On the first warm days in the spring, bees can be seen in bird feeders or on saw dust. The bees collect dust off of the bird seed and saw dust and pick it up as pollen. It is not pollen.
Beekeepers need to be careful sawing treated lumber in early spring, the bees can possibly bring back the treated lumber dust back to the hive. Treated lumber offers protection from insects in the treatment. This dust can possibly kill a colony.