mm)
albertaensis 362-426 mm (female - 362-402 mm, male - 378-426 mm)
As one can readily see, the difference in size between the sexes of a given subspecies is larger than the difference between the subspecies. Furthermore, since the sexes differ in size in all other gull species, using either or both of Ring-billed Gull and Herring Gull as a way to get at an individual California Gull's size is fraught with difficulty -- one would need to know the sex of the yardstick species! In fact, while most birders consider California Gull to be always larger than Ring-billed Gull, as can be seen below, that is not at all true:
Ring-billed Gull wing chord: 330-392 mm (female - 330-369 mm, male - 346-392 mm)
Mantle color in the Cal Gull subspecies is a bit more reliable:
Kodak gray scale values (lower is paler; data also taken from Pyle 2008):
Ring-billed: 4.0-5.0
californicus: 6.0-7.5
albertaensis: 5.0-6.0
Herring (smithsonianus): 4-5
However, given that the apparent tone of gray coloration in gulls is highly dependent upon both sky conditions (sun angle, cloud cover and color) and on precise angle of viewing (such that comparison of gray tones in compared individuals need to be in the very same plane relative to the viewer), certainty about field determination of gray tone is nearly impossible.
The only character that allows fairly certain differentiation of Cal Gull subspecies -- and this ONLY on full adults -- is the precise wing tip pattern (again, from Pyle 2008):
californicus: black on outer primaries extensive, that on outer web of p7 occasionally (~13% of individuals) and p8 often (~56%) reaching primary coverts
albertaensis: black on outer primaries reduced, that on outer web of p7 no reaching primary coverts and on p8 only occasionally (~15%) reaching primary coverts
One may now understand why once I had learned all of these things, I ceased trying to find albertaensis in Colorado.
Enjoy,
Tony
Tony Leukering
currently Guymon, OK
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