Diagnosis of Conowingo Gull (Part 2)

MHoff36100@aol.com
Sat, 14 Feb 1998 13:18:32 EST


Diagnosis of Conowingo "Common" Gull (Part 2)

By Mark L. Hoffman

Copyright Mark L. Hoffman 1998 All rights reserved.

DRAFT - 01/31/98

(Continued)


Greater Secondary Coverts

Description

The greater secondary covers were white, with a dark arrow shaped pattern
superimposed. The area along the feather veins was dark, and this darkness
expanded into a small pointed "arrow" at each feather's tip. A narrow white
fringe edged each feather. The degree of darkness of the "arrow" increased
from the inner wing to the outer wing, although the pattern was the same on
all the feathers. On the inner coverts the "arrow" was a dark gray, while on
the outer coverts in became black. The darkness along the vein also seemed to
expand broadly near the base of each covert, again with this being much more
noticeable on the outer coverts..

Literature

Grant (1986) "the inner great covers [of Ring-billed Gull] are usually not
plain grey-brown as on Common gull, but are marked or barred with dark". 

"Detailed Description [of juvenile Ring-billed Gull]  ... greater coverts pale
grey with usually obvious neat dark markings or bars, especially on innermost,
not uniform grey-brown as 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] ... greater coverts (except
three or four innermost) uniform pale grey-brown, forming pale midwing panel".
"Detailed Description [of first-winter Common Gull] ... as juvenile, but brown
and blackish areas [of wings ...] faded".

Harris et al. (1989) "[Ring-billed Gull's] usually appear pale grey, sometimes
barred on inners (unlike Common), and produce pale strip along bottom of
closed wing and noticeable pale mid-wing panel in flight".

Lewington et al. (1991) "the greater coverts [of Ring-billed Gull] generally
show dark subterminal marks (sometimes two or three rows), lacking in Common".

Tove (1993) "the greater secondary coverts (visible at the rear and along the
lower edge of the folded wing) have a broad region of white behind the dark
center. This arrowhead pattern to the coverts is distinctly different from the
paler brown, more rounded feather centers with buff fringes of the canus
complex. ...

This form [Common Gull] has the Mew Gull's wing-covert pattern of more
rounded, brown centers with narrow pale-buff margins".

Analysis

Unlike virtually every other character considered here (except the median and
lesser coverts), the greater secondary coverts present a clear-cut difference
between Common Gull and Ring-billed Gull. This difference is well illustrated
in Tove (1993, Figure 3). The pattern of the greater secondary coverts of the
Conowingo bird clearly matches that of the Ring-billed Gull, and is distinctly
different from that found in Common Gull. There is no indication that the dark
arrow-shaped marks observed on this bird are within the range of variability
of Common Gull.

These differences are well illustrated by comparing Figures 64, 65, 68 and 69
(of Common Gull) in Grant (1986) to Figures 121, 123, 124 (of Ring-billed
Gull).

Median and Lesser Secondary Coverts

Description

Although it is difficult to ascertain due to their small size, the outer
median coverts appear quite dark brown and angular at their tips. As you
progress out the wing, the median coverts become black and the distinctiveness
of the pointed tip increase. This also appear true of the next row of lesser
coverts. The darkness in the remainder of the lesser coverts appear more
rounded.

There is also a high level of contrast between the dark area of the center of
each covert and their white edges.

Literature

Grant (1986) "the dark centres of the inner median and lesser covers (which
form the inner part of the carpal-bar) [of Ring-billed Gull] have a pointed
shape at the tip, not rounded as on Common Gull, but these differences are
valid only in fresh plumage because wear and fading eventually make the wing
coverts uniform and whitish in both species". 

"Detailed Description [of juvenile Ring-billed Gull] ... carpal-bar darker,
less brown, and tip of dark central area of individual median and lesser
coverts (especially innermost) pointed, not rounded as 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] ... 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 centers of individual juvenal
wing covert feathers ... are actually darker in L. delawarensis than in L.
canus. The darker interior areas of the wing covert feathers in L.
delawarensis contrast with their light edges to produce a "brighter" or more
contrasting look to the plumage. This contrasting effect is analogous to the
differences between a Great Black-backed Gull and a Herring Gull in similar
plumage. 

In addition, the shape of the brown interior or central areas on individual
feathers is different on the two species. The typical Common Gull feather has
convex edges to the brown area, while the Ring-billed Gull often shows concave
or straight edges and "corners" on the brown area. Compared to standards
contained in Smiths's color guide, the shade of the interior area of a juvenal
covert feather in the Ring-billed Gull is number 28, olive brown, while the
Common gull color is number 27, drab, a shade lighter. These differences in
the interior areas of the juvenal covert feathers were consistent across 20+
specimens each of L. c. canus ... and L. delawarensis. ...

The difference in the shape of the centers of the brown centers of the wing
covert feathers should be helpful at close range and in photographs. The
presence of centers with angular or concave edges rules out Common Gull as far
as can be determined.

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 centers
of the juvenal covert feathers, ...".

Harris et al. (1989) "in fresh plumage, brown centres to median coverts are
pointed on Ring-billed, rounded on Common, but this distinction breaks down
with wear and fading and is of little use in worn plumage".

Lewington (1991) "the dark centres to the lesser and median coverts [of Ring-
billed Gull] are darker, more clearcut and more pointed at their tips and
contrasting more with the more distinct pale fringes than in Common. From late
autumn, the coverts are often too heavily worn/bleached to reveal any
pattern".

Tove (1993) "the dark centers of the wing coverts [of Ring-billed Gull] are
pointed ... this arrowhead pattern to the coverts is distinctly different from
the paler brown, more rounded feather centers with buff fringes of the canus
complex. ...

This form [Common Gull] has the Mew Gull's wing-covert pattern of more
rounded, brown centers with narrow pale-buff margins".

Analysis

This feature is supportive of Ring-billed Gull and would seem to preclude
Common Gull. However, due to the small size of the median coverts and the very
fine difference between the two species, I might not rule out Common Gull on
this feature alone. However, the observed pattern on the median and lower
lesser coverts is consistent with that observed on the greater secondary
coverts. Again, this is well illustrated in Tove 1993 (Figure 3).

Underwing

Description

The underwing is visible in a number of photos, but with the shading little
detail is visible. On some photos the secondary bar is clearly visible, on
other it is less so.

Literature

Grant (1986) "In first-year plumage, further differences from Common Gull are
... the usually more contrasting blackish outer primaries and secondary bar
from below". 

"Detailed Description [of Ring-billed Gull]  ... pattern on underwing coverts
similar, but markings perhaps darker on average, but some only faintly
marked".

"Detailed Description [of juvenile Common Gull] ... underwing whitish,
axillaries and most coverts with dark tips, forming lines". "Detailed
Description [of first-winter Common Gull] ... as juvenile, but brown and
blackish areas [of wings ...] faded".

Analysis

This feature provides no information.

Legs

Description

The legs are not visible in life or in the photos.

Literature

Grant (1986) "Ring-billed Gull ... [has] slightly longer legs".

"Detailed Description [of juvenile Ring-billed Gull] ... legs ... flesh-pink".
"Detailed Description [of first-winter Ring-billed Gull] ... legs ... as
juvenile".

"Detailed Description [of first-winter Common Gull] ... legs ... usually
greyish". 

Harris et al. (1989) "sometimes quite pink on first-year Ring-billed".

Analysis

Not observed.


Summary

Table 1 presents a summary of the features of the bird and whether or not they
support the identification as a Common or Ring-billed Gull. As expected, the
bird exhibits a number of features supportive of Common Gull -- without these
it is unlikely that it would have ever been identified as such in the first
place and "confirmed" by a number of skilled birders. Nonetheless, the
photographs of the bird make detailed study of its secondary coverts possible,
as these were hardly discernable in life. And the pattern of the secondary
coverts clearly supports Ring-billed Gull and precludes Common Gull. Other
factors, although supportive of Common Gull, do not seem to be outside the
range of variability of Ring-billed Gull. But clearly, it was at the extreme
end of the spectrum for a number of characteristics. Given the genetic control
of all these features, it is perhaps not surprising that as it was on the
extreme end for one character, it was also on the extreme end for a number of
characters.

In summary, I believe strongly that the characters strongly support the
identification of this bird as a Ring-billed Gull, or perhaps more
importantly, I believe the features of the bird do not support its
identification as a Common Gull.

Tove's (1993) comment is relevant "I cannot overstate the importance of always
assuming that the bird in question is the locally common form until proven
otherwise by strong evidence based on as many different field marks as
possible". The bird's features simply do not mutually reinforce the
identification as canus.

Hybrid

Although not a first consideration, given the known interbreeding among Larus
species and the expansion of the Ring-billed Gull's range into Europe, the
potential exists for hybrids between the two species. Given the close
resemblance of the two species, intermediate birds would present a difficult
identification problem. In many ways, this bird was intermediate, although
this is difficult to assess without a more complete knowledge of the full
range of variability within Ring-billed Gull. Assuming some of the features
(such as the tail) are indeed outside the range of variability for Ring-billed
Gull, then a hybrid original would seem more likely. Although the secondary
covert pattern does not seem to be intermediate, that would not necessarily
preclude a hybrid origin.

Birder Psychology

In hindsight, I suggest the original and subsequent misidentification were
very understandable. The features of the bird most similar to Common Gull -
the tail and body feathers - were readily discernable in flight. The bird
indeed did "look different" from an average first basic Ring-billed Gull. All
differences, whether diagnostic or not, reinforced the idea that the bird was
something different.

The critical identification feature, the secondary coverts, were only
discernable through the examination of many close photographs. Somewhat after-
the-fact, several skilled observers had qualms about the bird, but the
supposed diagnostic feature of the tail, combined with the other difference
make everyone believe it could not be a Ring-billed Gull. Lack of familiarity
with Common Gull by most observers undoubtedly contributed to the problem as
well.

Post-Script

I do not at all pretend to be an expert in this identification dichotomy - in
fact I have never seen a Common Gull. My sole interest is in trying to make an
accurate assessment of the bird with my observations, the photos I took, and
the reference material available to me. I was not totally comfortable with the
bird when first observed, but started this exercise hoping I would be able to
prove to myself, solely for my own satisfaction, that the bird was indeed a
Common Gull. Unfortunately, as of now, I have not been able to do that. I
welcome input from others who can educate me in the patient and understanding
way of a mentor.

Literature Cited

Grant, P. J. 1986. Gulls: A Guide to Identification. Buteo Books.

Harris, A., L. Tucker, and K. Vinicombe. 1989. The Macmillan Field Guide to
Bird Identification. Macmillan Press.

Lauro, A. J., and B. J. Spencer. 1980. A method for separating juvenal and
first-winter Ring-billed Gulls (Larus delawarensis) and Common Gulls (Larus
canus). American Birds 34:111-117.

Lewington, I., P. Alstrom, and P. Colston. 1991. A field guide to the rare
birds of Britain and Europe. HarperCollins.

Tove, M. H. 1993. Field separation of Ring-billed, Mew, Common and Kamchatka
Gulls. Birding  25:386-401.



Table 1. Summary of features (presented in text form).

Size	
Equivocal 	

Bill	
Common Gull-Not supportive, but within range of variation	
Ring-billed Gull-Supportive 

Eye	
Not observed/not relevant

Head/underparts	
Common Gull-Supportive 
Ring-billed Gull-Not supportive, but within range of variation	

Mantle/Back/Scapulars	
Common Gull-Supportive 
Ring-billed Gull-Not supportive, but within range of variation	

Rump/Uppertail/Undertail	
Common Gull-Supportive 
Ring-billed Gull-Not supportive, but within range of variation	

Tail	
Common Gull-Supportive 
Ring-billed Gull-Not supportive, but within range of variation			

Tertials	
Not observed/not relevant

Wings (in general)	
Common Gull-Not supportive, but within range of variation	
Ring-billed Gull-Supportive 

Greater Secondary Coverts	
Common Gull-Inconsistent	
Ring-billed Gull-Supportive
 
Median & Lesser Secondary Coverts	
Common Gull-Inconsistent	
Ring-billed Gull-Supportive

Underwing	
Not observed/not relevant

Legs	
Not observed/not relevant
	
TOTALS

Not oberved/not relevant = 4	
Equivocal = 1

Common Gull
Supportive = 4
Not supportive, but within range of variation = 2
Inconsistent = 2

Ring-billed  Gull
Supportive = 4
Not supportive, but within range of variation = 4
Inconsistent = 0