I have also noticed a substantial decrease in the number of birds visiting the backyard feeder when compared to previous years. Moreover, even when a few birds visit they will depart after a feeding for only a few minutes. Nothing may return for an hour, or more. Considering the variety of seed on offer and that the temperature is quite low it seems especially odd. Let's face it, when it's cold the birds need all the calories they can get.
Also, I will often see a bird "freeze" in position on the feeder, or on a perch for quite a long time. This generally suggests that there is a predator nearby so the bird chooses to remain motionless as if inanimate. I've been unable to detect a predator in the area. In fact, I have not seen a hawk or falcon in a very long time, despite all the perches offered by the street-side woodlot that a bird of prey could use as a look-out. Right now the snow is too deep for cats, so I know it's not the perceived threat. It's also too deep for squirrels and that part is great.