Plumage variability and field identification of Manx, Yelkouan and Balearic Shearwaters
By Marcel Gil-Velasco, Guillermo RodrÃguez, Stephen Menzie and JosÃ© Manuel Arcos
Abstract Three medium-sized Puffinus shearwaters breed in Europe: Manx P. puffinus, Yelkouan P. yelkouan and Balearic Shearwaters P. mauretanicus. These three, considered a single, polytypic species until relatively recently, can be difficult to identify in some situations, notably in a vagrant context. This paper presents a detailed review of the plumage variability of the three taxa; combined with an understanding of structural differences and flight characteristics, this should provide a sound basis for the identification of problematic individuals.
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A clean underwing (A) is the most commonly shown pattern in Manx Shearwater, but about a third of birds show diffuse dark marks around the ‘wrist’ area (B). Rarely (<6%), Manx Shearwater may show a dark diagonal axillary bar (C). This differs from the typical pattern of Yelkouan Shearwater (D, E) in being broadest towards the ‘wrist’ (narrowest towards the wrist in Yelkouan). The diagonal bar is absent in c. 20% of Yelkouans. The flanks under the wing are clean in c. 10% of Yelkouan (D) but more commonly (60%) birds show a fine line along the upper flank (E), sometimes broken in the centre. In c. 5% of Yelkouan the flank line is thicker, though still with some narrowing at the centre (F); typically these individuals also show a relatively thick diagonal bar and a dirtier-looking underwing. A few Balearic Shearwaters show an underwing similar to 3f, although pale birds (3%) typically show a uniformly thick flank line (G). Most Balearics show a dirty underwing with a broad diagonal bar (H). Artwork by MartÃ Franch.
Balearic (above) and Yelkouan Shearwater (below), Castellon, Spain, December 2014. This side-by-side comparison reveals the larger size and bulky, pot-bellied structure of Balearic, and the slimmer, more attenuated appearance of Yelkouan. Pic by Victor Paris.
Manx Shearwater typically shows a clean undertail, with dark restricted to the outer web of the outer row of undertail-coverts (A). Some fine dark markings may occasionally be found on the inner web of these feathers; one individual (B) was seen with solid dark markings on the inner webs – see text. The palest Yelkouan Shearwaters (c. 20%) can appear similar (C), though the dark/light contrast is less marked. More than 50% of Yelkouans show completely brown undertail-coverts, although the dark does not extend to the belly (D). In around 20% of Yelkouans, dark extends onto the sides of the lower belly (E); compare that with a pale Balearic Shearwater (F), in which brown usually extends right across the lower belly. Typical Balearics show a completely brown lower belly, which merges gradually into the pale central belly (G). Artwork by MartÃ Franch.