Where do they go?

I see I am not the only one whose garden is boycotted by the local birds when the BGB is on. How do they know? Or does the RSPB just always manage to choose a weekend when any sensible bird would be hiding in the holly bush. Which is what I suspect happened this weekend. We had snow on Saturday but I didn't think Sunday was too bad. The snow was gone, the food was out, I'd chased all the cats away but I only saw three birds in total.