A thorough assessment of four Pacific Swift subspecies has found that they may deserve species status.
Four subspecies of Pacific Swift are generally recognised, and the species as it is known traditionally has a wide breeding distribution throughout the eastern Palearctic. It is a long distance migrant, wintering south to Indonesia and Australasia, and consequently is a rare vagrant to Britain, the last being seen at Kilnsea and Spurn, East Yorkshire, on 26 June 2008.
Leader (2011) measured and assessed the plumage of 146 specimens of Pacific Swift from four different museums across Eurasia, as befits a species with pan-Palearctic records. The four forms were found to differ in wing and tail measurements, as well as the size and shape of their distinctive white rump patch, white throat patch, pale underpart fringes and the colour of the underwing coverts.
The new prospective species are as follows:
· Pacific Swift Apus pacificus: cleaner white throat patch, a slightly longer tail fork and tail length, and the broadest rump patch by a margin; breeds from Siberia through to Japan, winters from Indonesia south and east to Tasmania (incorporating the subspecies A p pacificus and A p kurodae - other subspecies were found to be invalid).
· Sàlim Ali's Swift A salimali: five to 10 mm longer tail but with similar wing length to A pacificus, throat patch forming a thin white strip half the width of the other three forms, thinnest at the bill end, and very little white to the underpart feathers; breeds at high altitude on the east Tibetan Plateau and west Sichuan, China, but its migratory range is unknown.
· Blyth's Swift A leuconyx: the smallest of the four forms, with the rump patch consistently narrow, brown-tinged crown and nape contrasting with the glossy black mantle, broad white thoat patch with black shaft streaks extending onto the upper breast, hardly any pale underpart fringing; mid- to high-altitude breeder across the Himalayan part of the Indian subcontinent into Bhutan and Nepal.
· Cook's Swift A cooki: shallowest tail fork, first primary the longest (the other three have P2 as the longest), narrow white rump patch with dark, club-shaped shaft streaking, overall black upper- and underpart-coloration (brownish tinge in the other three), broad well-defined fringes to the underpart feathers, throat patch off-white with broad black shaft streaks, black contrasting underwing coverts, and green iridescence to upperparts with some white fringed scapulars; restricted range in limestone caves in northern south-east Asia, and a short distance migrant to then south.
Though many of the differences are subtle, it is believed that even small differences in measurement are clearly adaptive and diagnostic in an almost totally aerial group of species like the Apodidae. Splitting is currently rife in the family, as it is predisposed to cryptic forms owing to these morphological adaptations. Though the splits seem sound on this basis, and a small sample of vocal recordings appears to add weight to the taxonomic divisions, genetic analysis will be needed to add an extra layer of confidence to the findings. Clearly, European records are overwhelmingly likely to be A pacificus, but confirmation of this would be ideal.
Leader, P J. 2011. Taxonomy of the Pacific Swift Apus pacificus Latham, 1802, complex. Bulletin of the British Ornithologists' Club 131: 81-93.