05 October 2014

A Note on Upperpart Color Variation in HY Ring-billed Gulls

I think it's fairly safe to say that adult Ring-billeds don't deviate from the Kodak Gray values that they've been assigned in the literature (KG value 4-5), but what about sub-adults, particularly hatch year birds?

Hatch Year Ring-billeds. Slightly darker scapulars shown by the individual on the left.
I'm most confident that post-juvenile scapulars on HY Ring-billeds do vary in color tones. I believe this to be primarily a result of these feathers not being "entirely" gray, showing a random mix of brown hues (on dark birds) and white hues (on pale birds).

Left bird shows slightly paler scapulars with whiter hues, while the individual on the right has browner hues.

These differences can be nearly impossible to detect under bright sunglight, and as always, observers should choose overcast days to accurately evaluate grays. Notice how the gulls in my photos aren't casting any shadows onto the ground - a good indication that the lighting is neutral, or mostly neutral.

The variation in upperpart "hues" on these two individuals should be noticeable.
Aside from gray, these post-juvenile scapulars show a mix of grown centers and white edges. These cololrs surely influence how the eye perceives the gray colors on HY birds.
A good example of a "pale" HY Ring-billed. This individual shows silvery-white hues on the post-juvenile scapulars. 

If it was later in the season, the pale gray on the individual above might be attributed to "bleaching", but being this early in the season, my guess is that there's actually very little bleaching - if any - that has affected these feathers. Further, it would seem that individuals with paler upperwing coverts would have paler scapulars, and individuals with more solidly brown upperwing coverts are more likley to show these darker scaps.

Perhaps this caveat also applies to other gull species - American Herring Gull being the first example that comes to mind. I've long suspected that sub-adult Herrings (3rd and 4th cycle types) can show slightly darker grays, and my explanation for this has long been that some of these birds show an excess amount of underlying brown tones within their gray feathers. More to come on this in an upcoming post...