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The Lesser Black-backed Gull in the Americas. Occurrence and subspecific identity

Post, P.W. and Lewis, R.H. 1995.
The Lesser Black-backed Gull in the Americas. Occurrence and subspecific identity. Part I: taxonomy, distribution and migration.
Birding, 27: 282–290.

The following sequel is appended to the PDF file:

Post, P.W. and Lewis, R.H. 1995.
Lesser Black-backed Gull in the Americas. Occurrence and subspecific identity. Part II: field identification.
Birding, 27: 370–381.

Abstract

We review the range expansion and current distribution of the Lesser Black-backed Gull in the New World (Part I) and discuss the field identification of the 3 subspecies of Lesser Black-backed Gull — graellsii, intermedius and fuscus (Part II). We offer criteria for differentiating the subspecies, although not every individual can be identified in the field.

  • L. f. graellsii have a grayish mantle that is clearly paler than that of any Great Black-backed Gull; it shows a marked contrast with the black of the outer primaries. (A bird with the same mantle color as a Great Black-backed cannot be graellsii.) L.f. graellsii have a bill that is similar to that of a Herring Gull and, from about September through January or later, dusky head streaking. A perched adult in fresh plumage (definitive basic) shows medium to large white primary tips. The end of the tail falls between the third and fourth primaries, but usually closer to the third.
  • L. f. intermedius have bill and body shapes similar to graellsii, but have a blackish mantle lighter than or equal to that of a Great Black- backed and showing some contrast with the black of the outer primaries. Winter head-streaking is similar in extent to graellsii but not as heavy. A perched adult bird in fresh plumage (definitive basic) shows very small white primary tips and wings extending well beyond the end of the tail, usually far enough to have the fourth primary tip opposite, or beyond, the end of the tail.
  • A Lesser Black-backed Gull can be identified as fuscus only if it has a thin, long bill, a mantle darker than that of a Great Black-backed, wings showing little contrast with the black of the outer primaries, long pointed wings, only one small white mirror on the outermost primary, a small, slender body, relatively long legs, and virtually no head markings at any time of year.

Although the >30 specimens of L. fuscus collected in the New World have been identified as graellsii, we present evidence that intermedius occurs in North America (Cover Photo and Figure 5, Part I; Figures 4, 13 and 14, Part II). There is no conclusive evidence that fuscus has occurred on this side of the Atlantic. Because of a serious decline in its numbers, the probability of its occurring here in the future is very low.

The criteria used to separate the 3 subspecies of Lesser Black-backed Gull need to be refined. Perhaps comparisons between eye diameter and bill and wing length, which led to the development of objective criteria for the field identification of Glaucous (L. hyperboreus) and Iceland (L. glaucoides) gulls (Grant and Mullarney 1989), may prove useful with Lesser Black-backed Gull subspecies as well.

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