Overwintering by chiffchaffs is not uncommon these days. Like the blackcap,this habit seems to have increased in the last forty years. To hear one singing,however,is remarkable-congratulations.
Living in East Anglia,i only wish the turtle dove could get into the habit of not migrating. Much of the reason for it's sad demise seems to be on migration and in it's winter quaters in Africa. Spending the winter in much safer Britain,perhaps feeding on farms and in gardens like it's close relative,the collared dove,could stem it's demise toward seemingly inevitable extinction in Britain.
I too have snowdrops in my garden,and the hedges are full of hazel catkins. A little early perhaps,but i feel the need to remind people that winter is only half-way through,and there is plenty of time for cold weather.