Anyone know what to make of these?

http://alexanderboyes.com/photos/201...llow_Backs.jpg

Context:
First year beekeeper just south of downtown Milwaukee, WI. Got a late start after two dud queens, but thanks to beesource member geebob I now have a healthy colony with a locally mated, first year queen. In early September I started noticing these yellow backed bees coming and going at the entrance. They behave normally, and I found them scattered throughout the hive. The pictured frame had more "marked" bees than most.

Is someone marking them? Are they marking themselves?