What Is That Sharpie Carrying?

While exploring the trails at Hawk Ridge with my mom and sisters, we noticed a sharp-shinned hawk catching a thermal (a warm current of air that raptors glide on).  This hawk appeared to be carrying something in its talons.

I wondered what it was carrying and my sisters wondered why a bird that wouldn't be feeding chicks would carry food during migration.  I wondered if it had just killed it's prey and was looking for a safe place to eat it--but why would it catch a thermal to go so high up?  Usually these hawks tuck into a tree to eat.  I also noticed that the sharp-shinned hawk appeared to have a full crop--why would it hunt if it had already eaten.  Sometimes at the hawk blind, Frank Taylor will get in sharp-shins with full crops that fly in for the bait pigeon.  Frank said that even though the crop is full, the message hasn't hit the rest of the body that it's full of food and further hunting isn't necessary.  Sharp-shins are accipiters like Cooper's hawks and that's very much a group of hawks that tends to act before it thinks.

In this photo you can really see that full crop.  Also, it looks like it's carrying a wing--a robin's wing maybe?  I wondered if the sharp-shinned had just killed and eaten a robin.  Since this is a smaller hawk, it could have eaten its fill and have some leftover.  Perhaps it didn't want to leave the remaining food behind?