Where have all the birds gone?

Over the last few days our garden has become almost bereft of birds. There is plenty of clean water and the feeders are full with fresh food. During the snow my wife kept steady supply of water and food and we saw birds’ not perhaps as many but usual suspects. Except yesterday one pigeon appeared strutted about for a few minutes and then left. Since then nothing. The garden usually goes quiet when a sparrow hawk has been around but never as quiet as this.