13 January 2013

First Cycle LWHG with Slaty-like Features

With the increasing number of adult Slaty-backed Gulls being sighted in the lower 48 states in recent years, one would think that more and more 1st cycle birds should be found as well. Don't young birds have a greater propensity for vagrating? Just as in most other gull species, the shortage of reports in "rare" 1st cycle gulls is in all likelihood due to observer bias. As I type this, there are no accepted records of first cycle Slaty-backeds anywhere outside of California, Texas and of course, Alaska. I've also been informed of an accepted record in Mississippi that I'm currently inquiring about. And so this post is an attempt at me sticking my neck out.

Lake County, Illinois; 02 December 2012.

My first impression was that of a heavy-set bird with a blocky head. It stood with its chest almost vertical most of the time, and the neck seemed long (without any apparent threats that would arouse it). The legs seemed short with a sagging appearance to the lower belly. The overall pale coffee-brown plumage seemed too neat for a Herring and the broad pale edges to the tertials and pale chevrons on the folded primaries reinforce that. The dark gray bars on the centers of the upper scapulars are also noteworthy. The plain greater coverts also fit very well with 1st cycle Slaty-backed.

Body size was no bigger or smaller than a Herring, but the jizz was of a more stocky bird - a bird that could bench-press twice as much as your average HERG.

The primaries show an elegant "venetian blind" pattern like Thayer's. The outer webs to the inner primaries show some contrast with the inner webs, and the subterminal tips to these feathers are dark.

In the photo above, a pseudo-string-of-pearls, that's found in adults, is mimicked (especially on the left wing). The upper tail shows a whitish barrier between the upper rump and the tail band. The secondaries have pale tips and the entire secondary bar contrasts with the greater coverts.

This photo reinforces the two-toned pattern on the primaries.

The silvery underside to the primaries and secondaries is also a feature that is not found on our Herrings. Note the goose-like structure of this bird's body and what look like shorter legs. 
As expected, there were mixed feelings about this bird's identity after I circulated the photos (including to 3 birders from Japan who have extensive experience with this species). Some birders thought it was without question a Slaty, whereas others felt there wasn't a single feature that completely contradicts SBGU, but there are enough oddities that make ruling out a hybrid difficult.
  • The bill base has paled relatively early in the season. 1st CY SBGUs tend to keep a rather solid dark bill into late winter/early spring.

  • The inner primaries are somewhat pale and show too much contrast with the outers. 1st CY SBGUs show dull inner primaries that gradually pale from outers to inners.

  • The wing projection seems longer than expected. 1st CY SBGUs have a stubby appearance from the rear.

  • The leg color lacks a deep pink color. Although variable, a majority of 1st CY SBGUs show pinker leg coloration.

  • The lesser and median upperwing coverts look just as washed out as the greaters. In Slaty, the greater coverts appear plain and the tracts above have some checkering/barring.  
And so this individual shall remain in the "unresolved" file. If I had to make an equally reasonable guess, I would pick American Herring x Glaucous. It's my hope that vigilant birders will continue to critically question these large Thayer's-like individuals that are clearly outside the range of American Herring Gull.