"Three blind mice..."

Captured on camera: now i know why  the bird feeder  empties  so fast. I had attributed it to hungry little birds. 
 
What is the story behind this macabre nursery rhyme ?
Well, today it would be regarded as politically incorrect and rightly  so .

But its origins have a dark and bloody history.  It is believed to refer to Queen Mary 1 of England blinding and executing three Protestant bishops.