To those readers who speak German, the following links may be of interest. To those who don't, the sound files will still provide pleasure, I presume.
Garden Warbler link here.
A link from Spain
Surfbirds Discussion Threat
On a German birding site (club300.de), a request is posted for information regarding abnormally calling Chiffchaffs: "Neu Aufruf: Abweichend rufende Zilpzalpe " .
Please note that the Chiffchaffs are definitely not Iberian or Siberian Chiffchaffs!
Interesting stuff. I've encountered several of the strange Chiffchaffs during one year now in Germany's South-West but have never encountered any during my 12 years in the North-East of the country. I might do a follow-up post on my latest encounter - time permitting.