Currently, the Kodak Gray Scale (KGS) value given for this taxon is 4-5 (Howell & Dunn 2007). For all intents and purposes, a one-point difference is typically thought of as minimal and negligible at best. Are there, however, adult smithsonianus individuals with KGS values that are potentially greater than 5?
Examples of Upperpart Variation
Adult aregentatus (front) with a typical argenteus (back). Katwijik, Netherlands. January 2018. Photo courtesy Mars Muusse.
The species is a bona fide black-backed gull, such that the occasional pale individual - potentially matching a pale graellsii Lesser Black-backed Gull - often raises doubt and confusion as to whether a hybrid is at hand or simply a pale-end bird.
Britian's first Slaty-backed Gull is of a paler individual. Greater London. January 2011. Photo courtesy Dominic Mitchell.
A Closer Look at American HerringAlthough widespread and the most abundant pink-legged species in North America, American Herring Gull remains largely understudied. Some authorities have suggested at least 2 distinct populations may exist, based on size & proportions, and wingtip primary patterns. However, I know of no exhaustive efforts that have been made to formally describe these types in known breeding colonies. Further, and most relevant to this post, little data is available on extreme dark-end and pale-end smithsonianus adults. Again, another consequence of little data from known-origin birds.
Below, I present 3 adult type Herring Gulls from east-central Florida. Each appeared to be 1.5-2 shades darker than surrounding adults. Note that the bare parts are in line with Herring Gull, as are the wingtip patterns.
All 3 birds were observed between 24-28 January 2018. The darker upperparts were striking, allowing me to easily refind them with the naked eye.
|Left of center.|
|In the center of the frame.|
|Left of center. Seen here with a large Herring (right).|
|Inner secondaries still not fully grown.|
|Seen on left with two Laughing Gulls.|
|On left with wings fully raised.|
The most important question to begin with is why aren't these hybrids/backcrosses? Great Black-backed x Herring and/or Lesser Black-backed x Herring are the most likely dark-backed Herring hybrids. I'm not ruling out that possibility. But outside of the darker upperparts, there are no obvious hybrid characteristics. As noted above, all bareparts (leg color, bill color and orbitals) are within range for American Herring Gull. The primary patterns also belong to smithsonianus. There may be some distant genes responsible for the darker grays we see here, however.
As for other taxa to eliminate, Vega Gull comes to mind when we think of darker Herrings with pink legs. I feel confident in eliminating Vega Herring based first and foremost on the orbitals. I don't know of any examples where Vega has been documented with an orange orbital.
If this is nothing more than variation found in smithsonianus, why isn't it detected/reported with more frequency? Perhaps many of them are written off as hybrids. It may also be a matter of not appreciating these differences in the field. I must admit, I've not detected such dark birds in my Herring flocks in the Lake Michigan region.