The bird flew over and landed near the woodchuck as it moved across the yard to the edge of the pond.
The woodchuck stopped and the bird landed on its back where it picked at the fur for about five minutes.
When the woodchuck moved on the bird followed it.
Reference that mentions this behavior for obtaining nesting material - under "Nest": Peterson
Taken on an overcast day through a dirty double pane sunroom window.
See article in Tulsa World newspaper