Re: But how do they spread fires?
It sounds like a pretty good strategy for a bird of prey, where it's prey would happily sit in a hide/nest/inaccessible location to prevent the birds of prey eating them.
Bird of prey drops burning branch on prey hide. Prey exits hide, to avoid being burnt to death and bird of prey gets lunch.
Presumably they started doing this before we started managing things to the point that wildfires didn't occour both frequently and naturally, so when there is a wildfire it ends up burning a huge area instead of a tiny one.