In Corso et al. (2014), the pattern and coloration of the underside of the outer primaries was described as one of the most important features in differentiating Slender-billed Curlews Numenius tenuirostris from Eurasian Curlews Numenius arquata. However, the authors noted that this feature was not always clear-cut, with some Eurasian Curlews showing a pattern on the outer prim

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