The Chiffchaff, Phylloscopus collybita complex
From The UAE Checklist:
Common Chiffchaff Phylloscopus collybita
Subspecies not clear, generally seem to most resemble P. c. abietinus
Very common migrant and winter visitor, end October to early April.
There is anecdotal evidence that the majority of wintering birds are somewhat different in appearance from occasional big spring falls of migrants that appear some years in early to mid March; in essence latter generally appear rather more obviously olive (less grey) tinged above and with more obvious yellow on underparts than normal wintering birds.
Note that the contact call of the common wintering Chiffchaffs in the UAE is very similar to Siberian tristis. However, song (heard regularly from wintering birds from January to March) is essentially the standard European song, and hence markedly different from tristis.
This indicates that the vast majority of wintering birds, irrespective of call or appearance, are not tristis.
Siberian Chiffchaff P. collybita tristis
Possibly a rare migrant and/or winter visitor. No proven records, but may in fact be a regular visitor.