Another Trip Around the Wildlife Loop

We were scouting the Wildlife Loop in Custer State Park and pulled off the road to watch the Bison as they were grazing in the early morning light.  Across the road a pair of Mountain Blue Birds were feeding their little ones in a nesting box and were landing on the fence before entering the nest.  When I took this image I saw the caterpillar she was caring but had no idea there was a spider sitting on her head.

Nikon D4, Nikkor 200-400f4, TC-20E