My notes on the hybrid gull at Falmouth last Saturday:
http://ebird.org/ebird/view/checklist?subID=S23895465
My photos are here too:
https://www.flickr.com/gp/lrbevier/8V86Mg
The slaty underside to the outer primaries plus the pale gray back and
ghost-like dark hood sweeping up the nape are Black-head Gull traits.
The large white "mirrors" on the outer primaries appear to be a result
of reducing the largely white pattern in those feathers on a normal
adult Black-headed with the mostly black primaries of Ring-billed,
which has smaller subterminal white mirrors. Other putative
Black-headed x Ring-billed hybrids have shown this pattern, which is
reminiscent of Mew Gull, another species that hybridizes with
Black-headed Gull. It would be expected that the darker gray
upperparts of Mew Gull might impart a darker gray tone on a hybrid,
but characters are not always intermediate between parent taxa. (The
Falmouth bird's upperparts were essentially the same pale gray as
Ring-billed and Black-headed.)
Other presumed Black-headed x Ring-billed hybrids differ from the
Falmouth bird in being larger, closer to Ring-billed in size, and in
having white tips to the primaries like those seen on adult large
white-headed gulls, e.g. Ring-billed. Given the uniformly pale greater
primary coverts and other characters, the Falmouth bird appeared to be
in adult (definitive) plumage.
Black-headed Gulls nest with Ring-billed Gulls in Newfoundland, where
the two species colonized in recent decades (Ring-billed first
breeding in 1945; Black-headed in 1977). Hybrids have been found in
Newfoundland in 1992 and 2012, for example. Black-headed Gull has
nested with Laughing Gulls, e.g. Petit Manan in Maine and Cape Cod in
Massachusetts, but hybrids between those two species are poorly known
and apparently very rare, with only one described (a bird at
Brigantine, NJ in September 1986). And not to be out-done, Ring-billed
has recently nested in Ireland...with a Mew Gull!
The first reported hybrid Black-headed x Ring-billed Gull was a bird
found in 1982. It was mated to a Ring-billed and nested in a
Ring-billed colony in eastern Lake Ontario. That bird differed from
most subsequently described hybrids in having yellow legs and a more
or less complete hood (other summer birds, like the Falmouth bird,
have had winter-like head patterns and orange-red legs). There is a
recent occurrence of an apparent backcross in Massachusetts (see links
below), i.e. a Black-head x Ring-billed backcrossed with a
Ring-billed. That bird was closer to Ring-billed in appearance. A bird
that appeared most similar to the Falmouth bird, but had more white in
the upperwing, was seen at the Avalon seawatch, New Jersey (Nov 2011,
see link below).
Some other putative Black-headed x Ring-billed hybrids include:
Jan 1992, MD
https://flic.kr/p/paYARt
Feb 2002, NY
http://www.birds.cornell.edu/crows/woodburne_gull.htm
Aug 2008, MD
https://flic.kr/p/5ehoy7
Jun 2010, MA
https://flic.kr/p/8e69xw
Feb 2011, MD
https://flic.kr/p/9iWsBH
Nov 2011, NJ
https://flic.kr/p/aJdsCa (similar to Falmouth bird)
Nov 2011, MA
https://flic.kr/p/aCtTr5
Jan 2013, PA
https://flic.kr/p/dLbznF
Jan 2013, MA
https://flic.kr/p/dRj3U9 (probable BHGU x RBGU backcross w/ RBGU)
Louis Bevier
Fairfield
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