The main difference between male samamisicus and phoenicurus is the presence (samamisicus) or absence (phoenicurus) of a white wing patch (e.g., Shirihai and Svensson 2018; Small 2009). Based on an initial evaluation of variation in this character, we defined five character states ranging from an all-dark wing to a bold white wing patch (Table 1).
Some male samamisicus are characterized by a much darker mantle compared to the nominate form, or even a black mantle (the so-called ‘incognita’ morph). To document the prevalence of this feature, we defined three character states for mantle coloration: (1) ‘normal’ dull blue-grey as in phoenicurus, (2) distinctly darker grey than phoenicurus, and (3) partially or completely black. It must be taken into account that grey shades can be difficult to assess on photographs without direct comparison under the same lighting conditions, thus we only scored as (2) those individuals that were obviously dark grey.
If possible, birds were aged based on moult contrast in the greater coverts and/or moult contrast between the wing feathers and mantle (Jenni and Winkler 2020; Svensson 1992). Ageing was performed mainly by NM but cross-checked by MS and by an experienced bird ringer (Fabian Schneider). Cross-checking was performed on a random sample of 100 birds plus all non-adult birds from the breeding areas of samamisicus (based on the results reported herein) and many supposed second calendar year (hereafter 2cy) males from the Balkans and Ukraine, as any errors in these regions would potentially have the greatest impact on the results.
All scoring was performed by the same person (NM) to achieve maximum consistency, but a random sample of 100 birds was cross-checked by MS. Mean difference was 0.05 scoring points and differences never exceeded 1 scoring point.
Data collection and analyses
To document plumage variation of male Common Redstarts during the breeding season across the species’ entire range, we searched for photographs on the internet, mainly on various citizen science databases (for a list of these, see Table S1). We considered only (i) photographs of birds that were obviously nesting (e.g., carrying food, copulating) or (ii) were taken between 15 May and 31 July, as these dates cover the species’ main breeding period (Glutz von Blotzheim 1988). Moreover, the migration of nominate birds through the breeding range of samamisicus should be effectively over by mid May (Handrinos and Akriotis 1997; Kirwan et al. 2008).
If multiple photos from a single site (i.e., the environs of a village) in the same year were available they counted as one bird, unless it was obvious that more than just one individual was involved. As we were mainly interested in the pattern of the wing (see before), only those photographs which permitted wing coloration/pattern to be critically assessed were considered.
Secondly, we consulted a number of museum collections, particularly to further enhance our sample from regions of potential introgression between the two subspecies of Common Redstart identified from the initial analysis of internet photographs, but also to confirm or refute patterns evident from the latter using material that is easier to categorise and reliably score using the characters selected. To this end, GMK photographed all relevant material (based on the dating parameter [ii] mentioned above) at the following museums (with acronyms and numbers of specimens eventually used for this study in parentheses): Natural History Museum, Tring (NHMUK, n = 11); Museum für Naturkunde, Berlin (ZMB, n = 8); Zoologisches Forschungsmuseum Alexander Koenig, Bonn (ZFMK, n = 10); and Museum of Zoology, University of Cambridge, UK (MZUC, n = 3). Among the material examined, but not analysed, for this study, was the type of Sylvia mesoleuca Ehrenberg, 1833, taken on spring migration in Jeddah, Saudi Arabia (at ZMB), which name is a synonym of Motacilla samamisica Hablizl, 1783 (= P. p. samamisicus). Furthermore, MS checked specimens in the Naturhistorisches Museum, Bern (NMBE, n = 2). In addition, photos of relevant specimens (fulfilling category [ii] above and originating from regions with scant material in citizen science databases and of specific interest, i.e., suspected intergradation zones, the main range of samamisicus, Central Asia, and North Africa) were solicited and received from the following museums (alphabetical order): American Museum of Natural History, New York (AMNH, n = 5); Field Museum of Natural History, Chicago (FMNH, n = 2); Museum of Comparative Zoology, Cambridge, MA (MCZ, n = 2), Manchester Museum (MM, n = 1); Slovenian Museum of Natural History, Ljubljana (ML, n = 1); National Museum of Natural History, Bulgarian Academy of Sciences, Sofia (NMNHS, n = 2); Natural History Museum, Belgrade (NHMBEO, n = 2); the Naturalis Biodiversity Center, Leiden (RMNH, n = 2); Darwin State Museum, Moscow (SDM, n = 13); University of Michigan Museum of Zoology (UMMZ, n = 1); University of Washington Burke Museum (UWBM, n = 4); National Museum of Natural History, Smithsonian Institution, Washington, D.C. (USNM, n = 7); Biodiversity Research and Teaching Collections (TCWC, n = 2); Museum für Naturkunde Berlin (ZMB, n = 8); Zoologisches Forschungsmuseum Alexander Koenig, Bonn (ZMFK, n = 14). Replies were received from four additional museums, confirming that they had no male specimens collected during the key mid May to end of July period and the above-mentioned regions: Muséum national d’Histoire naturelle, Paris (MNHN); Naturhistorisches Museum, Basel (NMBA); Yale Peabody Museum (YPM) and Zoological Museum, University of Athens (ZMUA).
In order to analyse frequencies of different wing scores and back colour, the following regions were used: Central Asia (Afghanistan, Pakistan, Tajikistan, Turkmenistan, Uzbekistan, Kyrgyzstan, southeastern Kazakhstan), Northern Asia (Siberia, Mongolia, China, northeastern Kazakhstan); Fennoscandia (Denmark, Finland, Norway, Sweden); European Russia and western Kazakhstan; Central Europe and UK (United Kingdom and countries from continental Europe including northern Ukraine, Italy north of Genoa, cf. Martinez and Martin 2020); Iberia and North Africa (Algeria, Morocco, Portugal, Spain, Tunisia); Italy south of Genoa; Croatia and Slovenia; the Balkans (Albania, Bosnia and Herzegovina, northern Bulgaria, Kosovo, Montenegro, southern and coastal Romania, Serbia); coastal Ukraine and Rostov Oblast, Russia; Crimea; Greece and southern Bulgaria; western Turkey; eastern Turkey; northern Caucasus (northern Azerbaijan, Georgia, Russian Caucasus), samamisicus core range (Armenia, southern Azerbaijan, Iran).