Murdered bluebird

Deceased male bluebird, box 2

Not what you want to see when you open a nesting box. I initially thought it was still alive but soon found out I was, sadly, wrong. Could be worse--a nesting bluebird hen, for example...

Pretty clearly murder and there are many possible suspects. These tree swallows, for example, quickly claimed the box after I cleaned it out. Could have been them. House sparrows are always a possibility, though I haven't seen them in this part of the field this year. Perhaps a sharpie dealt it a mortal wound. My prime suspect, though, is a rival male bluebird. I've seen two males go to the ground fighting this spring and I wonder if the violence reached fatal proportions.