Lindholm 2008. Mixed song of Chiffchaffs in Northern Russia. Alula 14(3): 108–115. [pdf]van den Berg, AB 2009. Calls, identification and taxonomy of Siberian Chiffchaff: an analysis. Dutch Birding 31(2): 79–85.
... It is presumably based upon the cline in colour within the range of tristis, or perhaps upon the incidence of mixed-song which, by definition, is more likely to occur in western areas where abietinus may turn up (Lindholm 2008). ...
Ref: Dean & Svensson 2005. 'Siberian Chiffchaff' revisited. British Birds 98(8): 396–410. [pdf]
[Open access to pre-2007 BB pdfs without registration/log-in wef 26 Dec 2012.]
Dutch Birding 31(2).In the latest Dutch Birding, Arnoud van den Berg (almost?) proposes species status for Phylloscopus collybita tristis:
Ref: van den Berg, AB 2009. Calls, identification and taxonomy of Siberian Chiffchaff: an analysis. Dutch Birding 31(2): 79-85.
Siberian Chiffchaff Phylloscopus tristis was recognised as a full species wef 1 Jan 2014 in Dutch Birding 36(1), but the most recent CSNA report was published in 2009 (as far as I'm aware).Christmas came early in the Netherlands: http://www.dutchavifauna.nl/species/siberische_tjiftjaf#en
Not sure this was every formally noted down in a CSNA report?
Marova, I M, Shipelina, D A, Fedorov, V V & Ivanitskii, V V 2013. Siberian and East European chiffchaffs: geographical distribution, morphological features, vocalization, phenonemon of mixed singing, and evidences of hybridization in sympatry zone. In: Rodríguez, N, Garcia, J & Copete, J L (redactie), El mosquitero ibérico. Léon.
Yes, the annual 'Redactiemededelingen: Naamgeving van taxa in Dutch Birding', which lists DB changes to WP taxa (including CSNA taxonomic changes affecting the Dutch avifaunal list).There are these yearly short notes in Dutch Birding (I think the first number of the year) with recent decisions.
My understanding is that (formally) DB follows the taxonomic decisions of CSNA for species on the Dutch avifaunal list (although sometimes provisional changes have been published before formal consideration by CSNA); and DB follows the advice of CSNA members for the taxonomy of other WP species. So, in practice, AvdB (DB Chief Ed) probably takes advice on WP taxa from AvdB (CSNA member).I thought the two were synonymous.
What four groups?There are four research groups actively working on Chiffchaff DNA, and a fifth group recently published a paper in a Spanish-language book on the Chiffchaff complex. The situation is more complex than most of us realise. Any change in rank at this time is premature. Best to wait until things are sorted out. CSNA has not decided on this, pending ongoing DNA work.