Our overwintering garden Chiffchaff is still around, feeding on gnats, and not singing - but of course it could be a female! This is par for the course - I have never heard our overwintering birds singing. They usually disappear in early March then there is a very small or no gap before the singing spring migrants - I am waiting with baited breath for the first ones to arrive here which is usually about 13th March. (Do we have any idea - e.g. from ringing recoveries - whether the overwintering birds here are male or female or both?)
Footnote: Just after I posted this, the lastest British Birds arrived and it contains a long article on over-wintering Chiffchaffs in Devon - so I may find out the answers to my questions! A quick scan and it appears that most of the birds in Devon were male ....
