The slender-billed curlew (Numenius tenuirostris) is an extinct species of curlew formerly native to Eurasia and North Africa. Isotope analysis suggests the majority of the former population bred in the Kazakh Steppe despite a record from the Siberian swamps, and was migratory, wintering in shallow freshwater habitats around the Mediterranean. This species has occurred as a vagrant in western Europe, the Canary Islands, the Azores, Oman, Canada, and Japan. In November 2024, the species was declared globally extinct, with the last irrefutable sighting of the slender-billed curlew identified from Morocco in February 1995.
Description The slender-billed curlew was a small curlew, 36–41 cm (14–16 in) in length with a 77–88 cm (30–35 in) wingspan. It was therefore about the same size as a Eurasian whimbrel, but was more like the Eurasian curlew in plumage. The breeding adult was mainly greyish brown above, with a whitish rump and lower back. The underparts were whitish, heavily streaked with dark brown. The flanks had round or heart-shaped spots. The non-breeding plumage was similar, but with fewer flank spots. Male and females were alike in plumage, but females were longer-billed than males, an adaptation in curlew species that eliminates direct competition for food between the sexes. The juvenile plumage was very similar to the adult, but the flank were marked with brown streaking, the heart-shaped spots only appearing toward the end of the first winter. Compared to the Eurasian curlew, the slender-billed curlew was whiter on the breast, tail, and underwing, and the bill is shorter, more slender, and slightly straighter at the base. The arrowhead-shaped flank spots of the Eurasian curlew also are different from the round or heart-shaped spots of the adults slender-billed. The head pattern, with a dark cap and whitish supercilium, recalls that of the Eurasian whimbrel, but that species also has a central crown stripe and a more clearly marked pattern overall; the pattern of the slender-billed curlew would be hard to make out in the field. This species shows more white than other curlews; however, the white underwing has been stressed too much as a relevant identification criteria along with the distinctive flank markings of adults (not helpful in juvenile and 1st year bird before post-juvenile moult). The most recent and most updated identification's paper, reports as clinching characters the uniformly dark underside of 4 to 6 outer primaries (the wing-tip or "hand"), the black (adults) or at least darker grey (juvenile and 1st year birds) legs, and the white tail with fewer dark bars.
Vocalisations The call was a cour-lee, similar to that of the Eurasian curlew, but higher-pitched, more melodic, and shorter. The alarm call is a fast cu-ee. The only recording of the slender-billed curlew known to date was made by the French ornithologist Claude Chappuis. After his death in 2021, his collection, including the slender-billed curlew, was donated to the Macaulay Library at Cornell University in Ithaca (New York).
Distribution The only confirmed breeding records of the slender-billed curlew were a small region of raised bogs north of Omsk, Russia in a period between 1909 and 1925. Isotopic analysis suggests that main breeding range of the species was in a narrow belt in Kazakhstan centered around the 50th parallel north. This area is predominantly steppe, with some areas of forest steppe. In recent history, it mostly migrated to the Mediterranean as well as southern Arabia, with claims in the northern reaches of the Persian gulf, in Kuwait and Iraq. There have been historical records of the bird elsewhere, as in an ornithological dictionary of Gibraltar, written in 1895, it indirectly states that the slender-billed curlew was recorded as a passage migrant in Málaga, Spain.
Behaviour Little is known about the breeding biology of the slender-billed curlew; however, the few nests that have been observed each contained four eggs. The species fed primarily by probing soft mud with its bill for small invertebrates, but it also picked small prey items from the surface when available. Outside the breeding season, the slender-billed curlew was highly gregarious and often associated with related species, particularly the Eurasian curlew.
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