The bees are collecting dust from the seed as if it were pollen.  Their
pollen stores are pretty much depleted at this time of year, and they need
it as a protein source for rearing young.  I've heard of bees collecting
sawdust in a similar way -- to them it's the closest thing they can find to
what they're really after.  As soon as the willows and maples begin
supplying the first pollen (soon), the bees will disregard the bird feeder.
You must have a colony nearby if they are present in large numbers.