Where are all the Birds?

Despite having an extended period of hard frosts, there have been few birds coming into the garden. I have six bird feeders well stocked with suitable food and positioned where I think there is sufficient cover for the birds to feel secure, yet I am only seeing the odd blue or great tit appearing. There are other birds around, such as robin, wren, hedge accentor, collared dove, magpie, pheasant, but these are not the birds you will generally get on the feeders.
I can only conjecture that the mild autumn has meant that there are still sufficient supplies of insects to keep many birds within the hedgerows and woodlands without a need to venture into the garden. I had hoped to entice woodpeckers and nuthatches, etc, but so far, I have only seen them fly over.