IBRC 23-B-10

Idaho Bird Records Committee
Rarities Report Form

Species: Lesser Black-backed Gull adult
Reporter: Jay Carlisle Idaho Bird Observatory, Boise State University, 1910 University Ave., Boise, ID 83725; jaycarlisle@boisestate.edu
Other Observers: Heidi Ware, Danette & Iris Henderson

Date Report Prepared: 2-27-10
Date Sighting Occurred: 2-27-10

Locality of Observation: Hidden Hollow landfill, Ada County
Habitat: sand/gravel, puddles, garbage
Conditions: partly cloudy, light breeze; I used Swarovski EL 10x42 binoculars and a Fujinon scope with 20-60x zoom

Did you take notes? not at all
Did you consult a field guide or other reference work? after the observation
What guide(s) or reference(s) did you consult? Sibley & National Geographic

Description: A moderately large gull (slightly larger than the average California) with a dark mantle. The bird was not ‘deep-bellied’ and appeared relatively long & slender. Bill thicker than those of nearby Californias and yellow with an obvious red spot in the gonydeal area. Also, very pale eyes and legs yellow. Dark streaking on the head. Aged as a full adult because of a uniformly dark gray mantle and the presence of small apical spots on several outer primaries.
Behavior: First seen bathing then flushed by vehicle and flew to a large roosting area on sand with other gulls. How and when did you positively identify the bird, and what clinched the identification for you? We’d been on-site for about 2 hrs, looking at well over 1,000 California Gulls. When we spotted this bird, I first noticed the dark mantle (a shade or two darker than nearby Californias) and noticed that the bird was not much bigger than Californias. Upon closer inspection, ID was clinched by moderately large size; long, slender shape; yellow legs; dark gray mantle color; and pale eyes.

How did you eliminate similar species, and what were they? Similar species include all dark-mantled gulls, including the moderately gray-mantled California Gull, but especially Slaty-backed and Western. This bird was about only slightly larger than nearby California Gulls, had a noticeably darker mantle, and had very pale (almost white) eyes. Both Slaty-backed and Western (darker eye) are larger on average, more bulky in body proportions, have pink legs, and have larger bills. Also, Slaty-backed adults usually have a more prominent white crescent separating the primaries from the mantle. Lastly, Lesser Black-backed has a longer primary projection which gives a more attenuated shape (hard to judge in the attached photos but we could see this the few times we observed the bird standing)

Experience with this species (and similar species): I have observed 20 or more Lesser Black-backed Gulls, mostly on the east coast but also in South Dakota and, most recently, in Kenya (subspecies with darker mantle than this bird). This is my 4th in Idaho; all in the last few years.
General birding experience: extensive; 15+ years in US and Latin America

Were photo(s), video, and/or audio obtained by you? (If yes, please include or attach) Yes – 2 attached pics