Rare Bird Report 22-B-09

Idaho Bird Records Committee
Rarities Report Form

 

Species: Thayer’s Gull – 3 individual 1st-cycle birds

 

Reporter: Jay Carlisle

Idaho Bird Observatory, Boise State University, 1910 University Ave., Boise, ID 83725; jaycarlisle@boisestate.edu

 

Other Observers: Heidi Ware

 

Date Report Prepared: 3-25-09

 

Date Sighting Occurred:  3-21-09  Also, I should note that at least 3, 1st-cycle Thayer’s – up to 4 or 5 – have been seen at this location since at least Feb 26, 2009 (my first time there this year)  – it’s just that I/we were paying a lot more attention to the rarer species being found.  This was my first attempt to document these guys.

 

Locality of Observation: Pickle’s Butte landfill, Canyon Co.

 

Habitat: sand/gravel

 

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:  Note – I am attaching photos of 3 individual birds that varied in the relative paleness of their overall coloration and am noting here characters observed on all 3. 

 

The birds were all medium-sized gulls (larger than Ring-bills, comparable to nearby California Gulls, and smaller than the many Herring and the single Lesser Black-backed present) with relatively rounded heads and medium-sized bills without prominent gonydeal bulges.  All had primaries in the dark brown range and each visible outer primary had pale terminal edging; primaries were darker than the mantle but definitely not black.  All birds had dark eyes, dark bills, and pink legs.

 

Behavior:  Seen roosting and/or walking around on sand/gravel away from the main dumping area; curiously, a single bird was seen to be actively picking up and moving ‘tumbleweed’ a few times (see photo).

 

How and when did you positively identify the bird, and what clinched the identification for you?  The clues were the combination of rounded head shape, smaller size than Herring, paler overall coloration than Herring 1st-cycles, brown primaries with pale edging, and moderate-sized bill. 

 

How did you eliminate similar species, and what were they?  Possibilities include 1st-cycle birds (immatures) of Herring Gull (larger, blockier head, black primaries), Iceland Gull (similar size/shape but distinctly paler wingtips), Glaucous-winged Gull (larger, bigger bill, primaries paler), and hybrids – especially between Glaucous-winged x Western or Glaucous-winged x Herring (both generally larger-billed & bulkier). The generally small body size (comparable to California), moderate bill size, mostly rounded head, and the color of the primaries pattern all point to Thayer’s. 

 

On the same day, we saw an apparent 1st-cycle Glaucous-winged x Western hybrid that was huge and very big-billed compared to these birds.  Likewise, we observed at least one apparent Glaucous-winged x Herring hybrid the same day and, though similar in primary color, it was larger with a blockier head and larger bill with more prominent gonydeal bulge.  We also saw at least 10 1st-cycle Herring Gulls (other ages as well), and they were larger (with one exception) and with black primaries and blockier head shape.  The exception was an apparent 1st-cycle Herring at the small end of the spectrum (see attached) that also appeared round-headed but clearly had darker primaries than any Thayer’s present.

 

Experience with this species (and similar species):  I have observed countless Herring Gulls, 100s of Glaucous-winged, 10s of Icelands, and 100s of Thayer’s (including many adults, especially in winter in Oregon, but also many inland sightings – esp. 1st W birds - from Idaho to South Dakota).  I have also observed a number of Western x Glaucous-winged hybrids on the west coast and, on the same day, an apparent 1st-cycle Glaucous-winged x Western hybrid.  I am just becoming familiar with Glaucous-winged x Herring hybrids and one or more were also present the same day.

 

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, of three individuals plus a small (apparent) Herring for comparison.

 

Individual "A":

 

Individual "B":

 

Individual "C":

 

Photo below is a presumed Herring for comparison:

 

Here a Thayer's is in the foreground with a Lesser Black-backed: