I have heard more reports of "missing" birds at feeders this winter than in any previous year, at least that I can recall. So much so that I've kept a folder of such complaints, in case some disease or other culprit comes to light. But in general, my answer is that birds are co-evolved with native food crops, and have been for FAR longer than they've had benefit of relatively new food sources provided by people and their feeders. If natural food crops are available, many birds will forsake your handouts in favor of nutritious native fare. At least, I believe that is usually the case. If it is a boom year for certain natural food crops, the species that feed on those may be especially apt to be AWOL at your feeders.
In general, it seems that the wintertime woodlands and meadows are bursting with food crops, at least in southeastern Ohio. I observed many examples of birds utilizing natural foods last Saturday, and managed to document a number of these relationships with my camera.
Some day, I will write an essay in support of poison ivy, a plant with a great many virtues. It seems that only humans abhor this valuable plant, which is of vital importance to many species of animals.