Identification notes of a hybrid Glaucous-winged Gull (GWGU; Larus glaucescens) x Herring Gull (HERG; Larus argentatus) from Wheatley, Ontario - April 27, 2022.
In the field, this bird was recognizable as a Glaucous-winged type Gull both standing and in flight due to it's unique "west coast" proportions and bulk. The dark gray primaries indicated a hybrid, yet it retained a strong impression of a "white-winged" gull. The mantle shade initially appeared slightly darker than nearby Herring, but at other times appeared similar. Some signs of immaturity such as mottling on the underwing suggests a 4th cycle plumage which is frequently shown by GWGU.
With the dark primary markings extending up to the base of the feather, from P10 through P7, it initially reminded me of the primary pattern shown by Western Gull (WEGU; Larus occidentalis); however, the markings only occurred on the outer web of each feather which is not shown by WEGU . I also believe it would be unlikely for an Olympic Gull (GWGUxWEGU hybrid) to show an extensive "string of pearls" such as this bird, which are as large to larger than what is expected on many GWGU, particularly with large pearl/mirror on P9. Finally the darkness of the primaries compared to the near-HERG mantle tone seemed like an odd combination for a hybrid involving a dark mantled species like WEGU.
The heavy structure of the bird, the extensive string of pearls, and the "venetian blind" affect to the primary markings provides some reason to consider a GWGU hybrid with Slaty-backed Gull (SBAG; Larus schistisagus) also known as Commander Gull, which (yes is a thing, and...) can seemingly look quite similar to the Wheatley bird; however, the HERG-like mantle tone would likely exclude this hybrid combination.
The combination of darker primaries with a paler mantle is suitable for "Cook Inlet" Gull (GWGUxHERG hybrid) which is remarkably common in the Gulf of Alaska; however, the "string of pearls" primary pattern is still likely rare with this hybrid (personal observation from extensive experience in Prince Rupert, BC).
Above: A hybrid GWGUxHERG from Prince Rupert, lacking pearl or venetian blind patterning in the primaries.
Despite the differences in primary pattern, I still felt that GWGUxHERG may be a reasonable explanation. Hybrids from the Gulf of Alaska, while common, may not be an ideal source for occurrence in Ontario. Could a GWGU at the northern/northwestern limits of occurrence hybridize with a "northern" HERG (i.e., with a reduced black primary pattern) to produce a primary pattern like the Wheatley bird? Probable GWGUxHERG hybrids from the Yukon were surprisingly similar!
With further reflection, GWGUxHERG hybrids from more northerly populations are likely longer-distance migrants and therefore potential vagrants to the interior/east, especially when considering prevailing westerly winds and avoidance of the Rocky or Coastal Mountains. Ultimately I felt comfortable with a GWGUxHERG label for the Wheatley bird.
I hope this note provides some insight into the potential occurrence and identification of vagrant GWGUxHERG hybrids, but also as an example of how a vagrant bird may not always have the appearance of a typical individual from the core of their expected range.
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