Just back from a birding conference in Northern Italy where I got slightly confused by the Chiffchaffs. All of the birds I heard (30 - 40 individuals) were making a disyllabic call with the first note rising in pitch and the second falling ie. completely the opposite from the normal huuit/tuuit calls of western collybita. I was alerted to the following web article:
Copete, J.L- & Armada, R.(2004) Unusual calls of Chiffchaffs Phylloscopus collybita in NE Spain in autumn-winter: an alert to Spanish observers. Rare Birds in Spain. Retrieved from http://www.rarebirdspain.net/arbsi026.htm
The birds were calling in a way very similar to that described in the article as the Tsi-lip call. Having lived in that part of Italy for 5 years I have never heard (or noticed?) that type of call and have never heard it elsewhere in Europe. The authors suggest that these birds are Eastern collybita as they have been recorded breeding in Finland/Estonia in some years. Certainly, plumage did not suggest anything other than normal collybita collybita although some observers thought that perhaps the supercillium was quite well defined/marked.
I would be interested to hear from other European observers as to whether they are hearing similar birds at the moment or if they are restricted to Northern Italy.