Identification Quiz #10

Answer courtesy of Stephen Davies:

The chunky body, deep chest, thick neck and broad wings of this bird identify it as a large gull (Larus). The overall brown color of the underparts, and the contrast between the underparts and the paler, more worn head, point to it being a bird in its first calendar year, probably in juvenile/first basic plumage. With no opportunity to view the upperparts, we are left to glean what we can from its overall structure and the appearance of the flight feathers.

The general lack of dark pigmentation in the remiges and tail suggests it is one of the 'white-winged' species or their lookalikes. This process of elimination leaves us with a manageable shortlist of species/forms to consider: GLAUCOUS (hyperboreus, including barrovianus), ICELAND (glaucoides, including both nominate glaucoides and kumlieni), THAYER'S (thayeri) and GLAUCOUS-WINGED (glaucescens). GLAUCOUS is easily eliminated on the basis of bill pattern and extent of dark in the outer primaries, visible on the right wing. On first year GLAUCOUS, the bill is pink with a sharply demarcated black tip - not mostly dark fading paler at the base, as on this bird. The bill is also not substantial enough for GLAUCOUS, which typically has a long, straight, parallel-sided bill.

The upper surfaces of the outermost primaries (P8/9-10) in the right wing are just visible, and they are clearly dark brown. The juvenile primaries of GLAUCOUS (retained throughout the first calendar year, as in other large larids) are pale gray-brown when fresh, looking paler than the rest of the wing, and often with darker internal mottling or chevrons. The outer primaries of 1st year hyperboreus are never uniformly dark as on this bird. The underwings of hyperboreus, particularly the axillary region, would also not appear as dark as on the quiz bird, regardless of the effects of backlighting in this photo.

Although the bill of the mystery bird resembles that of many glaucoides and kumlieni, both in structure and pattern, the coloration of the outer primaries eliminates both of these forms, for the same reasons as for hyperboreus. The outer primaries of glaucoides resemble those of hyperboreus, though sometimes with slightly more prominent internal markings. On average, kumlieni is darker than glaucoides on the outer webs of P7/8-10, with darker subterminal chevrons, but this is variable and many are indistinguishable from glaucoides. Neither form approaches the level of pigmentation present in the outer primaries of the quiz bird. GLAUCOUS-WINGED can also be eliminated on the basis of outer primary pattern. The primaries of first year glaucescens are pale gray-brown, though not as pale as hyperboreus, usually appearing concolorous with the rest of the wing, not contrastingly darker as on this bird. The bill also appears too slight for glaucescens, which has a rather deep bill. The wings appear too long and tapered for glaucescens, being most similar in shape to those of glaucoides/kumlieni of thayeri.

By a process of elimination, we are left with THAYER'S GULL, but can this bird be positively identified as thayeri? As mentioned above, the relatively long, tapered wings and the shape and pattern of the bill are consistent with that species. Thayeri differs from glaucoides and kumlieni in being uniformly dark on the narrow outer webs to P5/6-10, producing a dark wing tip that is darker than the rest of the wing. Superficially, this would appear to match the primary pattern shown by the mystery gull. However, there are several features that argue against identifying this bird as thayeri. The outer primaries appear too dark - on thayeri, the inner webs of the outer primaries are pale, producing the well-known 'Venetian blind' effect of alternating dark outer webs and pale inner webs, which is visible even when the hand is in a relatively closed posture. This is not evident on the quiz bird, which appears to have dark pigmentation on both the inner and outer webs of P8/9-10. On thayeri, the dark on the outer web of each outer primary extends across the tip of the feather, as a dark subterminal bar, producing a dark trailing edge to the outer primaries when viewed from below - this is not evident on the underside of the left wing of the quiz bird.

A possibility we have not considered yet is that of a hybrid, of which several possible combinations are commonly encountered and must be taken into account. On the Pacific coast of the U.S., hybrids of WESTERN x GLAUCOUS-WINGED, HERRING (smithsonianus) x GLAUCOUS-WINGED, GLAUCOUS x GLAUCOUS-WINGED and smithsonianus x GLAUCOUS are all possible, but only WESTERN x GLAUCOUS-WINGED, HERRING x GLAUCOUS-WINGED and HERRING x GLAUCOUS could potentially show his degree of pigmentation in the primaries. WESTERN x GLAUCOUS-WINGED can probably be eliminated on the basis of wing shape and bill size and shape, but this mix can produce a primary pattern similar to the one shown by the quiz bird, particularly when hybrid backcrosses to GLAUCOUS-WINGED are considered. HERRING x GLAUCOUS usually shows a bill pattern similar to that of GLAUCOUS, and is therefore an unlikely candidate in this case. HERRING x GLAUCOUS-WINGED, however, cannot be ruled out. This particular hybrid can look very THAYER'S-like, both in shape and plumage. The lack of definition in the outer primary pattern is typical of this hybrid combination, and the contrast between the pale head and darker underparts is very suggestive of smithsonianus parentage.

As a last ditch attempt to identify this bird, we could consider the fanciful possibility of Slaty-backed Gull (schistisagus). Identification of extralimitial 1st year schistisagus in the U.S. is problematic, because of its potential similarity to WESTERN x GLAUCOUS-WINGED and particularly HERRING x GLAUCOUS-WINGED hybrids. Although variable, it seems unlikely that a worn schistisagus would appear pale enough to match the appearance of this bird. The hefty proportions of schistisagus would also probably be inconsistent with the quiz gull. In conclusion, the absence of several key pro-THAYER'S features should argue against definitively identifying this gull as THAYER'S. Consequently, the bird is best left unidentified, but it is probably a THAYER'S-like hybrid, such as a HERRING x GLAUCOUS-WINGED hybrid. This hybrid is more common on the northern California coast than most birders would like to admit, and can outnumber genuine THAYER'S GULLS at some coastal locations. A close examination of the scapulars, to determine their exact pattern and state of molt would have allowed us to progress further with this bird's identification.

The image below, again courtesy of Stephen, illustrates the "venetian blind" effect of alternating dark outer webs and light inner webs in thayeri:

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Last updated 13 May 2000