It IS a Common Gull
Mary Gustafson (Mary_Gustafson@usgs.gov)
Fri, 20 Feb 1998 16:57:49 -0700
I was asked to forward the following to the MDOsprey list.
I wish to respond to the criticism of the Common Gull report that has
appeared in Maryland Osprey. At the outset, I wish to emphasize a
fact known to all careful birders: Any challenging field
identification should be based on a careful examination of all
characteristics, not just a single feature. The fallacy of basing
criticism on just a single field mark (in this case, the shape of the
dark tips to some of the upper wing coverts) is illustrated below. I
have no doubt that the very vocal critics of this report will continue
to howl after the posting of this response. I hope that the more
objective readers will realize there are two sides to this issue.
Mark Hoffman deserves thanks for having the courtesy of making his
description of the bird available. I will use his own words, plus
information in various literature sources, to demonstrate that all
diagnostic field marks are consistent with the identification as a
Common Gull. Mark's biases are very obvious in his uneven "analysis"
of the various "diagnostic" field marks; those favoring Common Gulls
are summarily dismissed because of unsupported, theoretical variation
in Ring-billed Gulls, while those supposedly favoring Ring-billed
Gulls are always 100% definitive. Importantly, one has to wait until
the last paragraph before he admits that he has no previous experience
with Common Gull. Yet, throughout this summary, he expresses with
100% confidence statements that certain field marks are (or are not)
more supportive of a Common Gull.
Using bill color as an example, I agree with his description of the
bill color on this individual and that this pattern is consistent with
either species. None of the references he cites gives any specific
indication of the proportion of Common Gulls with this pattern, or how
this proportion may vary seasonally or geographically. Based on the
lack of specific data in these literature sources and his inexperience
with Common Gulls, how can he claim that this feature is more
supportive of a Ring-billed Gull? I disagree with this conclusion,
based on observations of numerous first-winter Common Gulls in the
Netherlands during the past month, and this particular bill color is
not at all helpful in the identification of this individual.
I will not go into a lengthy discussion on the variability of each
potential field mark for this identification, but will concentrate on
those characteristics that may be diagnostic or are claimed by Mark to
be diagnostic. It is these features where the true biases of the
critics of this report become very evident. For example, the photos
and Mark's description indicate the pattern of the tail and upper tail
coverts consist of unmarked white upper tail coverts, a sharply
defined black sub-terminal tail band without any extension into the
white tail base, and an outer rectrix that is largely white with only
a small, irregularly-shaped mark on the inner web.
According to the literature sources (e.g. Birding 25(6): fig. 11 on
p.391; the discussion in Lewington et al. {reference in Mark's
disscusion} etc...), this is a diagnostic tail pattern of a Common
Gull and outside of the described range of variability for a
Ring-billed Gull. Mark tries to dodge this issue by claiming he is
unfamiliar with the range of variability in the tail pattern of
Ring-billed Gulls, and cannot state with 100% certainty that a
Ring-billed Gull cannot have this pattern. He can observe Ring-billed
Gulls any day of the year, yet is not familiar with their tail
patterns--but confidently tells us which field marks are not
supportive of a species he has never seen, the Common Gull?
Of course, negative evidence is impossible to prove. Neither Mark nor
myself nor anybody else can prove that within the past millenia, there
has not been a single first-winter Ring-billed Gull that may have
matched this tail pattern. However, the critics of this report have
provided no proof that Ring-billed Gulls can have this tail pattern.
Did they cite specimen evidence? No. Did they cite photographs? No.
Did they cite any literature references? No. Until they provide some
undeniable physical proof, the statements in the literature remain
valid and this tail pattern provides some diagnostic evidence that the
the identification as a Common Gull is correct.
With regards to the shape of the tips of all upper wing coverts, the
literature indicates that these features are valid only on individuals
in fresh plumage. To counter the claims in this forum that the
pattern on these feathers are diagnostic in all individuals, a very
brief search of the literature has turned up the following photographs
of Common Gulls where some or most of the greater and/or median
coverts have pointed dark tips to these feathers:
Grant, Gull identification (2nd ed.): Figs. 66, 67, 69, 72, 77
Harrison, Photographic guide to seabirds: Fig. 517
Tove, Birding 25(6): 386-401: Figs. 7, 8
A more detailed search of the literature would undoubtedly uncover
more photographs. Either British birders cannot correctly identify
this species, or the tips to the various wing coverts of Common Gulls
can appear pointed with wear. Obviously, this characteristic IS NOT
diagnostic as claimed by Mark and others.
A detailed discussion of feather wear is beyond the scope of this
discussion. However, feather wear is shown in various means in addition
to the actual fraying of feathers. On the wings of gulls, wear is
indicated through the bleaching of the feather tips, so that the tips
become paler and the remaining darkness is concentrated along the
shafts. Hence, on worn wing coverts, the shape of the dark tips
becomes more pointed with increasing wear.
Assessing wear in the field is very subjective, and it is easy for
knowledgeable birders to disagree. The plumage of the Conowingo gull
is actually moderately worn (compare with Grant, op cit. Fig. 124 for
a fresh-plumaged Ring-billed Gull). My experience and photos in the
literature indicate that plumage of first-year gulls tends to wear
rapidly, and by mid-December, only a very tiny fraction (less than
0.01%) of the individuals still have fresh plumage and usually appear
strikingly different from their worn conspecifics.
There is a potentially diagnostic characteristic on the greater coverts
of Ring-billed Gulls that is very helpful in the assessment of wear on
the Conowingo bird, a characteristic that Mark conveniently did not
discuss in his summary. In fresh plumage, the greater coverts of
first-winter Ring-billed Gulls have a brownish bar across the middle of
the feather (illustrated in Fig. 3, line #2 in Tove's Birding article).
In fresh plumage, this bar can appear as a narrow dark band across the
greater coverts (worn but still somewhat visible on Fig. 5 in Tove).
This bar actually disappears fairly quickly with wear, although may
remain on the inner coverts for a longer period of time. When present,
it would support the identification as a Ring-billed Gull; its absence
could readily be found on individuals of either species. Neither the
photographs nor Mark's description provide any indication that this bar
was present on any of the greater wing coverts, to which I concur. If
the bird were a Ring-billed Gull, then the absence of this barring
indicates the wing coverts are worn and hence the shape of the dark tips
to the coverts is not necesarily reliable. Its absence does not support
the identification of either species.
While it is apparent that the upper wing coverts were worn, the
relevant question is are there some relatively fresh coverts that may
still provide a clue for this identification. Since the greater and
median coverts tend to were fastest, the key feathers are the lesser
coverts. To quote from Mark's own description: "The darkness in the
remainder of the lesser coverts appear more rounded". The literature
cited in Mark's summary states that Ring-billeds always have pointed
tips to the lesser coverts, and rounded tips are found in ... COMMON
GULL. Mark even describes the diagnostic shape to the tips of the
lesser coverts of a Common Gull!
To summarize, the tail pattern of this gull perfectly matches the
diagnostic tail pattern of a Common Gull; no physical evidence was
provided to contradict the extensive literature on this point. The
fact that the tips to the upper wing coverts of Common Gulls can
become pointed through wear is clearly established in a number of
photographs, and Mark's own description of the rounded tips to the
lesser coverts is also diagnostic for a Common Gull and does not
support the claim of a Ring-billed Gull. Through Mark's own
admission, the other characteristics of this gull are consistent with
or suggestive of a Common Gull. This individual is a Common Gull.
Will this discussion convince the highly vocal critics of this report
that the original identification was correct--probably not. They will
probably continue to submit their biased criticisms, since they know
that if they throw enough mud at this report, they will eventually bury
it. Hopefully, people willing to take an objective view of this report
will carefully examine the photographs and all relevant literature
(there is an extensive European literature on the subject not cited in
Mark's summary), and reach their own conclusions that will not be based
on the one-sided, cynical, combative, and mean-spirited atmosphere that
has existed in the Maryland Osprey forum in recent weeks. If this
atmosphere represents the future for birding in Maryland, then please
count me out.
Bruce Peterjohn