Very few birds on feeders

We have several bird feeders in our garden with a variety of different seeds. In the spring, summer and autumn they are in constant use by goldfinches, various tits, house sparrows .............. et al and they consume a lot of seed. In the winter however, there are very few to virtually no birds around and the seeds start to rot and go black. Where have all the birds gone? Surely they don't all migrate, where are the Robins? Anybody else have the same? Thanks.