Identification of American Herring Gull in a western European context

More documents

Recommendations

Info

Identification of American Herring Gull in a western European context123456FIGURE 1Variation in rump and tail pattern of first-winter American Herring Gull / Amerikaanse Zilvermeeuw Larussmithsonianus (Pat Lonergan & Killian Mullarney). Note that tail pattern varies from wholly dark at one extreme(1 and 2) to, rarely, well-defined tail-band (8). More often, however, pattern is intermediate, with obvious vermiculationon both webs of outer two or three rectrices.78a minority of first-year smithsonianus really do havewhat might justifiably be described as an ‘all-dark’ tail;2 an absolutely all-dark tail (including the outer websof the outermost rectrices) has been observed in firstyearEuropean Herring Gulls; 3 the difficulty in establishingthe exact tail pattern in the field, as opposed tofrom critical examination of good photographs, is oftenunderestimated. Many smithsonianus have an extent ofdark on the tail that is very rarely matched by EuropeanHerring. Those with the most extensively dark tailshave a solid-dark upper tail surface, as viewed fromabove (plate 33, figure 1), the only relief being the paleshaft-bases. Close examination will often reveal a verynarrow strip, barred black and white, along the outermostedge to the tail (plate 3, figure 1) and it is notunusual for there to be some limited pale barring orvermiculation on the bases to the outer two or threepairs of rectrices. In addition, the fully spread tail (bestlooked for as a bird takes off or just before it alights), ora view of the tail from below, will often reveal quiteextensive areas of pale barring or vermiculation on thebases to the inner webs of the outer rectrices. Thevariety of tail patterns featured in figure 1 clearlyillustrate the point that an ‘all-dark’ tail should notnecessarily be considered a prerequisite of smithsonianus.Some European Herring Gulls can show a tail patternapproaching that of classic smithsonianus but onlyexceptionally are the outer rectrices wholly dark (PeterAdriaens pers comm). Similarly, it is not exceptional forsmithsonianus to have extensive white (usually barreddark) at the base to the outer rectrices and it may evenbe that this variation is more frequent in some populationsthan in others.Vent and undertail-coverts The ventral area andundertail-coverts of juvenile smithsonianus are generallymore densely patterned with dark than in EuropeanHerring Gull, in which these areas are sparsely markedand usually appear predominantly whitish. ’Classic’smithsonianus are so extensively dark on the undertailcovertsthat the white ’bars’ may be reduced to littlemore than paired spots, and the longest undertail-covertsmay be almost solidly dark-centred; such bold andextensive dark markings are probably never shown byEuropean Herring, although a few are more heavilymarked than usual and may resemble smithsonianus.Of course, some smithsonianus are not so heavilymarked on the undertail-coverts but such birds are verymuch in a minority in North America, at least inAtlantic seaboard populations. Any suspected juvenilesmithsonianus in Europe which does not exhibit reasonablydense or bold undertail-covert-markingsshould be considered very critically before being positivelyidentified.Pattern of rump and uppertail-coverts In smithsonianus,the rump and uppertail-coverts are characteristicallydensely patterned with dark brown bars, chevronsor large spots, the overall tone being close to that of therest of the upperparts but clearly contrasting with theoften ‘all-dark’ tail (plate 3). The overall look of the tailand rump may prompt comparisons with that of a paleorintermediate-morph juvenile Pomarine JaegerStercorarius pomarinus. On well-marked birds, this is astriking difference from any typical European HerringGull but the degree of variation both ways means thereis considerable overlap. The longest uppertail-covertsmay have a completely dark centre – a pattern similar10
Identification of American Herring Gull in a western European contextto that of the lower scapulars, and very probably notoccurring in first-year European Herring.Uniformity of underparts The strikingly smoothtextured,evenly dark underparts of many juvenilesmithsonianus (plate 1 and 4) are probably never trulymatched by European Herring Gulls. Sometimes,however, European birds are more evenly saturatedand darker-looking below than usual (plate 16), so careshould be taken not to place too much importance onthis feature alone.Upperparts The pale fringes and notches to the upperpart-feathersaverage slightly less extensive than inEuropean Herring Gull, contributing to an overall darkerappearance. At the darker end of the range (plate 1),the patterns are probably never matched by juvenileEuropean Herring but lighter smithsonianus could easilyescape detection, at least on the basis of upperpartspattern, among their European counterparts.Tertial pattern In juvenile plumage, the tertial patternis rather similar to some graellsii (and therefore unlikemost argenteus) with little notching which is generally‘finer’ and is usually confined to the tips of the feathers.Juvenile argentatus tertials are often even more extensivelypale-notched than in argenteus but, significantly,a high proportion of Finnish argentatus, which mightbe described as ‘dark type’, have much reduced palefringe-markings, their tertials being similarly patternedto graellsii (plate 18).Greater-covert bar Most smithsonianus show a variableamount of solid dark-brown at the base of thegreater coverts which is clearly visible on the restingbird (plate 28, 36 and 37), forming an additional ‘bar’in flight (plate 33), similar to that shown by most firstyeargraellsii. The significance of this feature has, perhaps,been over-emphasized since it is by no meansexclusive to smithsonianus and is often shown byargentatus and sometimes by argenteus.Inner primaries On average, the inner-primary windowof first- and second-year smithsonianus is slightlyduller than in European Herring Gulls of the same age,adding to the uniformity of the upperwing. While thereis usually no strong contrast between inner and outerwebs on these feathers in either smithsonianus orEuropean Herring (thus differing from graellsii), theground colour of the inner primaries is rather dullbrownish grey in smithsonianus, while more pale greyish(with less of a muddy brown tinge) in EuropeanHerring Gull. This is not a terribly useful character, ofcourse, but it may be of some significance in evaluatingcontentious birds.Confusion with graellsiiThere are many similarities between juvenilesmithsonianus and dark first-year graellsii, someof which can show a confusing combination ofrather dark underparts, heavily patterned rumpand uppertail-coverts and sometimes an apparently‘all-dark’ tail. Most graellsii can, however,be quickly recognized by their more lightweightbuild, narrower, more pointed wings andslimmer bill than smithsonianus but, since not allsmithsonianus are ‘heavyweights’ (plate 5) andsize is not always easily determined, the followingcharacters should be checked:Lack of pale window on inner primaries Much as inEuropean Herring Gull, smithsonianus show a prominentpale window on the inner primaries (plate 2and 3). In graellsii, the inner primaries are almost asdark as the outer.Uniformity of underparts Generally, never matchedby graellsii but some of the latter are potentially confusing(plate 17).Vent and undertail-coverts The ventral area andundertail-coverts of juvenile smithsonianus are generallymuch more densely patterned than in graellsii(plate 7).Pattern of rump and uppertail-coverts Most graellsiishow an obviously whitish rump contrasting witha black tail-band but a few show a pattern and densityof markings that is undeniably close to that of lessheavily marked smithsonianus. It is all-too-easy, whenconcentrating too much on this one feature, to overlookthe other identification clues and mistake suchdark graellsii for smithsonianus.First-winter (plate 24-58, figure 1)With more than 90% of the records of smithsonianusin Europe being made up of birds in firstwinterplumage, this is certainly the bestrepresentedplumage type on the European sideof the Atlantic. At least in Ireland, where the vastmajority of European Herring Gulls are of thepaler argenteus type, first-winter smithsonianuscan be rather conspicuous amongst a mid-wintergull flock. Smithsonianus appears to exhibit evenmore individual variation than argenteus of thesame age but even so, we had difficulty findingbirds in the eastern USA that would not attractsome attention in a routine search of a gull flockon the European side of the Atlantic.Many of the characters which help differentiatejuvenile smithsonianus and European HerringGulls, most notably those relating to the upperandundertail-coverts, wings and tail, remainessentially unchanged in first-winters and thereforedo not need to be repeated here. Additionalcharacters on which to concentrate whenconfronted with a possible first-winter smithsonianusinclude:Uniformity of underparts By mid-winter, many firstyearsmithsonianus are a little faded and less evenlydark than they would have been as juveniles but are stillmore uniform below than the great majority ofEuropean Herring Gulls. The latter generally have paler,more mottled or streaked underparts but can occasion-11
Page 3 and 4: Identification of American Herring
Page 5: Identification of American Herring
Page 14 and 15: 36 American Herring Gull / Amerikaa

Identification of American Herring Gull in a western European context

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?