A Murder Mystery

I have a real life murder mystery on my hands.  I headed out this morning to find a dead bird (I think an American robin) on my doorstep.  How did it get there?  How did it die?  There are many possibilities.  Was it killed by a predator?  Unlikely, as the bird looked untouched after it's death.  Did my landlord's cat leave it as a present for me?  Similarly unlikely, although I have recently discovered that it is the cat that has been setting off the motion sensor lights above my front door during the night.  Then an inspection of the crime scene revealed some telling evidence.  A paintball-like splatter on the side of my house above where the bird was found.  So my working theory is that the bird few into the side of my house, and either the shock or the impact from that was the cause of death.  Why would it fly into the side of my house?  Perhaps it was being chased by a crow (another reason to hate them) or a bird of prey (I don't see them very often, but they are around - they killed a rabbit in my yard a few years ago).  Perhaps it died of old age - I remember Basil, our family budgerigar, was prone to flying into walls and suchlike in his last days (RIP).  But the mystery, and it's a big one, is the color of the splatter on the side of my house.  It is green/grayish.  I can't explain that.  What would cause it to be that color, and how would the bird have caused that?  Any possible explanations welcome.