In this cold weather, the food people provide for birds can make the difference between life and death. You can find some useful information about feeding birds here:
http://www.rspb.org.uk/advice/helpingbirds/feeding/index.aspx
Don't forget to provide fresh water for them daily too (any shallow container will do!)
It is now generally accepted that feeding birds all year round is beneficial (I have been doing this for years!) so please don't stop providing food once the winter is over.