The majority of these GBBGs are adults and 1st cycles (as is often the case with gulls). They're usually seen associating with Glaucous Gulls and both species are quite skittish. They seldomly come in for chum even when the more common gulls stir a huge commotion (although there's always at least one young bird that takes the bait).
This particular individual caught my attention as I watched it fly in from a distance:
|2nd Cycle GBBG; 13 Feb, 2011. Whiting, IN (Lake Michigan)|
My initial impression was first cycle because of the almost all dark bill and the neatly patterned upperwing coverts, but then important field marks of an older bird presented themselves. For one, 1st cycles will usually have darker secondaries and darker heads. This individual has an increasing amount of white on the secondaries and a clean white head. The upper scapulars seem to have a good amount of gray coming in, showing a "smudgy" look that contradicts 1st cycle feathers. The inner and outer primaries have a strong contrast mainly because of the pale second generation inner primaries. This is exaggerated by the darker black outer primaries which are a result of the 2nd prebasic molt. 1st cycles have a much more concolorous look to the primaries like on this individual:
|1st cycle GBBG; 13 Feb 2011. Calumet Park, IL.|
Did you notice the mirror on P10 in the 2nd cycle GBBG? This dispels any suspicion of a Great Black-backed Gull in its first plumage cycle. Although it should be noted that not all 2nd cycles show a mirror on P10. Here's that same bird again:
|Same 2nd cycle as first photo. Note the mirror on P10 on the right wing. Also note the strong contrast between the inner and outer primaries.|