Pollen substitute feeding

A few days ago I posted a picture of returning bees with what I thought was pollen.  I was convinced it is too early for pollen and the bees were probably returning with propolis.  Feeling sorry for my bees I made up a quick and dirty pollen substitute feeder from a cardboard box (see picture).  Nothing happened immediately, but 15 minutes later the bees had found the protein source and were packing it away.  Some were packing it on so heavy they had a hard time getting airborne.