Gulls, hybrids, and votes (LONG)
Rick Blom (rblom@blazie.com)
Sat, 14 Feb 1998 03:59:16 -0500
Fran Saunders wrote:
>Since some of us were so certain of some of the
>field marks and since there seems to be a confusion of field
>marks, could the bird be a hybrid?
Of course, but the odds are against it. Theoretically, Common and
Ring-billed do not overlap in breeding season. From a practical point of
view, there are about 80 Ring-billed records a year in this decade in Great
Britain. Although most are in the period of fall through spring, some are
in summer and it is reasonable to assume that some wintering birds hang
around. It is also reasonable to assume, on the basis of published
information about all kinds of birds, that the urge to breed is powerful
and that when no appropriate mate can be found, birds make do with a
closely related species. So there is a chance that Common and Ring-billed
Gulls have hybridized.
Now comes the howevers. I can find no reports from Europe of
suspected hybrids and no reports of mixed pairs. Remember, a far greater
percentage of gull colonies in Great Britain are monitored than are here.
If hybrids were occurring, one would assume there would at least be reports
of suspicious birds there before they were reported here. Second, it is not
necessarily logical that a hybrid is more likely to be a vagrant than a
pure Common, although it would probably have more conflicting migratory
urges than either species.
On this side of the pond, there are typically a few records of
Common Gull in the Maritimes in winter, and a very few farther south. I
know of no summer records and no records within the breeding range of
Ring-billed at any season. Hybridization in North America, while not
impossible, would seem less likely than hybridiztion in Europe.
Another argument against the bird being a hybrid is that it seems,
depending on your point of view, to have conflicting rather than
intermediate characters. Where known hybrids exist among gulls, the birds
are intermediate between the species involved. For example, hybrids between
Glaucous-winged and Western Gulls are intermediate in mantle colorand other
characters. The same is true of hybrids between Herring and Great
Black-backed Gulls and Herring and Glaucous Gulls. What we do not have
evidence for is birds that show typical characters of one species in some
areas and typical characters for the other in different areas. It does not
seem to be the way the genetics of gulls works, although genetics is not my
field and others may have additional information. Given how similar the two
species are in this plumage, field identification of a hybrid might be
impossible.
The argument for hybrid rises because the bird appears to show
Common Gull characters in the tail and upper and under tail coverts, but to
otherwise show characters more typical of Ring-billed. As noted, you would
not normally expect that dichotomy in a hybrid. A more urgent line of
inquiry is the question of variability in both species in 1st year birds.
As is often the case, the literature is weak in this area, in part because
both birds are common (we look far more closely at rare birds) and because
there is no real identification issue with either one except in the rare
instances when a vagrant is found. That happens far more frequently in
Europe, where they do not have large numbers of Ring-bills to study. The
fact is, the tail pattern of young Ring-bills is amazingly variable and not
much is known about that variability.
The pattern of the wing coverts, which some of us have used to
conclude that the bird was most likely a Ring-billed, is better documented,
having been studied in museum skins and published in at least two sources
(cited by others). There is no "known" variability in this character.
The other charcaters most widely discussed - size, shape, back
color, bill shape, size, and color, and upper and under wing pattern, all
seem more typical of Ring-billed, although all are subject to variability
in both species. For many of us who looked at the bird, the primary
question is whether a Ring-billed can have a tail pattern like this bird.
That is not easy to resolve. I saw birds with tail patterns very like, but
not identical to, this. Without specimens and banded birds, it may will be
difficult to resolve the issue with 100% certainty because the debate
becomes circular. If it has a tail pattern like this bird, then we assume
it is a Common because the charcater is supposed to be diagnostic...and
around we go again. My own feeling is that Ring-billeds can show tail
patterns very close to the one on this bird and that is based on looking at
only a few hundred 1st winter Ring-billeds. I think, and it is only my
opinion, that the evidence argues for a Ring-billed with a probably rare
variation in tail rather than a Common with a number of variations, at
least one of which there is no evidence for.
As for the records committee deciding this bird is a hybrid, it is
important to remember that hybrid is not a dump category for difficult or
contradictory birds. From the perspective of a committee, a hybrid is the
same as a third species to be considered, and would carry all the burdens
of proof that either Common or Ring-billed would. With no evidence of
hybridization and no descriptions of hybrids, drawing that conclusion would
be extremely difficult.
Several people have raised the question of whether multiple birds
may be under discussion. It is possible, but I do not think it is likely.
The photos and video I saw came from a number of people and I could not see
any variation. The photos were taken on different days, but they were
directly compared to each other. They appeared to be identical. If it
turned out there were more than one bird with this tail pattern, it would
add to the argument that we are seeing a variation in Ring-billed becasue
that is more likely than there being multiple Common Gulls, especially
multiple birds that show so many characters indicating Ring-billed.
I am not dismayed by "dogmatic" comments like "This is definitely a
Ring-billed." Anyone who has reached such a conclusion, or an alternative
one, should say so. It is after all, opinion, not a vote. Many of the
people who are commenting are knowledgeable and have investigated the
problem and I am grateful for their opinion and have no trouble remembering
that it is just opinion and putting it in context. Because I have the same
faith in the intelligence and judgement of all the subscribers I am not
embarassed to offer my own view: The bird is a Ring-billed. If it turns out
that I am wrong it means we will have learned something significant about
both species and in that case I will happily eat gull.
Rick
"Everywhere I go I'm asked if the university stifles writers. My opinion is
that they don't stifle enough of them. There's many a bestseller that could
have been prevented by a good teacher." Flannery O'Connor
Rick Blom
rblom@blazie.com
Bel Air, Maryland