THE MONTEREY COUNTY LIST
Annotated checklist and data resource
text © Don Roberson
photos copyrighted by photographer(s) credited
all photos taken in Monterey County, California [with one exception noted]
Thayer's Gull Larus thayeri
    with comments on Iceland Gull Larus glaucoides
Thayer's Gull is an Arctic breeding gull that winters regularly in MTY. Most arrive in Nov-Dec and depart in Feb-Mar; spring & summer records are few. A classic basic-plumaged adult is shown below: pale gray mantle (paler than California Gull; just a tiny bit darker than Herring Gull), heavily streaked head & neck (in basic plumage), rather short, thin bill for a largish Larus, bright pink legs, and dark eye (usually; but see right). There is substantial variation in eye color of adults, ranging from dark to darkish (below top) to quite pale yellowish (below bottom). These photos of different adults were taken just minutes apart in the same light conditions in January. Note that both have purplish orbital rings. King & Howell (1999) showed that most adults have pale brown eyes with extremes ranging from yellow to dark brown.
Where to locate a Thayer's Gull in MTY? Thayer's Gulls in MTY apparently forage mostly offshore, although some also use the Marina dump. They are most regularly found, in season, at large gull roosts at Moss Landing and the Little Sur River mouth. Thayer's are much less common at other gull roosts, although they are regular in small numbers at Pt. Pinos, Pebble Beach, Roberts Lake in Seaside, Salinas R. mouth, and Pajaro R. mouth.
all photos above & below © Don Roberson 
except as otherwise indicated
all above: 8 Jan 2004 Moss Landing
The standard identification problem in adult Thayer's is separating it from Herring Gull, the other pale-backed, black-primaried, pink-legged, large adult Larus gull.  The composite (below) compares the head and bill shape of these two species. I have chosen to use a pale-eyed Thayer's because dark-eyed Thayer's are less similar to adult Herring Gull:
Compare the following features:
  • Thayer's had shorter, slimmer bill than Herring. This is a critical field mark and, once learned, is useful even if flight of birds some distance away;
  • Thayer's has a "gentler" head profile caused by more rounded forecrown; Herring looks comparatively angular and therefore "fierce" in comparison. However, as discussed below, females look more round-headed and short-billed than males, and large male Thayer's can approach the size and shape of small female Herring;
  • Adding to the "fierce" aspect of Herring is a very bright angry-yellow eye surrounded by a yellow orbital ring (bare eye-ring), while the orbital ring of Thayer's is dull purple or purple-red;
  • Thayer's may have a tendency to have a bit more duskiness around the eye;
  • Thayer's back color is slightly darker than Herring, but this difference is not apparent in bright sunlight and is better noted on overcast days; and
  • Thayer's tend to have brighter pink legs (as adults) than Herring, but this is an "average" difference that is not useful on individual birds.
Head streaking is very variable in winter. Shown here are a February Herring Gull and a January Thayer's Gull with rather similar head/neck streaking, but both can have much heavier streaking than this. Later, as they advance to alternate plumage, they will lose the streaks altogether.

For another nice comparison page of all four winter plumages of Herring vs. Thayer's Gull, see Greg Gillson's web site

For our local MTY purposes, these comparisons are with American Herring Gull L. argentatus smithsonianus. Many European authorities already split this taxon as a separate species from various Eurasian subspecies of Herring Gull, based primarily on biochemical evidence.
In flight, adult Thayer's Gull show restricted black tips to the outer primaries compared to other large Larus gulls. The white apical spot is large and prominent on p10 (the outermost primary). In most birds each outer primary has a mostly black outer web and mostly white inner web, sometimes giving a "black-and-white striped" effect to the flight pattern on the outer wing (Lehman 1980, Gosselin & David 1975). There is much variation in adult wingtip pattern in Thayer's Gull. Sketches in Snell (2002) are particularly useful as they are linked to known breeding locations on high Arctic islands, while sketches in Howell & Elliott (2001) catalogue variation among 123 winter birds in California. You can see some small variation in the photos on this page, but they do not cover the known range of variation.

left: 25 Jan 1980 Pt. Pinos
On Thayer's Gull (two photos just above), the underside of the primaries are much paler than the upperside, recalling Glaucous-winged Gull. This is because one sees the mostly white and gray inner webs from the underside, rather than the mostly black and white outer webs when viewed from above. Thus a good flight character in adults is a large pale-backed Larus whose upperwings have black-tipped outer primaries but the underwings do not (only very narrow black tips at close range).

On Herring Gull (right), both the upperside and the underside of the primaries show much black and white at the tips.
upper left flight photo © Jeff Poklen ~4 nmi W of Pt. Lobos 8 Feb 2004
two right-hand photos © Don Roberson ~5 nmi NW of Pt. Sur 8 Feb 2004
Note: the upper right gull in flight is actually a third-winter bird (more on it below) but illustrates the same basic point about the underside of the primaries
Another identification problem faced by MTY birders is separating adult Thayer's Gull from adult Glaucous-winged x Western Gull hybrids/intergrades. Glaucous-winged and Western Gulls hybridize and intergrade widely in w. Washington and n. Oregon, and many of these birds winter here in MTY. Birders colloquially call them "Olympic" Gulls. As these are large, pale-backed, pink-legged, dark-eyed gulls, birders using field guides can easily mistake an "Olympic" Gull for a Thayer's Gull, especially at Pt. Pinos or Pebble Beach, where hybrid gulls are common and Thayer's are scarce. In the photos above, compare these features on adult (all white-tailed) gulls:
  • The introgressant gulls ("Olympic") are big billed with a pronounced, swollen gonys; Thayer's have slim bills with less swollen gonys;
  • Many "Olympic" gulls, like the one shown here, have extensive smudging to the head, neck, and sides of breast; Thayer's can be heavily streaked but they look distinctly streaked, not smudged, except around the eye;
  • "Olympic" Gulls are big-bodied and block-headed — they are the size of Western and/or Glaucous-winged Gulls while Thayer's is decidedly smaller and slimmer;
  • The wingtips of "Olympic" Gulls range from slaty to charcoal to dark gray, but not usually black; on Thayer's, the wingtips are black;
  • "Olympic" Gulls tend to have slightly darker mantles than the pale-mantled Thayer's;
  • The bills of "Olympic" Gulls often have dark markings while fully adult Thayer's (i.e., with all-white tails) generally do not; and
  • True adult Thayer's usually have bright pink legs while most intergrades have duller pink legs.
A more serious problem is separating Thayer's from Glaucous-winged x Herring Gull hybrids/intergrades. Glaucous-winged and Herring hybridize widely in southeast Alaska and some of them winter here in MTY. They are much less common than "Olympic" Gull but some can resemble Thayer's Gull more closely. Adult Glaucous-winged x Herring Gull hybrids tend to be larger than Thayer's; have bigger and heavier bills (but can overlap male Thayer's); tend to have dark markings on the bill; and can have yellowish in the orbital ring. For more details see Howell & Corben (2000), Howell & Elliott (2001).

Photo of "Olympic" Gull Nov 1979 Pt. Pinos © Jeri M. Langham; inset Thayer's same bird as first shot at top
Thayer's Gull is a four-year gull. A third-winter bird is shown above in flight (upper right in preceding panel) and to the left. Note these points that are different than adults:
  1. Bill still two-tone or pale with black ring near tip;
  2. Dark spots in otherwise while tail;
  3. Duskiness on primary coverts; and
  4. Primary pattern with more black (and less white) than full adults.
Howell & Elliott (2001) recently showed that the extent of white in the outer primaries increases in each molt during years 3 through 7. Further, one must always recognize that our discussion of plumage (e.g., the idea that this is a "third-year" bird) refers to plumage, not age. Studies of known age gulls have shown that individual birds may be actually a year older or a year younger than is suggested by their plumage state (Monaghan & Duncan 1976).
Thayer's in second basic plumage (=2nd Winter; right) are identified by pale gray back, size, head and bill shape, all-dark tail, dark tertials, and black primaries. The bill is two-toned as typical in most large second-winter gulls.

right: 8 Jan 2004 Moss Landing; © D. Roberson
Thayer's Gull retains juvenal plumage well into the winter; often birds in Feb or Mar are still in full juvenal plumage, as is this Jan bird (left). There is much variation in juvenal plumage from dark birds to light birds; this one is a darkish individual with dark brown (but not black) primaries, mostly dark tertials, a darkish eye patch, and typical Thayer's bill shape (short; it looks a little thick here only because the bill is slightly open to preen its feathers). Same bird is below, right-hand bird.

left & below right: 11 Jan 2004 Pebble Beach
below left: 7 Feb 2004 Roberts Lake
In juvenal plumage there is a wide range of variation between paler birds and darker birds; those below might be called "mid-range" (left) to darker end (right). There is much variation in details of tertial pattern but Thayer's do tend to have dark-centered tertials. There is also size and bill variation due to sexual dimorphism: the left-hand bird below is probably a larger billed male and the right-hand bird a smaller billed female. As noted by Zimmer (1990), our "image" of the jizz of Thayer's Gull is probably based on a more delicate, dove-headed female rather than on both sexes, and this bias can temper our analysis of birds in the field.
As in adults, the main identification problems with juvenal-plumaged Thayer's Gulls in their first winter is with similar aged Herring Gulls and hybrid/introgressant Glaucous-winged x Western Gulls (below).
First-winter Herring Gulls differ from Thayer's by:
  • Herring is a bit larger and heavier-billed, but I've chosen to illustrate what is likely a small female Herring (above) to minimize this character;
  • Herring is black-winged; Thayer's has brownish to tan primaries;
  • The bill of first-year Herring becomes pink at the base much earlier than Thayer's, so mid-winter birds with pink bases are likely Herring while Thayer's shows only the barest hint of of paleness well into winter;
  • Herring often looks decidedly white-headed in its first winter; Thayer's does not;
  • Herring tends to have a less-checkered pattern to the wing coverts; Thayer's looks quite checkered and can sometimes be dramatically checkered; and
  • Herring tends to show show new gray feathers to back (pre-alternate molt using terminology of Howell 2001) and thus is molting out of juvenal plumage by mid-winter, while Thayer's remains in juvenal plumage throughout the winter and often well into spring.
Photo above 10 Feb 2004 Pt. Pinos; © Don Roberson		 First-winter intergrades or hybrids of Glaucous-winged x Western Gull ("Olympic" Gulls), of which the photo above shows a probable individual, can recall Thayer's because they have brownish (not black) wingtips. Among the ways they differ from Thayer's are:
  • "Olympic" Gull is much larger than Thayer's; it is size of Glaucous-winged or Western
  • "Olympic" Gull has much heavier bill with swollen gonys
  • The greater coverts (yellow arrow points to this set of feathers) are typically unpatterned and plain; Thayer's has very patterned and checkered greater coverts
  • "Olympic" Gulls are in molt much earlier than Thayer's and usually look worn or ratty, rather than having the crisp patterns of unmolted, juvenal-plumaged Thayer's Gulls
Remember that here in MTY, "Olympic" Gulls are more common than Thayer's Gull at popular sites like Pt. Pinos or Roberts Lake.

Photo above 7 Feb 2004 Roberts Lake, Seaside; © Don Roberson
In flight, juvenal Thayer's Gull recalls Glaucous-winged Gull because the primaries are not black. and many birds, like this one, have paleish tan primaries than give a translucent feel. Thayer's is a smaller gull overall than Glaucous-winged, and note particularly the distinctive tail band effect (Glaucous-winged has a uniform tail in juvenal & 1st winter plumage). Note also the short, slim bill and delicate head, rather than the big-billed, big-headed look of Glaucous-winged Gull.

As winter progresses, young Thayer's become more worn and can look almost "white winged" by late February, like this one on Crespi Pond (below). Most remain in juvenal plumage well into spring and, like other high latitude gulls, there is no pre-basic molt in the first winter (Howell, King & Corben 1999). Almost all depart MTY by the end of March, with only a few migrants noted later in spring.

right 17 Nov 1979 ~50 nmi SW of Cape San Martin; above 24 Feb 1998 Crespi Pond; both © D. Roberson

The Kumlien's konundrum: As the AOU taxonomy currently (2004) stands, Thayer's Gull is considered closely related to Iceland Gull L. glaucoides, and this latter species has two races: nominate glaucoides (nesting mostly in Greenland) and kumlieni (nesting mostly in extreme northeastern Canada). There is much current debate as to whether Thayer's/Iceland should be considered a single variable species, and whether the taxon kumlieni is a hybrid population between nominate glaucoides (to the east) and thayeri (to the west). Some studies support this scenario; others suggest that biochemically these groups are not each other's closest relatives. There is still much to be learned about the relationship and taxonomy of these groups, which have a long and checkered history (Pittaway 1999).

There is abundant evidence, however, that a few gulls, mostly juvenals, from eastern populations within the range of kumlieni occur as rarities along the west coast of North America. Identifying such vagrants has been a major challenge and the subject of long debate [e.g., see my earlier page on an apparent Kumlien's in Orange Co., California, and also read the contrary opinions]. In Monterey County there have been several claims of Iceland Gull, presumably assignable to Kumlien's, including a specimen at U.C. Davis and several sight records. My own evaluation of these is that only one was probably Kumlien's: a small, dainty, frosty-white juvenal at Moss Landing 15-20 Feb 1998, discovered by Rich Stallcup. My photos of (presumably) this bird on 17 Feb 1998 are below. They have been enlarged to the limits of the film. Compare these with a known juvenal Iceland for Connecticut, shown on purple background to the right:
 



Three different views of the MTY bird 17 Feb 1998 are shown to the left; the bird below is a juvenal Kumlien's Iceland Gull from Connecticut [© Louis Bevier 15 Dec 1986]. This East Coast bird, bordered in purple, is the only photo on this page not taken in Monterey Co.
On the CT bird note the very white primaries with slight dusky spots at the tips; the pale and thinly-barred tertials; the dainty but pot-bellied shape; and the very "dove-headed" appearance.
It is difficult to directly analyze the MTY bird because the photos are so distant and not crisp. To my eye, the bill size, shape, and color pattern (slight pale to base of lower mandible) is very similar, and to my eye the MTY bird is "dove-headed." However, to me it looks less pot-bellied and looks longer-legged than the CT bird. It is difficult to determine the exact primary and tertial pattern of the MTY bird, and we can't see the tail pattern at all. My field notes describe the primaries as pale, frosty "coffee-with-cream-colored;" the tertials as also pale coffee-colored; and the tail in flight as having "ghost image" of a tail band created by fine mottling on the whitish tail. I did not note a secondary bar in flight; note that in the middle photo the pale bird is next to a "dark-end" Thayer's above it and to its left with blackish primaries and secondary bar apparent.

The February 1998 MTY bird was evaluated by details over the Internet on "I.D. Frontiers." It was evaluated by almost all East Coast birders as being well within the range of Kumlien's Iceland Gull, but it was almost near unanimously rejected by the California Bird Records Committee. To date, the only CBRC-accepted juvenal-plumaged Iceland Gull has been an entirely white-winged bird in San Diego Co. in Jan 1986. [I also saw that bird.]

My own view is that the February 1998 MTY bird likely originated from within the population in eastern Canada that is currently assigned to Kumlien's Iceland Gull (e.g., see Zimmer 1991, Snell 2002). However, it is not quite a "classic" Iceland, and is thus best left undetermined. In my book Monterey Birds 2d ed. (2002) it is discussed but not included on the official MTY checklist. For more on Kumlien's Gull, see Zimmer (1991), Snell (2002), and Howell & MacTavish (2003).

Bibliography: these are probably the most important and/or accessible text references to date. Much additional information is available on line; see my links [find the gull specialty sites in the list]:

Gosselin, M., and N. David. 1975. Field identification of Thayer's Gull (Larus thayeri) in eastern North America. American Birds 29:1059-1066.
Howell, S.N.G. 2001. A new look at moult in gulls. Alula 7:2-11.
Howell, S.N.G., and C. Corben. 2000. Identification of Thayer's-like gulls: the Herring x Glaucous-winged Gull problem. Birders Journal 9:25-33.
Howell, S.N.G., and M.T. Elliott. 2001. Identification and variation of winter adult Thayer's Gulls with comments on taxonomy. Alula 4:130-144.
Howell, S.N.G., J.R. King, and C. Corben. 1999. First prebasic molt in Herring, Thayer's, and Glaucous-winged Gulls. J. Field Ornithology 70:543-554.
Howell, S.N.G., and B. MacTavish. 2003. Identification and variation of winter adult Kumlien's Gulls. Alula 6:2-15.
King, J.R., and S.N.G. Howell. 1999. Featured photo: variation in iris color of adult Thayer's Gull. Western Birds 30:55-56 (and back cover).
Lehman, P. E. 1980. The identification of Thayer's Gull in the field. Birding 12:198-210.
Monaghan, P., and N. Duncan. 1976. Plumage variation of known-age Herring Gulls. British Birds 72:100-103.
Pittaway, R. 1999. Taxonomic history of Thayer's Gull. Ontario Birds 17:2-13.
Snell, R.R. 2002. Iceland/Thayer's Gull (Larus glaucoides/thayeri) in The Birds of North America, No. 699 (A. Poole & F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.
Zimmer, K. J. 1990. "The Thayer's Gull complex," pp. 114-130 in Field Guide to Advanced Birding by K. Kaufman. Houghton Mifflin, Boston.
Zimmer, K. J. 1991. Plumage variation in "Kumlien's" Iceland Gull. Birding 23: 254-269.
Use these links to reach other portions of the Monterey County list:
Part 1: Waterfowl through Grebes
Part 2: Albatrosses through Frigatebirds
Part 3: Herons through Cranes
Part 4: Plovers through Sandpipers
Part 5: Jaegers through Alcids
Part 6: Doves through Woodpeckers
Part 7: Flycatchers through Larks
Part 8: Swallows through Pipits
Part 9: Waxwings through Warblers
Part 10: Tanagers through Sparrows
Part 11: Grosbeaks through Finches
or just the plain Checklist (no annotations)
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