This bird was first seen swimming alongside Herring Gull on 12 March 2013 at Black Hollow Reservoir in northwestern Weld County, CO. Size was similar; bill was bigger than Herring Gull. Wingtip barely extended beyond tail, seemed too short for LBBG. Clean white head. Another adult LBBG present was smaller, longer-winged, and fairly heavily streaked all over head. Later I photographed it staging with 1000+ gulls at dusk at Windsor Reservoir, about 4 miles south. Here, in addition to the other marks noted, we observed large red gonydeal spot, bright yellow legs and feet (seen briefly), 4 primaries extending beyond tertials, and barely beyond tail. Is this bird too large-bodied, large-billed, short-winged to be a Lesser Black-backed Gull (the expected black-backed species in Colorado in 2013)? Alternatives would be:
1) Western Gull in high breeding condition (3-4 previous records of this species in Colorado)
2) Yellow-footed Gull (no previous state records)
3) Hybrid? (perhaps LBBG x WEGU??)

Images are digiscoped. Click to enlarge. Obviously, better photos are needed.