19 February 2011

2nd Cycle GBBG in Flight

The Illinois/Indiana stateline has been seeing an increasing number of Great Black-backed Gulls in the last several years. I had a high count of 37 individuals between Calumet Park, Hammond Marina and the Whiting BP Refinery on 13 Feb, 2011. A weather related note that I will keep in mind next winter is the amount of ice on the lake. Just one week prior, most of the lake was frozen with only about 12 individuals found between all 3 locations. Numbers rocketed once the "heat wave" swept through (including one day of 58 degree temps). I wonder if these birds went unnoticed way out on the ice over the lake, or if they were birds that moved in from the south. More interesting, on 20 Feb, 2011 there was virtually no GBBGs left at Calumet Park, Hammond Marina and Whiting. In fact, it seemed that these birds were moving in between the rainstorm. I observed 2 adults at Whiting (during the rain) that disappeared just a few hours later (no rain). Coincidentally, Eric Walters had a high count of 4 adults at North Point Marina the same morning. He observed these birds come in one after the other in under a minute; ironically, we had "zero" the day before at the IOS Gull Frolic. Keep in mind that Great Black-backed Gull is still a good find along the lakefront in Lake County. No one seems to know why they favor the Illinois/Indiana stateline but it goes without saying that it must be food-related. This is also true of Glaucous Gulls, but not as noticable as with GBBG.

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.
Note the fine streaking on the head and the "neater" look of the upperparts. The literature mentions that tail patterns and uppertail coverts are variable in these two ages. I agree wholeheartedly. I've found tail patterns to be of little use as some 2nd cycles could have heavier tailbands than 1st cycles and vice versa.

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.