10 December 2017

Variable Widths to Inner Primary Tips in Iceland Gulls

A few years ago I began to notice a small percentage of adult-type Iceland Gulls (thayeri & kumlieni) with relatively broad white tips to the inner primaries.

The most extreme of these types recalls a pattern not unlike the trailing edge on Slaty-backed Gull:

An unusual pattern with white tips to the inner primaries congruent in width to those of the secondaries. Also note how white eats into the feather shafts on p2-p5. Chicago. January.

I've only seen 3-4 birds similar to the one above. Others generally have slightly more narrow tips to p1-p2, but widen thereafter.

 Superior, Wisconsin. November.

Michigan City, Indiana. December.

Chicago, Illinois. February. 

Whiting, Indiana. March.

Seattle, Washington. January.

New Buffalo, Michigan. November. 

In general, it's much more common to see narrower tips on the inner primaries in this species:

The inner primaries have white tips that are noticeably thin when compared to the trailing edge to the secondaries. This appears to be the most common pattern in this species. Winthrop Harbor, Illinois. March. 

The tips to the inner primaries are merely thin slivers here. Could this "look" be a result of behavior and the way the feathers are being held? Hammond, Indiana. December.

So why do some Iceland Gull show this? I believe it is nothing more than variation at the species level (similar to how some Glaucous-winged and Mew Gulls show it, and others don't). One question to ponder: Does the apparent shape or "width" of the white tips change depending on behavior and/or the way the hand is held?

I don't believe, however, that these wide tips to the inner primaries are a result of fresh basic primaries that haven't suffered wear. Generally speaking, the inner primaries stay intact throughout most of the year and my examples above are all from the winter season.