Hi all,
The breeding biology of Larus gulls is very well studied. Large and noisy, they
often breed abundantly in large, conspicuous colonies where it is relatively
easy to observe their strictly socially-monogamous mating systems: the members
of pairs share elaborate display patterns exclusively with each other (and not
with the members of other pairs) over a period of many weeks as they cooperate
in raising their young. In most places, whatever the species, the individuals
within a colony are very uniform in appearance, and one almost never observes a
pair in which the partners are of different species, or even where one partner
appears intermediate toward a different species. Over more than forty years
watching tens of thousands of breeding pairs of ten-plus species of gulls and
terns, I have only once witnessed strongly pair-bonded behavior and copulation
between individuals of different species (a Roseate Tern x Common Tern). Among
non-breeding birds, I have found a larger, but still small, number of
individuals that I concluded were likely hybrids (LBBG x HERG, GBBG x HERG,
GLGU x HERG, COTE x ROST). Meanwhile, in the course of being out there on the
outer coast, I have found a considerably larger number of extralimital gulls
and terns of varying degrees of rarity, including some very rare.
I was going to begin this essay with a statement like, “We simply don’t
know how frequently hybridization occurs in Larus gulls”—but this is
insufficient, because we actually DO know how rarely it is observed in most
contexts. Most of what little we know about hybridization in these birds comes
from genetic data revealing that, here and there, the genes of one species are
present in a typical-looking individual of another species, implying past
hybridization (let’s ponder the WEGU that turned out to have RBGU mtDNA). The
rest of what we actually know about it comes from a very small number of
observed hybrid pairings and an even smaller number of marked offspring of such
pairings, whose appearance and behavior were available for study as they
matured and reached adult-hood.
These facts are sufficient to imply that we might expect to see hybrid
Larus from time to time; to support the tentative identification of
intermediate-looking individuals as potential hybrids, and to factor this
possibility into the identification of potential vagrants. After, all Larus is
such a grab-bag of mix and match combinations of leg color, eye color, mantle
color, etc., and also so basically similar ecologically and anatomically, that
appearing intermediate between species A and species B will often result in
resemblance to species C. For instance, the various taxa of Kelp Gulls were
initially described as subspecies of Lesser Black-backed Gull because they
share a combination of color values; California Gull and Armenian Gull share a
combo; Yellow-legged Gull and omissus-type European Herring Gull share a combo;
etc.
The Old Field Point Larus has inspired a lot of interest and at least a
moderate amount of public discussion. I have made what I regard as a strong
case for European Herring Gull because every observable characteristic of the
bird matches trait values that are common in northern (and possibly eastern)
breeding populations of L a. argentatus. This conclusion has received, in what
has reached me so far, a lot of mostly quiet support and no explicit
contradiction. The main obstacle is natural caution, the perceived
improbability of a European Herring Gull appearing on Long Island, and the
perception that hybrid LBBG x HERG are common enough that one might match this
bird’s appearance. I believe the question of European Herring Gull vs. LBBG x
HERG hybrid can be settled with a reasonable amount of confidence, by a careful
logical critique of both hypotheses.
On the one hand, I hear people say, “It’s reasonable to identify birds
that are intermediate in multiple characters between Herring and Lesser
Black-backed Gulls as hybrids.” And on the other, I’ve heard things like this,
“Given that hybrids are often variable and in some cases are known to deviate
from both parents in some respects, it’s difficult to be sure that a hybrid
could NOT match the appearance of the Old Field Point bird.” Both these
statements (I’ll call them #1 and #2) might sound reasonable, but they are not.
The first should be qualified, “It is reasonable to regard the birds that are
intermediate in multiple characters as likely including actual hybrids, at
least in some cases—and records of such birds should be studied closely for
patterns such as timing, spacing, and unimodality (do they show a single body
of variation or more than one tendency?).” The second statement is deceiving
because it accepts the weaker part, and abandons the stronger part, of the
first: i.e., it assumes that hybrid LBBG x HERG are common, but it no longer
expects them to be intermediate. It must be remembered that statement 1 is not
based on comparing the putative hybrids with birds of known identity (of which
almost none are known); it is an hypothesis regarding birds of unknown
identity, chosen because of their intermediate appearance. I fully accept that
these probably include actual hybrids, but they also probably include a motley
by-catch of pale-end graellsii (North American LBBGs are oddly variable in
mantle color, so tossing off the pale ones as hybrids is a wrong emboyo),
European Herring Gulls, Yellow-legged Gulls, and hybrids involving completely
different taxa. But even if all of the putative LBBG x HERG hybrids are really
hybrids, they are by no means common. Furthermore, just as we are probably
overstating the frequency of hybrids by lumping in other things, we are
probably NOT over-looking actual hybrids. In closely related species like
these, hybrids are unlikely NOT to be intermediate in general, and especially
in quantitative characters such as size and shape. These considerations also
have implications for statement 2, which must now be understood as referring to
extremes of variation (by definition infrequent) within an already small number
of presumed hybrids.
In contrast, the European Herring Gull hypothesis stands on a
completely different logical basis. First, it is based on comparing the
detailed appearance of the focal bird to large series of birds of known
identity in the northern and eastern breeding populations of L. a. argentatus.
Second, although we do not know how often European Herring Gulls occur in
eastern North America, the small number of proven cases is not tiny, and surely
the actual rate of occurrence greatly exceeds this number. Adult L. a.
argenteus would almost always be overlooked as smithsonianus—as would adult
argentatus with paler mantles and/or pink legs; darker adults with yellowish
legs would be overlooked as hybrids; and all immatures would be overlooked as
Lesser Black-backed Gulls.
The thing that is unusual about Larus is not its supposed promiscuity,
but rather its strong reproductive isolating mechanisms, which allow groups of
local populations to show remarkable phenotypic cohesion, even when they are
very recently isolated and not yet sorted out genetically. In such a system,
where the animals are very closely related, hybrids will be viable and
intermediate in appearance. Thousands of Lesser Black-backed Gulls are right
now migrating north past hundreds of thousands of already-paired-up Herring
Gulls and Great Black backed Gulls. None will drop in and pair with a HERG or
GBBG. Maybe one in a million. No not even that.
________________________________________
From: Shaibal Mitra
Sent: Monday, February 28, 2022 12:36 PM
To: [email protected]; NYSBIRDS ([email protected])
Subject: RE: A Long Island, New York Larus with Yellow Legs
Hi all,
We studied the yellow-legged Larus at Old Field Point again yesterday, 27 Feb
2022. For convenience, here are links to some checklists with useful photos and
descriptions of the bird:
https://ebird.org/atlasny/checklist/S103596988
https://ebird.org/atlasny/checklist/S103599196
https://ebird.org/checklist/S103599677
https://ebird.org/atlasny/checklist/S103711048
https://ebird.org/checklist/S103758350
https://ebird.org/checklist/S103798052
https://ebird.org/atlasny/checklist/S103770855
https://ebird.org/checklist/S103820434
https://ebird.org/checklist/S103880419
I’m increasingly convinced that this gull is a European L. a. argentatus. There
are many serious obstacles to the alternative interpretations.
The extent of white on P10 and P9, the restricted amount of black on PP6-8, the
absence of black on P5, and the large size of the apical spots on all these
feathers strongly counter-indicate Lesser Black-backed Gull, its potential
hybrids with various Herring-type taxa, Yellow-legged Gull, Caspian Gull, and
also most “Herring Gulls” (e.g., western/interior North American L. a.
smithsonianus and European L. a. argenteus). These features of the wingtip are
most consistent with northeastern North American smithsonianus (which is
locally abundant) and vagrant argentatus (not yet documented in New York, but
with records from Newfoundland). Published resources and series of photographs
from known sites and dates indicate that the wingtip pattern wherein these two
taxa approach each other most closely is very similar to that of the Old Field
Point bird. There are several very subtle distinctions in primary pattern
between the two taxa, and the assessment of these in the Old Field Point bird
seems to me to be the primary remaining task (see below).
But even if this bird’s wingtip pattern is equivocal, it must be noted that it
shows numerous other characters that closely match birds from the northern
breeding areas of L. a. argentatus, and that specifically point away from L. a.
smithsonianus:
1. Mantle tone. The bird’s mantle is definitely slightly darker than in
smithsonianus, the pale tone of which is extremely consistent and not prone to
variation (one could examine a thousand breeding Herring Gulls on Long Island
without finding a single bird approaching the mantle tone of the Old Field
Point bird. Conversely, argentatus is darker than smithsonianus and argenteus,
is furthermore described as being variable, and includes populations described
as closely resembling Yellow-legged Gull in mantle tone (and other features,
see next).
2. Leg color. The bird’s legs and feet are yellow, which is atypical (but
not unknown) for smithsonianus, but quite typical for populations of argentatus
in the northern and eastern parts of its breeding range. Birds with varying
amounts of yellow in the legs and feet occur among smithsonianus more
frequently than do birds with noticeably dark mantles, but very rarely approach
the condition shown by the Old Field Point bird. in contrast, this feature is
common in the very populations of argentatus that match the Old Field Point
bird most closely in terms of wingtip pattern and mantle color.
3. Bill pattern. The bill is intensely orange, lacks black markings, and
shows an elongated red gonys spot. The former point is probably equivocal, as
it covaries with leg color in variant smithsonianus:
https://flic.kr/p/T15pGz
But the large gonys spot is possibly important, as it definitely points away
from smithsonianus. On Long Island, we are accustomed to interpreting an
elongated red gonys spot as indicative of Lesser Black-backed Gull, and this
was a source of confusion in initial assessments of the present bird.
Interestingly, this feature is apparently not unexpected among those argentatus
that most resemble the OFP bird (dark-mantled, bright-billed, and restricted
black in the wingtip):
http://www.gull-research.org/hg/hg5cy/adapr47.html
4. Orbital ring. The orbital ring appears to be red based on photos and
some descriptions, though I have not been able to confirm this fully to my own
satisfaction. If so, this points strongly away from smithsonianus, but again,
it is expected, in correlation with all the characters discussed above, among
northern argentatus.
Before concluding with a brief description of our remaining work regarding the
minutiae of the wingtip pattern, I feel the need to emphasize again that this
bird’s resemblance to a hybrid LBBG x HERG in several ways (mantle tone, leg
color, and gonys spot) is nevertheless superficial. For one thing, the bright
yellow leg color is brighter than that observed in putative hybrids. But more
importantly, its overall structure is Herring-like, and its wingtip pattern is
at the extreme end of variation in smithsonianus, in the direction away from,
not toward, the condition in Lesser Black-backed Gull.
Here is what remains to be done:
5. Nail down the color of the orbital ring and gape.
6. The shape of the large, broken mirror on P9 is distinctive—what does it
mean? (Example of a similar wingtip and details of p9 from Belgium 1 Mar:
https://macaulaylibrary.org/asset/212753731)
7. Do the lengths or shapes of the pale tongues in PP7-8 favor either
taxon?
8. The black band on P6 shows a very slight W shape. This is ascribed to
smithsonianus but is readily found in photos of European Herring Gulls, at
least of ssp. argenteus:
http://www.gull-research.org/hg/hg5cy/adfeb66.html
For the sake of thoroughness, these points should be resolved and assessed with
regard to northeastern smithsonianus vs. northern argentatus. But it seems to
me that characters 5-8 could only weakly support smithsonianus or
counter-indicate argentatus, whereas characters 1-3 pose very serious obstacles
for smithsonianus and match northern argentatus to a surprisingly detailed
degree.
Finally, in going back through my photos of variant Herring and Great
Black-backed Gulls with yellow legs, I found another bird (from 3 May 2014)
that is suggestive of argentatus:
https://flic.kr/p/RV27qh
Shai Mitra
Bay Shore
________________________________________
From: Shaibal Mitra
Sent: Thursday, February 24, 2022 8:47 AM
To: [email protected]; NYSBIRDS ([email protected])
Subject: A Long Island, New York Larus with Yellow Legs
Dear ID Frontiers NYSBirds,
I would appreciate feedback regarding an adult yellow-legged Larus found 22 Feb
at Old Field Point, on the north shore of central Long Island, New York.
Broadly speaking, it is a Herring Gull type, but there are reasons to doubt
each of the usual (and less usual) interpretations of Herring-like gulls with
yellow legs in this region at this time of year.
Most often, such birds prove to be otherwise typical smithsonianus Herring
Herring Gulls, which regularly show some degree of yellow color in the legs and
feet in late winter and early spring (as do small numbers of local Great
Black-backed Gulls). Another frequent interpretation is Herring Gull x Lesser
Black-backed Gull hybrid, which the original finder, Patrice Domeischel,
considered in the present case because of the bird's slightly darker than
smithsonianus mantle. A third possibility, always present in the minds of New
York gull aficionados, is Yellow-legged Gull, which was considered by Patrice,
and also by Peter Osswald, who independently found the bird on 23 Feb.
Patricia Lindsay and I studied the bird yesterday afternoon (23 Feb), and I
have reservations about all three of these hypotheses, which are explained in
my eBird report (link below). Briefly, Yellow-legged Gull is counter-indicated
by this bird's heavier than expected head and nape streaking; it's notably
large (larger than typical smithsonianus) apical spots on the primaries; and
other details of the wingtip pattern (more white, less black than typical for
smithsonianus, let along Yellow-legged Gull). The latter two points regarding
the primaries also point away from Lesser Black-backed Gull ancestry. Finally,
American Herring Gull is problematic by virtue of the subtly (but clearly)
darker than typical mantle tone; the completely clear yellow tones of the the
legs and feet; and details of the wingtip pattern. I could not see the orbital
ring color. Photos by Patrice and Barbara Lagois seem to show it is red, but
are not decisive on this point, in my opinion.
I wonder if this bird might represent a yellow-legged example of northern
European L. a. argentatus, which I do not know well in life, but which is
described as having a slightly darker mantle tone than smithsonianus, a wingtip
pattern very similar to the present bird, and a relatively high incidence of
yellow leg color.
My preliminary analysis (with photos by Barbara Lagois) and two of Patrice's
checklists, also with excellent photos can be found here:
https://ebird.org/checklist/S103599677
https://ebird.org/atlasny/checklist/S103599196
https://ebird.org/atlasny/checklist/S103596988
Shai Mitra
Bay Shore, New York
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