This second cycle gull was at Erie Reservoir in Boulder County, Colorado, on 26 Dec 2009. This Herring Gull-sized bird resembles Thayer's Gull (Larus thayeri) but its size seems too big even for the largest male Thayer's Gull. Does this bird's pale plumage fall within range of genetically pure American Herring Gull (L. argentatus smithsonianus, AHGU)? The mostly dark bill and pale remiges suggest Glaucous-winged Gull (L. glaucescens, GWGU), but I suspect that the primary outer webs and the tail may be a bit too dark for pure heritage. The most likely explanation for this "morphotype" would be hybridization (glaucescens x smithsonianus). However, a 2nd cycle GWGU or AHGU both have solidly dark tails, so therefore, a hybrid among these species would also. The pale bases to R5 and R6 creat the appearance of a broad tail band, a common feature of Thayer's Gull (L. thayeri) and Vega Gull (L. argentatus vegae), but also present on some "barrovianus" Glaucous Gulls (L. hyperboreus) and some Slaty-backed Gull (L. schistisagus). Seems like a large male THGU is a candidate, as well as GLGU x SBGU, although neither of these options should have a dark bill at this age. GWGU x VEGU could be the best explanation. What do you think?

The photos have been cropped and sometimes resized but otherwise unedited. Some are underexposed, some overexposed. A few were digiscoped. Click on an image to enlarge.