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