Diagnosis of Conowingo Gull (Part 1)
MHoff36100@aol.com
Sat, 14 Feb 1998 13:13:07 EST
Diagnosis of Conowingo "Common" Gull (Part 1)
By Mark L. Hoffman
Copyright Mark L. Hoffman 1998 All rights reserved.
DRAFT - 01/31/98
Background
On 1 January 1998, a first basic plumaged Mew (=Common) Gull (Larus c. canus)
(hereafter "Common Gull") was reported at Conowingo Dam, Harford and Cecil
Counties, Maryland. I traveled to Conowingo on the afternoon of the 1 January,
and observed this bird for about two and one-half hours, and took at number of
photographs of the bird. I went to Conowingo Dam again on January 4th and
again observed the subject bird with my wife, Amy Hoffman. This bird continued
to be reported later in January (based on e-mail/Voice of the Naturalist
reports, although I have no knowledge of the validity of these reports).
During this period the subject bird was observed by, I suspect, the vast
majority of the active birders in Maryland, and many observers from DC and
adjacent states.
Because of the presence of numerous first winter Ring-billed Gulls (L.
delawarensis) at this location, it is critical to determine that reports,
photographs, etc. were of the bird originally claimed as a Common Gull. Based
on personal observation and discussion with other observers the key feature of
the subject bird differentiating it from other first basic plumaged Ring-
billed Gulls was the tail pattern. In the field, the bird exhibited a clean-
cut black tail band, in particular with no black on the outer web of the outer
rectrices. Additionally, the bird exhibited clean white upper and under tail
coverts, and a white "triangle" extending from the rump into the lower back,
not unlike a dowitcher (Limnodromus sp.). These features allowed this
particular bird to be differentiated from the numerous first basic Ring-billed
Gulls. Additional characters related to this bird are discussed under below.
When I originally observed the bird on January 1st, I was surprised by how
similar it appeared to a first basic plumaged Ring-billed Gull. Although the
tail band was certainly distinct, other features such as the bill size and
color and the extent of contrast on the wing seemed closer to delawarensis. I
felt that identification may not be possible visually, and took great effort
to take a large series of photographs of the bird to allow much closer
scrutiny than would be possible in life. As the bird was almost always in
flight (landing on the water for several brief periods only), it was not
possible to study in detail the critical pattern of the secondary coverts.
I have now received my photos, and present the following description and
analysis.
Observation details
I observed the bird on 1 January 1998 at Conowingo Dam, Harford County,
Maryland, from about 1400 to 1630. The bird was mostly closely observed from
the "cat-walk" along the base of the dam, during which time it actively fed in
the western corner of the spillway/river. Distances ranged from about 100 feet
to several hundred yards. I observed the bird with Zeiss 10x40 binoculars and
took 243 exposures with a Nikon 6006 with a Sigma 400 f5.6 lenses. Film used
was Kodachrome 200 - 1 roll (pushed to ISO 400), Sensia 100 - 4 rolls (pushed
to ISO 200) and Kodachrome 64 - 2 rolls (pushed to ISO 200). The day was over
cast and with the bird in flight, I took most photos at a 1/500 shutter speed
and f5.6. The majority of the photos were of a fuzzy bird, but approximately
20 provide very close (some frame-filling) in focus shots that allow
considerable closer scrutiny of the bird than was possible in life. The
observations on 4 January where at a greater distinct and did not
substantially add to the details observed on 1 January.
Description/Review/Diagnosis
In the following analysis, each feature of the bird is considered in three
parts. Firstly, a description of the bird, normally in comparison to a
"typical" first winter Ring-billed Gull, is given. Then a review of the
pertinent literature is presented, in regards to the differences and
usefulness of this feature in separating delawarensis and canus. Then an
analysis is presented as to whether or not the description supports the
identification as one species or the other.
Age determination
Based on the bill coloration (two-toned), wing pattern (brownish carpal bar,
dark secondary bar) and tail (complete band) this bird is considered to be in
first basic plumage.
Elimination of L. c. brachyrhynchus
The tail pattern (distinct tail band), white rump and uppertail coverts, and
white underparts eliminate the "Mew" Gull (L. c. brachyrhynchus) from
consideration.
Molt and feather wear
No signs of active molt are visible in the photos. The ends of the tail
feathers show substantial wear, but the primaries and secondaries appeared
relatively unworn.
General comments in literature
Harris et al. (1989) "First-years The most difficult to identify, as many of
the subtle differences are inconsistent. First-year Ring-bills have
distinctive "jizz" once learnt, but should always be identified by a
combination of minor differences. Close views and detailed notes essential,
and observer should always bear in mind possible occurrence of odd Common
Gulls (for example, unusually pale individuals)."
Size
Description
The bird appeared similar in size to a Ring-billed Gull. It was suggested by
some observers that the bird appeared slightly more narrow-winged then an
"average" Ring-bill, and I would share this impression.
Literature
Grant (1986) "Ring-billed Gull ... is usually slightly larger and heavier-
bodied".
Lauro and Spencer (1980) "examination of museum skins and the observation of
small Ring-billed Gulls in the field suggests that size differences are of
little assistance in determining the identity of an individual bird".
Lewington et al. (1991) "[Ring-bill] slightly larger and heavier".
Analysis
Although a noticeable smaller size would be supportive for Common Gull, this
bird was not noticeable smaller. Given the viewing conditions (in flight, not
standing among typical Ring-billed Gulls) size was difficult to assess. This
feature is judged to be equivocal.
Bill
Description
The bill seemed similar in length to a Ring-billed Gull, but slightly more
narrow, particularly along the basal half. It not particularly thin or short.
It also appear somewhat swollen or "blob-tipped" at the end.
The basal one-half was a pink-orange color, with the remainder being black.
Literature
Grant (1986) "Ring-billed Gull ... usually [has] obviously thicker bill".
"Detailed Description [of first-winter Ring-billed Gull] ...as juvenile except
... bill pink, yellowish or greenish-yellow with clear-cut black tip: extreme
tip occasionally develops mall pale area toward end of first winter, rarely as
early as February".
"Detailed Description [of juvenile Common Gull] bill blackish with diffuse
dull flesh, flesh-pink or greyish-flesh base sometimes confined to lower
mandible". "Detailed Description [of first-winter Common Gull] ...as juvenile,
but bill with clear-cut black tip. ... base of bill usually grayish".
Harris et al.(1989) "The best character, [Ring-billed Gull's bill] being
heavy, thick, "parallel" and blunt-ended (Common's bill looks slender, pointed
and weedy). Usually pale orangy-pink with prominent black tip, reminiscent of
bill of first-winter Glaucous Gull L. hyperboreus (some Commons have similar
bill colour, but many have a duller, grey or greenish base)."
Lewington et al. (1991) "[Ring-billed Gull] with an appreciable heavier bill.
... In 1st-winter plumage, the bill [of Ring-billed Gull] is usually pink with
a clearcut dark tip, occasionally more extensively dark as in the juvenile.
... In Common, the basal part of the bill is generally more greyish but
sometimes pink or yellowish as in Ring-billed, a broad dark band is sometimes
seen in Common too, especially in summer".
Tove (1993) "Common Gulls in first basic plumage have a bill color and pattern
closer to that of Ring-billed than to Mew. The bill of Common Gull typically
appears slender and pointed in contrast to Ring-billed's heavier and more
blunt-ended bill. But, again, beware of Ring-billeds that are small in overall
size and have small bills".
Analysis
Some Common Gulls (e.g. Figure 7 in Tove 1993, Figure 71 in Grant 1986) have
clearly diminutive bills compared to a Ring-billed Gull. These could be
females, which as in all gulls, are smaller than males. The Conowingo bird
clearly does not at all fall in this category.
Other Common Gulls (e.g. Figure 8 in Tove 1993, Figures 66, 67 and 77 in Grant
1986) would appear to have considerable more substantial bills, approaching
Ring-billed Gull. The Conowingo bird could perhaps approach the upper end of
this spectrum. However, to quote Harris et al., the bill did not look
"slender, pointed or weedy" at all. Bill size and shape is considered more
supportive of Ring-billed Gull, but not outside the potential range of
variation for Common Gull.
As described above, some Common Gulls have bills with greyish bases, but this
bird did not. That does not rule out Common, but again is more supportive of
Ring-billed Gull.
Eye
Description
The eye was dark.
Literature
Grant (1986) "Detailed Description [of juvenile Ring-billed Gull] ... iris
dark brown". "Detailed Description [of first-winter Ring-billed Gull] ... as
juvenile".
"Detailed Description [of juvenile Common Gull] ... iris dark brown".
"Detailed Description [of first-winter Common Gull] ... as juvenile".
Analysis
This feature is shared by both species.
Head/underparts
Description
The head and underparts were predominately white. A dark smudge "surround"
enveloped the area immediate around the eye. The top of the head was marked
with faint brown-gray streaking, that increased in density on the back of the
head and onto the nape, where it appear as more of a dark mottling. In some
photos, the nape mottling seems to extend towards the breast. The throat and
rest of the underparts are white.
The shape of the head did not appear to be petite or "rounded".
Literature
Grant (1986) "Ring-billed Gull ... [has] more fierce expression caused by its
less rounded head, often faint dark furrow or brow over eye". "In first-year
plumage, further differences from Common Gull are ... the usually clearly
spotted (rather than mottled) lower hindneck, and the usually more defined
spots or crescentic markings on the breast-sides and flanks rather than the
usually indistinct mottling on Common Gull, although these may be reduced or
lacking in first-summer plumage".
"Detailed Description [of first-winter Ring-billed Gull] ... as juvenile
except ... head and underparts generally whiter, with distinct blackish spots
on lower hindneck, and defined spots and crescentic markings on breast-sides
and flanks".
"Detailed Description [of first-winter Common Gull] ... lower hindneck, breast
or breast-sides and flanks with variable grey-brown mottling, streaks or
spots, most dense on lower hindneck and breast-sides; underparts otherwise
white".
Harris et al. (1989) "[Ring-billed Gull] usually well mottled and spotted
about head and breast and more heavily marked below. ... Both species,
however, variable".
Lewington (1991) "the breast/flanks [of Ring-billed Gull] are generally more
distinctly marked with dark spots, crescents, and bars. On average, the
nape/hindneck show slightly more distinct, darker spotting than in Common".
Analysis
The relative lack of marking on the underparts, and pattern of marking on the
head and nape are more supportive of Common Gull, however, given the
variability of Ring-billed Gull, this feature is not consider to be outside
the range of variability for that species. Both species can be relatively
well-marked below, or relatively white below.
Mantle/Back/Scapulars
Description
The back and scapulars were a uniform gray, perhaps a half shade darker then
the average Ring-billed Gull. No brown mottling was visible in these areas. On
the photos, white fringes are visible on many of the scapulars, creating a
look similar to a series of waves rolling in from the ocean. Additionally, the
white of the rump extended up the mid-back, to about the level of the
insertion of the secondaries on the wing. This white areas was reminiscent of
a similar mark on a dowitcher (Limnodromus sp.). The extent of this white area
varied greatly based on the position of the bird and the degree to which it
was covered by the overlapping gray scapulars.
Literature
Grant (1986) "The grey of the upperparts [of Ring-billed Gull] is much paler".
"Because of the paleness of the grey upperparts, Ring-billed lacks the
contrasting dark grey saddle which is obvious on first-year (especially first-
summer) Common Gulls."
"Detailed Description [first-winter of Ring-billed Gull] ... as juvenile
except ... mantle, scapulars and back pale grey, some individual feathers
often with dark subterminal crescents and pale fringes: mantle and scapulars
thus paler grey and often with obvious faint barring or mottling, not uniform
as on typical Common Gull. A few brown juvenile scapulars are sometimes
retained".
"Detailed Description [of first-winter Common Gull] ... mantle, back and
scapulars uniform blue-grey, latter rarely with faint brownish subterminal
marks".
Lauro and Spencer (1980) "comparison of museum specimens of L. delawarensis
with L. c. canus ... shows that ... the gray feathers in the male of L. canus
are a shaded darker than the gray feathers in the mantle of L. delawarensis".
Harris et al. (1989) "[Ring-billed Gull] pale grey, lacking dark 'saddle'
effect of Common. Whites tips to many of the scapulars and retention of some
dark juvenile feathering may create more variegated pattern than on Common,
but dark feathers are moulted and pale tips wear off as winter progresses."
Lewington et al. (1991) "the grey on the upperparts [of Ring-billed Gull] is
distinctly paler than in Common Gull. ... The grey mantle/scapulars [of Ring-
billed Gull] can show more dark subterminal markings and paler fringes than in
Common Gull".
Tove (1993) "as the bird [Ring-billed Gull] enters its first basic plumage,
the back acquires gray feathers. Compared with the canus group, these feathers
are much paler gray.
The gray of the mantle [of Common Gull] is darker than on Ring-billed, giving
more of a "saddle" effect ...".
Analysis
The uniformity and shade of the back seem more supportive of Common Gull,
however, given the variability of Ring-billed Gull this feature is not
considered to be outside the range of variation for that species.
Rump, Uppertail and Undertail Coverts
Description
The rump, uppertail, and undertail coverts appeared pure white.
Literature
Grant (1986) "Detailed Description [of juvenile Ring-billed Gull] ...
resembles juvenile Common Gull except ... uppertail- and undertail-coverts
more strongly barred on average". "Detailed Description [of first-winter Ring-
billed Gull] ... as juvenile except ... brown and blackish areas [of ... tail]
becoming faded".
"Detailed Description [of juvenile Common Gull] ... rump and uppertail- and
undertail-coverts white, with dark arrowhead markings or bars". "Detailed
Description [of first-winter Common Gull] ... dark marks on rump and
uppertail- and undertail-covets less prominent than on juvenile or lacking."
Lauro and Spencer (1980) "among other field marks, the tail characteristics
are the most helpful ... L. c. canus generally has ... pure white upper and
under tail coverts. A few birds show some spotting on the upper tail coverts,
but never as much as most L. delawarensis.
The coverts [of Ring-billed Gulls] are spotted and extend down over the
rectrices".
Harris et al. (1989) "[Ring-billed Gull] often with heavy dark barring or
spotting on upper- and undertail-coverts, which Common usually lacks. Both
species, however, variable".
Tove (1993) "the rump and uppertail coverts [of Ring-billed Gull] are spotted
with brown, but may sometimes appear completely whitish in the field.
[Common Gull has] ... a bold white rump. At close range, birds in fresh
plumage may show faint tan markings on the rump".
Analysis
The lack of markings on the rump, uppertail and undertail coverts is
supportive of Common Gull, however given the range of variability this feature
is not considered outside the potential range of Ring-billed Gull.
Tail
Description
The tail was white, except for a broad black subterminal band. The band became
slightly narrower toward the edges of the tail. The band was complete, except
on the outer tail feathers. On the birds left, the outer tail feathers showed
a small black smudge on the inside of the vain, at the same level as the rest
of the tail band. On the bird's right, a comparable black smudge was present,
which bled slightly on to the outer web of the feather and extended a distance
equal to the length of the tail band. I do not believe this was visible in
life.
The bird also had a fairly broad white terminal band below the black and
considerable wear was evident on the ends of the rectrices.
Literature
Grant (1986) "In first-year plumage, further differences from Common Gull are
... the usually more variegated pattern of the tail band".
"Detailed Description [of juvenile Ring-billed Gull] ... subterminal band
rarely solid black and clear-cut as on Common Gull, but usually broken by pale
mottling of highly variable pattern; remainder of tail often shaded with grey
of variable pattern. Tail pattern of Ring-billed Gulls highly variable, with
any two individuals rarely identical, unlike comparatively standard pattern of
Common Gull". "Detailed Description [of first-winter Ring-billed Gull] ... as
juvenile except ... brown and blackish areas [of ... tail] becoming faded".
"Detailed Description [of juvenile Common Gull] ... tail white, typically with
clear-cut, broad, solidly blackish-brown subterminal band: outer pair of
feathers often with dark only on inner webs or occasionally all white.
Sometimes, blackish-brown, occasionally greyish, extends up the side of each
feather, giving a notched or diffuse leading edge to the tail band". "Detailed
Description [of first-winter Common Gull] ... as juvenile, but brown and
blackish areas [of ... tail] faded".
Lauro and Spencer (1980) "among other field marks, the tail characteristics
are the most helpful ... L. c. canus generally has a pure white tail with a
clear-cut dark subterminal band ... an occasional L. c. canus shows a little
shading on the outer rectrices.
Ring-billed Gulls usually have mottled tails above the band, particularly
toward the outer edges".
Harris et al. (1989) "[Ring-billed Gull's] dark of tail band usually extends
up outer web of each tail feather to intrude into tail base, which usually
shows delicate greyish mottling or shading; tail therefore looks messy
compared with clear-cut band and white base shown by most Commons. On some
Commons, however, dark also intrudes into the white, while minority also show
grey mottling at base, so differences not absolute. Ring-billed has dark
mottling or barring on outer web of outer tail feather, which Common seems to
lack."
Lewington et al. (1991) "the dark tail band [of Ring-billed] is rather
variable, but is hardly perfectly clearcut, as in Common, and this is a very
good character".
Tove (1993) "the tail [of Ring-billed Gull] is white with a broad (but
variable), dark subterminal band. Although this band is often depicted as
solid, on close examination it is usually a series of narrow bands that fuse
distally with a broader one. Given a reasonable view, this feature is visible
in the field. ...
The outermost rectrices [of Common Gull] are almost entirely white, so the
solid brownish-black tail-band does not extend fully across the tail. This
bold tail-and-rump pattern is the most distinctive feature of birds in ...
first basic plumage. On a few Commons, however, some dark intrudes into the
white areas or there is some mottling present at the base of the tail. Ring-
billed Gulls may show white borders to the tail, but, at best, less than half
the width of the outer web will be white".
Analysis
The tail pattern is more supportive of Common Gull. It is clearly not the
"typical" pattern of Ring-billed Gull. However, it is unclear to what extent
the tail pattern is absolutely diagnostic for Common Gull. In any event, the
supposed key feature (lack of black on the outer web of the outer tail
feathers) does not hold for the tail feather on the bird's right. Regardless,
this feature is not considered outside the range of potential variability for
Ring-billed Gull.
The white terminal band is considerable wider than that shown in published
photos of Common Gull (e.g. Tove (1993) Figure 9; Grant (1986) Figures 64, 68,
69 and 72). Many Ring-billed Gulls have a fairly broad area below the main
part of their subterminal tail band, but this area is usually mottled with
black (e.g. Grant [1986] Figure 124, 125). The significant of this is unknown.
Tertials
Description
The tertials are not visible in the photos, as the are covered by the
scapulars.
Literature
Grant (1986) "The brown centres of the tertials [of Ring-billed Gull] tend to
be darker, with thinner white fringes".
"Detailed Description [of juvenile Ring-billed Gull] ... tertials and
adjacent greater covert darker than Common Gull, with thinner pale fringes on
average". "Detailed Description [of first-winter Ring-billed] ... as juvenile
except ... brown and blackish areas [of wings ...] becoming faded, and white
tips and fringes often reduced by wear".
"Detailed Description [of juvenile Common Gull] ... carpal-bar brown, with
rounded brown feather-centres and pale fringes". "Detailed Description [of
first-winter Common Gull] ... as juvenile, but brown and blackish areas [of
wings ...] faded".
Lauro and Spencer (1980) "comparison of museum specimens of L. delawarensis
with L. c. canus ... shows that ... the brown tertials ... are actually
darker in L. delawarensis than in L. canus".
The most consistent determinant we found in the skins of first winter birds
was the above-mentioned difference between the shades of brown in the ...
coverts ..., in the tertials, and in the primaries".
Harris et al. (1989) "[Ring-billed Gull's are] solidly dark brown, narrowly
edged white (on Common, paler brown, with thick white edgings, but beware of
abrasion)."
Tove (1993) "the tertials [of Common Gull] show a broad buff-white crescent
that Ring-billed Gull typically lacks".
Analysis
Unable to make any determination.
Wings (in general/primaries)
Description
Although variable with lighting conditions, the wings showed a high degree of
contrast between the pale greater coverts and inner primaries, and the black
secondaries and outer primaries.
The visible portion of primaries 1-4 were entirely black, while 5 showed some
gray in the basal portion of the inner web. Primaries 6-10 were gray with
black tips (and some black extending up the vain) which decreased in size
toward the inner wing. These inner primaries also had narrow white fringes,
and small white spots on their outer webs at the juncture of the gray "tongue"
and black tip.
The secondaries were black with narrow white edges, although the base of the
outer 2-3 secondaries were visibly gray.
Literature
Grant (1986) "In first-year plumage, further differences from Common Gull are
... more contrasting upperwing pattern (due mainly to the darker brown carpal-
bar, the blacker outer primaries and secondary bar, and the paler inner
primaries and greater coverts)."
"Detailed Description [of juvenile Ring-billed Gull] ... dark areas on outer
wing generally blacker, and more defined and less extensive on inner
primaries: inner primaries and midwing panel basically paler grey, giving more
contrasting upperwing pattern than on Common Gull". "Detailed Description [of
first-winter Ring-billed Gull] ... as juvenile except ... brown and blackish
areas [of wings ...] becoming faded, and white tips and fringes often reduced
by wear, so losing covert pattern differences from Common Gull".
"Detailed Description [of juvenile Common Gull] ... outer greater primary
coverts and outer three to five primaries wholly blackish-brown except for
pale fringes at tips: dull grey on outer webs increasing in extent (and
subterminal blackish areas decreasing) from base of 5th or 6th primary
inwards, forming pale division between outer primaries and secondary bar".
"Detailed Description [of first-winter Common Gull] ... as juvenile, but brown
and blackish areas [of wings ... ] faded. Occasionally, fading produces pale
subterminal spot on 1st primary, forming sometimes well-marked mirror as in
Photo 72".
Lauro and Spencer (1980) "comparison of museum specimens of L. delawarensis
with L. c. canus ... shows that ... the brown ... primaries are actually
darker in L. delawarensis than in L. canus".
The most consistent determinant we found in the skins of first winter birds
was the above-mentioned difference between the shades of brown in the ...
primaries".
Lewington et al. (1991) "the wing pattern [of Ring-billed Gull] is distinctly
more contrasting, with the dark parts darker and the pale areas paler and
greyer, less brownish-tinged, than in Common Gull".
Analysis
Some Common Gulls (e.g. Figure 9 in Tove 1993, Figure 68 and 69 in Grant 1986)
appear quite dark winged, with seemingly very little contrast between the
paler and darker areas of the spread wing. This bird did not match that
pattern at all. It is not clear that the observed boldness of the wing was out
of the range of variability for Common Gull, but this feature is more support
of Ring-billed Gull.