Idaho Bird Records Committee Rarities Report Form
Rare Bird Report #: 8-B-08

Species: Glaucous-winged Gull, adult

Reporter: Cliff Weisse, 4125 Beaver Springs Rd., Island Park, ID 83429, 208-558-7789, cliffandlisa@octobersetters.com

Other Observers: Darren Clark (on 12/9), clarkd@byui.edu; Steve Butterworth (on 12/9)(photos included with report), tntbutters@cableone.net; Lisa Weisse (on the 12/18), contact as above.

Date of Observation: 12/9/2007 & 12/18/2007

Date Report Prepared:1/20/2008

Locality of Observation: On the Snake River below CJ Strike Dam

Habitat: River

Conditions: Can't remember details about wether but it was cold on the 9th, single digits above zero as I recall, and we were there in the early am, just after daybreak. On the18th we arrived in mid afteernoon. The bird was mostly viewed in shade on the 9th and in sunlight on the 18th. Distance was about 100 yards with Nikon 10x42(Cliff) & 10x32 (Lisa) Premier bincos and Swarovski 80mm scope with 20-60x eyepeice.  Also observed on 12/18 at the upper end of Bruneau Arm from the end of the road at Jacks Creek WMA.  Distance was several hundred yards so details were not attainable but mantle shade was compared with Herring and California Gulls at this location, with Ring-billed at the dam.

Did you take notes…  Not at all

Did you consult a field guide or other reference work?

            during the observation?  Yes

What guide or reference did you consult?  Peterson's Gulls of the Americas and Olsen and Larsson, Gulls of Europe, Asis and North America

Description: Mostly taken from photo but Primary extension is from memory and was specifically looked at on both dates.  Primary color also from memory and compared to photos in O&L and Peterson's during observation.  Mantle shade from memory.

Large white-headed Gull in adult basic plumage.  
- Larger than California Full and similar to Herring Gulls in overall size
- Body heavier/bulkier compared to other gulls present
- Head heavily streaked with gray covering head and neck down to the breast giving hooded look.  Streaking was smudgy or not well defined,not crisp/sharp. Head shape was rounded without squared off fore- and hidnd-crown. Eye more centrally located than on Herring Gull.
- Mantle gray with no hint of brown.  Shade oviously darker than Ring-billed and Herring Gull, not as dark as California Gull.
- Bill was heavy with slight buldge formed by strong gonydeal buldge, and was yellow with black ring around gonydeal buldge, similar to Ring-billed Gull.  Hint of red was visible on lower mandible adjacent to the rear of the black ring.
- Large white tertial crescents
- Tertial stack was wide/tall and square.
- Primaries were wide based and round tipped.  Color was dark gray with white apical spots, slightly darker than mantle, but appear darked in the photos.
- Two primary tips were beyond end of tail and P8 did not reach the end of the tail.  
- Tail white with no apparent dark markings.
- Iris dark
- Underparts clean white

Behavior: Sitting on water and trying to steal food from diving ducks including Common Goldeneyes and Common Mergansers.

How and when did you positively identify the bird, and what clinched the identification for you?  This bird immediately struck me as a Glaucous-winged Gulbecause of the darker mantle than Ring-billed Gull and the gray primaries.  Close examination of primary projection and comparison of primary shade to photos in references, especially to #33.2 in Gulls of the Americas, which this individual matched almost exactly in primary shade, convinced me that it was a Glaucous-winged and conclusively ruled out hybrids with Herring and Western.

How did you eliminate similar species, and what were they? Other gulls likely to be confused with an adult Glaucous-winged include various hybrids, and possibly Thayer's, Herring, Ring-billed, or Iceland.  

- Iceland Gull is eliminated because this individual has a heavy bill, mantle obviously darker than Herring Gull, heavy/bulky body, short primary extension, dark iris, and heavily streaked head.
- Ring-billed Gull is much smaller, has yellow iris, long primary extension, black primaries, and paler mantle.
- Thayer's Gull is similar in having mantle darker tha Herring and sometimes gray primaries.  However Thayer's has long primary extension, narrower primaries and tertial stack, less bulky body, and much finer bill.
- Herring Gull has paler mantle, less heavy bill, slender/elegant body shape with longer primary extension, yellow iris, and black primaries.
- Glaucous-winged x Western Gull has heavier bill, darker mantle, darker primaries, and longer primary extension than this individual.  Usually less head streaking too.  
- That leaves the main contender, Glaucous-winged x Herring Gull.  Hybrids from that combination are expected to have paler mantle, darker primaries, longer primary extension, and paler iris.  Head and bill shape should show influence of Herring Gull.  Head is flatter with more squared of hind- and fore-crown, eye set further forward, and bill narrower than this individual shows. '

In summary there is nothing about this individual that is not consistent with what's expected for Glaucous-winged Gull.  The primaries are on the dark side of normal range and everything else is quite typical.

Experience with this species: I've seen one adult Glaucous-winged Gull in Idaho and several on the west coast on a trip to CA.  I also see a few GWGUs and GWGUxHEGU annually in Idaho.

General experience birding: 15 years

Were photo(s), video, and/or audio obtained by you? Photos were obtained by Steve Butterworth and are included below.