Rare Bird Report 23-B-09

Idaho Bird Records Committee Rarities Report Form

 

Species: Thayer’s Gull (adult)

 

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. 

 

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:  An adult Thayer’s Gull transitioning from basic plumage. 

 

The bird was standing amongst Ring-billed & California Gulls and was notably larger than the Ring-bills but comparable to the Californias.  I first noticed the dark eye combined with very pink legs.  Mantle shading was lighter than California and slightly darker than Ring-bills.  The bill was yellow with a red gonydeal spot and was relatively thin for a medium-sized to large gull.  The bird had black outer primaries with prominent white terminal spots and, when it opened the wing (see photo), it showed large ‘mirrors’ in the outer primaries.  The head was also relatively rounded with the eye placed towards the bill (i.e., not centrally-located).  The head pattern was a little ‘dirty’ but this was mostly limited to the nape and around the eye (much less dirty than an adult I photographed 5 weeks prior at Ted Trueblood; separate report) suggesting a transition towards alternate plumage.

 

Behavior:  Seen roosting on sand/gravel away from the main dumping area.

 

How and when did you positively identify the bird, and what clinched the identification for you?  The initial clues were the combination of dark eyes, rounded head shape, very pink legs, and moderate-sized bill.  When I saw the wing open and could see the extent of white in the apical spots and in the mirrors, I was more convinced.

 

How did you eliminate similar species, and what were they?  Possibilities include Herring Gull (larger, blockier head, pale eyes, less white in primaries), Iceland Gull (similar size/shape but distinctly paler wingtips), and hybrids – especially between Glaucous-winged x Western or Glaucous-winged x Herring (both generally larger-billed & bulkier).  The generally small size (comparable to California), slender bill shape, and the primary pattern all argue for a pure adult Thayer’s.  Lastly, on the same day (at the far end of the same group of roosting gulls), we saw an apparent 1st-cycle GW x Western hybrid that was huge and very big-billed compared to this bird.

 

Experience with this species (and similar species):  I have observed countless Herring Gulls, 10s of Icelands, and large #s of Thayer’s (including many adults, especially in winter in Oregon, but many inland sightings – esp. 1st W birds - from Idaho to South Dakota) and this was my 2nd adult in Idaho.  I have also observed a number of Western x Glaucous-winged hybrids (including adults) on the west coast and, on the same day, an apparent 1st-cycle GW x Western hybrid.

 

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, including one with the wing open.