Herkenning van Vorkstaartmeeuwen Xema sabini in de hand op het strand

During beached bird surveys, especially when dealing with incomplete, dirty or rotten carcasses, the identification of birds found is not always easy. In Dutch beached bird surveys, it was noted that Sabine’s Gulls were found rather less frequently than expected, considering frequencies of sightings during seawatches in autumn. Possibly, Sabine’s Gulls are overlooked or confused with for example juvenile Black-legged Kittiwakes. In this paper, diagnostic characteristics of Sabine’s Gulls are listed and compared with characteristics of superficially similar species (juvenile Kittiwakes, Little Gulls and Ross’s Gulls). Confusion was considered least likely with juvenile Little Gulls, which do not feature a flashing white wing panel, but have dark grey subterminal spots on inner primaries and secondaries (Fig. 7). Sabine’s Gulls have a distinctly smaller bill than Black-legged Kittiwakes, and the wing length, even of adult Sabine’s Gulls, should be shorter than even very small juvenile Kittiwakes (some of the juvenile Kittiwakes used for Table 2 had still growing primaries). Features to separate Sabine’s Gulls from juvenile Kittiwakes include the dark shaft of white inner primaries in the latter (pure white in Sabine’s Gull; compare Figs 4 & 6), a rather narrow white edge of the inner vane in the blackish outer primaries of Sabine’s Gulls (Fig. 5-6) versus a broad white tongue on primaries in Kittiwakes (Fig. 4), white tipped outer primaries in adult Sabine’s Gull (may wear off; Fig. 5), more or less entirely white primaries 5-6 in Sabine’s Gulls, with at best a very small subterminal spot on P5 (subterminal bands on P5 and P6 in Kittiwake), completely dark primary coverts in Sabine’s Gulls, dark coverts with a distinct light inner vane in Kittiwakes (Fig. 8), and a clear colour contrast between mantle/secondary coverts (grey) and primaries/ primary coverts (blackish) in Sabine’s Gulls as opposed to black feathers in a rather narrow band running diagonal over the arm towards the equally black primary coverts and outer primaries in Black-legged Kittiwakes.