Red in tooth and claw (and bone)

Alfred Lord Tennyson used 'red in tooth and claw' to describe nature in 1850. Two episodes in my garden confirm that this is a fair point. The first involves a blackbird's nest. The parent birds have been running themselves ragged feeding the young these last few days.  A cacophony of alarm calls brought me to the window and in seconds it was from this...

to this......

via this.....

The crow, having taken the chick to the roof ridge and torn it apart, hopped off and just left it there.

Last week I had been watching a blue tit bringing nest material into a nestbox.  Hour after hour every few minutes, in and out of the hole.  I watched it come out and fly directly over a low hedge into a neighbour's garden.  Seconds later, the flash of a sparrowhawk over the hedge and a silent nest box.  The box is re-occupied now - maybe with a smarter, faster and more vigilant blue tit.

Meanwhile, the red bone in the title still remains a mystery.