Rare Bird Report 38-B-08

Report prepared by Doug Ward

Coeur d’Alene 2007 Christmas Count Rare Bird Report – 4 January 2004

Species:            Thayer’s Gull (Larus thayeri)

Sex/Age:          Adult (female ?)

Dates:               16 December 2007

Location:          North Idaho College, Spokane River/Lake Coeur d’Alene; Coeur d’Alene, Kootenai County, Idaho

Description:      Medium sized “white-headed” gull in adult basic (winter) plumage.

                        Plumage:  Underparts from lower chest and sides through to the lower tail coverts, white.  Rump, upper tail coverts and tail white.  Head and upper chest white with limited tan colored smudging (indistinctly marked feathers) around the crown, nape and sides of upper chest.  Mantle evenly colored grey; only slightly darker in shade than the adult Ring-billed (Larus delawarensis) and Herring Gulls (L. argentatus) with which it was sitting.  Upper wing:  All coverts same shade of grey as mantle.  Secondaries and tertials also same shade of grey as rest of back but showing a medium width, white trailing edge / “tertial cresent”.  Primaries also mostly grey, though outer 4 sharply marked sub terminally with dark, slate-grey; most extensive on outer two primaries (P9 & P10) and concentrated on the outer web of each feather.  Outer two primaries also marked with white “mirrors” within the slate-grey sub terminal markings, and mostly confined to the inner web of the feather, with the outer primary (P10) showing the largest “mirror”.  All primaries were also tipped with white; narrowly (outer) to medium width (inner).  Under wing:  Entirely silvery-white from auxiliaries to tips of primaries except for a white trailing edge, and a slightly darker “reflection” of the primary pattern shown on the upper wing.

                        Bill / Feet / Eye Color:  Bill was pale yellow with a red gonydeal spot.  The iris was dark yellowish-brown and the orbital ring was dark though color could not be definitely determined although the bird was seen at close range.  Legs and feet were pink; while darker in shade than the flesh colored legs of the Herring it was sitting with, they were not bright.

                        Structure:  Medium sized gull.  In direct comparison, slightly smaller in size and bulk than an adult Herring Gull, with which it was sitting.  The head was smallish, and very rounded in general shape with a fairly high angled forehead.  The bill was also slight, both in length (~¾ head length) and weight, which was highlighted by a straight to curved culman and a fairly shallow gonydeal angle.  Wings were moderately long (extended slightly beyond tail when perched) and appeared somewhat narrow in flight.

Behavior:          This bird was resting (standing), though alert with head up, on floating “log booms” just offshore.  It allowed close approach (~30’) and it took a little effort to harass it into flight to verify identity; primary pattern and under wing marking.

Similar Species:  Other species considered for comparison and elimination were Herring Gull (Larus argentatus), “Kumlien’s” Iceland Gull (L. glaucoides kumlieni), and California Gull (L. californicus).  This bird was associating closely with adult Ring-billed (L. delawarensis) and Herring Gulls allowing for direct comparison.

                        Herring Gull was eliminated initially by this bird’s general structure and dark iris. The structural features were highlighted by the fact this bird was standing immediately adjacent to an obvious adult Herring; small, rounded head and a short, light weight bill with a straight culman.  Conformation came as the bird took to flight and revealed its completely silvery-white under wing.

                        “Kumlien’s” Iceland Gull struck me as a possibility as this bird seemed small and generally pale for a typical Thayer’s Gull (L. thayeri); mantle only slightly darker than the Herring and Ring-billed Gulls present, as well as the limited amount of “winter smudging” on the head.  This consideration was the principal reason I wanted to flush the bird into flight to check its primary pattern.  When it did take off, it was apparent the markings on P9 and P10 (outer two primaries) were those of a Thayer’s and not a “Kumlien’s” – the white “mirrors” on those two feathers were largely confined to the inner web of the feather where an Iceland should show fairly extensive white across to the outer web as well.  In addition, the dark iris of this bird is apparently inconsistent with the majority of Kumlien’s.  On a more subjective note, this bird did not exhibit the “silvery” quality of back coloration that the Icelands (including “Kumlien’s”) that I have seen previously.

                        California Gull was eliminated by this bird’s pink legs, and bill coloration (yellow bill with red gonyal spot, no black on gonys).  Structure also inconsistent with California; high forehead, short neck, flat back).

References:       1) “The Sibley Guide to Birds”; Sibley; 2000

                        2) “Complete Birds of North America”; National Geographic; 2005

                        3) “Advanced Birding”; Ken Kauffman; 1990

                        4) “The Monterey County List; Thayer's Gull Larus thayeri, with comments on             Iceland Gull Larus glaucoides”; D. Roberson; 2003 (?);           (www.montereybay.com/creagrus/MTYlistTHGU.html)

                        5) Discussion of Irish Rare Bird Committee (“IRBC”) with reference to “Howell,           S.N.G., and B. MacTavish. 2003. Identification and variation of winter adult Kumlien's Gulls. Alula 6:2-15.”; 4 January 2004;   (www.netfugl.dk/news_print_version.php?news_id=4423ff895751f380)

Photographs:    None