Possible adult Vega Gull (Larus vegae) at Lake Loveland, late afternoon, 17 December 2012. Darker eye and darker gray mantle, and thicker white tertial crescent than American Herring Gull (Larus argentatus smithsonianus). Mantle is very slightly lighter than that of California Gull (Larus californicus; last photo below).
These four photos were digiscoped by holding my 70 mM lens to my 20x Kowa telescope eyepiece, and bringing in light with a 1600 ISO setting. Light was low due to a setting sun (which was behind clouds). Birds was backlit too. Photos have not been edited other than cropping.
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Status of Vega Gull in Colorado: Vega is considered a subspecies of L. argentatus by the AOU, and breeds in northwest Alaska westward into Siberia. There are no accepted records in Colorado. This bird was not accepted as Vega Gull by the Colorado Bird Records Committee.
I have photographed other candidates (see for example photos of a white-headed adult at the same location 1 year earlier here: https://pbase.com/quetzal/mysterygullac). Some other candidates I have photographed are heavily marked on the head like this individual. One candidate first-cycle bird - also at the same location two winters ago - was also not accepted as Vega Gull by the Colorado Bird Records Committee: https://pbase.com/quetzal/mysterygully. Another candidate 1st-cycle gull from November 2014 (seen at nearby lakes several miles north) is at https://pbase.com/quetzal/mysterygullai.
