Monday, November 24, 2014

Hybrid Herring x Great Black-backed Gull

Gull hybridization occurs rarely, but it is most frequent in large white-headed gulls. Hybrids are highly variable and can show almost any combination of parental characters. The resultant hybrid offspring often look intermediate between the parent species. This combination of features can produce a hybrid gull that closely resembles another species entirely.
In Newfoundland, there are Herring x Lesser Black-backed Gull hybrids that resemble the sought after Yellow-legged Gull. Bruce Mactavish likens differentiating between a real Yellow-legged Gull and its hybrid imposter like splitting an atom in his blog.
On November 22, 2014 I found a rather odd gull at Dennis Point Wharf in Pubnico, Yarmouth County. It was large, perhaps about the size of a Herring Gull or larger. Its mantle was similar in shade to a nearby Lesser Black-backed Gull. Its legs were lead-pink and it had a fairly large bill.
Experienced larophiles on the North American Gulls Facebook Group have labelled it as a probable hybrid Herring x Great Black-backed Gull. I use the word "probable" because one can never be sure of a hybrid's exact lineage without having direct information on the parents (which would be available if a mixed pair's chick was banded). The speculation on the parent species of a hybrid are simply educated guesses.
HERG x GBBG - Dennis Point Wharf - Nov. 22, 2014.
HERG x GBBG - Dennis Point Wharf - Nov. 22, 2014.
The following is a list of features of this hybrid and their probable origins.
GBBG-like Features
  • faintly streaked nape and head
  • heavy bill with extreme gonydeal angle
HERG-like Features
  • White mirror on P9 and P10
  • Black on P5-P10
Intermediate Features
  • Mantle shade (half way between HERG and GBBG)
Unique Features
  • Lead pink legs 
The upper wing tip pattern shown below more closely resembles an average HERG. The hybrid has a large white mirror on P10 and a small one on P9 like a HERG. The black in the primaries goes from P5 to P10. A GBBG in comparison typically has fewer primaries with black (P6-P10), P10 has a completely white tip and P9 has a white mirror. See the wing tip comparison image below which shows these features.
Wing tip comparison
This hybrid gull is superficially similar to a Slaty-backed Gull. A few missing features are the bright pink legs, broad white tertial crescent, broad white secondary tips and dark streak through the eye. A Western Gull would show much less white on the wing tips and an even whiter head and neck.
A similar gull also believed to be a HERG x GBBG was found in Glace Bay in the spring of 2002. The initial thoughts of it possibly being a Slaty-backed Gull soon vanished after the gull was thoroughly analyzed. The lack of deep pink kegs and extensive window in the outer primaries ruled out Slaty-backed and it was then presumed to be a hybrid. [NS Birds, Vol. 44, No. 3]

HERG x GBBG at Glace Bay - March 7, 2002 - Ian McLaren Photo
Howell, S.N.G. and J. Dunn. 2007. Gulls of the Americas. Houghton Mifflin Company, New York, N.Y.

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