this is not uncommon. first the bees will fly and live to tell about it any
day its 50F or warmer.  what draws them to the birdfeeder, as far as i can
tell, is pollen!  some small amount of pollen is probably carried along with
the seed in  processing (plausible?).  they will ignore the seeds in your
feeder as soon as the 'natural' stuff makes its appearance.  and at this time
of year they have started brooding their spring/summer family and need the
protein available in pollen.
 
Rick Haver