Bloody hell, last year it was a brazen and semi-psychotic squirrel in roof and now it's crows! 
I managed to evict the squirrel (humanely) then we glued back every PVC vent thing (most new) so that the air vents/duct things were not open and exposed.
Few days ago I hear squealing and screeching. I investigate and see an almighty mess of twigs and small stones and various debris in my insulation in the attic. On closer scrutiny, I can see the hatchlings huddling and ducking down but their steady breathing gives them away.
How the hell did they get in?!?what do I do now???!
Aaaargh they're noisy buggers and they're wrecking the loft.