16 November 2014

A Tale of Two Thayer's

I made my first visit of the season to the Lake County Fairgrounds on Saturday. I was happy to find the gulls comfortable and resting as they should be. I tallied 6 Thayer's Gulls for the day (3 adults, 1 fourth cycle type and 2 first cycles). Highlights were the 2 first cycles on opposite sides of the color spectrum - one dark individual and one light - which the title of this post is referring to.

Thayer's Gull (1st cycle). Hampton Scale Score 28.

Thayer's Gull (1st cycle). Hampton Scale Score 21.
The lighter Thayer's falls squarely between a solid Kumlien's and Thayer's, and so it wouldn't be unreasonable to call this a Kumlien's/Thayer's type, or KT (for a review of these scores, click here).

Although given the relatively dark outer primaries, along with the darker tertials and darker secondary bar, I'm leaning heavily towards Thayer's.

It's always fun seeing definitive adult Thayer's, but finding a near adult is much more exciting. Here's an open wing with plumage features that are suggestive of a 4th cycle "type":

Note the less advanced wingtip pattern. For instance, the mirror on P10 is faint and smaller than that on P9. Also seen is relatively extensive black streaking on the alula and outer greater primary coverts. 

The upper tail shows black markings and the upperparts are tainted with brown tones - both sub-adult features.
The prebasic molt is finishing up with the inner secondaries and the outermost primary (still about 1" from fully grown).
Lesser Black-backed Gulls are the 4th most common species here (with Herring, Ring-billed, and Thayer's being the other three, in that respective order). Here's an individual that appears to be a 1st cycle Lesser Black-backed. I never did get to see the open wing and so I'll have to think about it a bit more:

Possible LBBG. It's my hope that observers aren't looking the other way when they see these brown types with paler bill bases. I've learned that using the descriptions in field guides, and not allowing for variation at the species level, hampers one's ability to really get to know a species. Large gulls can not be contained by a field guide. 
Of all the species I see here in Northern Illinois, adult type LBBGs are always in heaviest, and latest, flight-feather molt. Even more interesting is that birds at this landfill, in particular, seem to be farther behind in their PB molts than those near the lakefront in Michigan City and New Buffalo (data is insignificant, based on 3 years of observations). Could it be that birds under higher demands of molt-stress resort to a more reliable and "convenient" food source, such as landfills? Whereas others with mostly grown flight feathers may be more inclined to fly out on Lake Michigan and keep to a more aquatic diet?

Miscellaneous Notes:

A 1st cycle Herring with a paling iris. This is not something I often see. Same individual above. 
Herring Gull (1st cycle). Not uncommon is this bill color aberration found on 1st cycle HERGs.
These darker brown Herrings appear to be more common out West. Birds this uniform likely originate from farther north (or away) from the general Great Lakes region.
Many adult type HERGs are growing out their outermost primary right now which gives those birds with a larger mirror on P10 a thayeri feel.  Couple that with the large, fresh, apicals and I had to stop to closely examine these birds a few times yesterday - especially on a couple that had darker, amber-brown eyes.
Thayer's Gull (adult type). Outer two primaries still growing.
One of my most interesting Herring types of the day was this bird:

Herring Gull (2nd cycle).
The mostly black bill and white rounded head give it an interesting look. The pointed brown centers to the newly grown scapulars and long-winged look had me wondering if there was something else going on here. More to come on this bird in my next post!