Table S1. Detailed comparison of songs of different taxa in the Arctic Warbler complex.

Table S2. Syllable order (syntax) in examinandus and xanthodryas.

Table S3. Detailed comparison of calls of different taxa in the Arctic Warbler complex.

Appendix S1. Information on sound recordings and sequences analysed.

Figure S1. Mitochondrial cytochrome-b tree estimated by Bayesian inference using the GTR + I model.

Figure S2. Song of borealis: (a) Abisko1, Sweden, June (Krister Mild; same individual as in Fig.  3a); (b) Khovsgol lake8, Mongolia, June (Geoff J. Carey); (c) Respublika Tyva7, Russia, July (Magnus Robb); (d) Respublika Tyva7, Russia, May (Nicolai Formosov/BL 46264); (e) Respublika Tyva7, Russia, June (Veprintsev 1982); (f) Ulan Bator10, Mongolia, June (Dieter Wallschläger/BL 14496); (g) Huzong14, Heilongjiang province, China, June (Per Alström/BL WA 1994/82; same individual as in Fig.  3f); (h) Magadan12, Russia, June (Ulf Hassel); (i) Magadan12, Russia, June (Ulf Hassel); (j) Magadan12, Russia, June (Ulf Hassel); Magadan12, Russia, June (Trevor Price); (k) Khabarovsk16, Russia, May (Anvar B. Kerimov/BL 46280). Superscript numbers represent localities (details in Appendix  S1).AUTHOR: pls chck that this is the correct definition of the superscript numbers: Superscript numbers represent localities (details in Appendix  S1).

Figure S3. Song of kennicotti: Alaska.

Figure S4. Song of examinandus: (a) Kamchatka19, Russia, July (Veprintsev 1982, same individual as in Fig.  4a); (b) Sakhalin20, Russia, July (Toshio Oda/Oda 2005, same individual as in Fig.  4b); (c) Mt. Rausu23, Hokkaido, Japan, July (Takema Saitoh; same individual as in Fig.  4e); (d) Mt. Shari23, Hokkaido, Japan, August (Takema Saitoh). Superscript numbers represent localities (details in Appendix  S1).

Figure S5. Song of xanthodryas: Honshu, Japan.

Figure S6. Factor analysis of song variables.

Figure S7. Canonical discriminant analysis of four measurements of 58 adult Arctic Warblers, including three type specimens.

Audio S1.Phylloscopus borealis call, Heilongjiang, China, Per Alström.AUTHOR: Audios S1–S15 have not been mentioned in the text. Please cite the Audios in the relevant place in the text.

Audio S2.Phylloscopus borealis song, Huzhong, Heilongjiang, China, Per Alström.

Audio S3.Phylloscopus borealis song, Khovsgol lake, Mongolia, Geoff J. Carey.

Audio S4.Phylloscopus examinandus call, Japan (migration), Per Alström 1AUTHOR: pls indicate what numbers after Per Alström indicate in audio s4 and s5.

Audio S5.Phylloscopus examinandus call, Japan (migration), Per Alström 2.

Audio S6.Phylloscopus examinandus call,  Mt Rausu, Hokkaido ind1-PG5 Takema Saitoh.

Audio S7.Phylloscopus examinandus song, Mt Rausu, Hokkaido ind4-PG3 Takema Saitoh.

Audio S8.Phylloscopus examinandus, song Mt Shari, Hokkaido ind3-PG9 Takema Saitoh.

Audio S9.Phylloscopus examinandus song, Opala river, Kamchatka ind2-PG4-2 Takema Saitoh.

Audio S10.Phylloscopus xanthodryas call, Karuizawa, Honshu Urban Olsson 1.

Audio S11.Phylloscopus xanthodryas call, Karuizawa, Honshu Urban Olsson 2.

Audio S12.Phylloscopus xanthodryas song, Iwate, Honshu ind6-PG1 Takema Saitoh.

Audio S13.Phylloscopus xanthodryas song, Iwate, Honshu ind12-PG1 Takema Saitoh.

Audio S14.Phylloscopus xanthodryas song, Karuizawa, Honshu Urban Olsson.

Audio S15.Phylloscopus xanthodryas song Shikoku, Japan ind2-PG2 Takema Saitoh.

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