On the taxonomic status of the Siberian Chiffchaff Phylloscopus [collybita] tristis (Phylloscopidae)

The Common Chiffchaff Phylloscopus collybita is a common, widespread and polytypic species of the Palearctic comprising six recognised subspecies: collybita (Vieillot, 1817) breeding in most of western and central Europe and wintering in western and southwestern Europe; abietinus (Nilsson, 1819) breeding from Scandinavia and northwestern Russia to the Black Sea and wintering in southeastern Europe, the western part of the Middle East and northeastern Africa; caucasicus Loskot, 1991, breeding in the Caucasus and Iran; menzbieri Shestoperov, 1937, breeding in northeastern Iran and Turkmenistan; brevirostris (Strickland, 1837) breeding in northwestern Turkey; and tristis Blyth, 1843, breeding in most of Siberia from the western foothills of the Urals to the Kolyma River in eastern Siberia and wintering mostly in the Indian subcontinent and potentially marginally in the eastern part of the Middle East (Shirihai & Svensson 2018; Rakovic et al. 2019). The wintering grounds of menzbieri, caucasicus and brevirostris are much less well-known (Shirihai & Svensson 2018). The taxonomic status of the taxon tristis, widely known as the Siberian Chiffchaff, has always been hotly debated, some authors arguing that it deserves specific status based on morphological, acoustic and mitochondrial divergence, whereas others prefer to keep it as a subspecies of the Common Chiffchaff because of unclear levels of reproductive isolation in the contact zone (Helbig et al. 1996; Talla et al. 2017; Shirihai & Svensson 2018; Rakovic et al. 2019). Currently, only one of the four main global ornithological taxonomic authorities, the HBW & BirdLife Taxonomic Checklist (del Hoyo et al. 2016), considers the Siberian Chiffchaff as a valid species, whereas the IOC World Bird List (Gill et al. 2024), the Clements Checklist of the Bird of the World (Clements et al. 2023) and the Howard & Moore Complete Checklist of the Birds of the World (Dickinson & Christidis 2014) keep it as a subspecies of the Common Chiffchaff. The aim of this article is to review the published knowledge on phenotypic, acoustic, genetic, biogeographical and migratory divergence between the Siberian Chiffchaff and the Common Chiffchaff to allow a re-evaluation of species limits.