What would posess someone to set alight a stump in the middle of a dam. If it were a house/shed/dwelling it would be arson. Nest or no nest, setting fire to a tree is pure stupidity - (actually a word from Chongs' dictionary would fit well in here).
If you were to drive past anything on fire, why would you not try to put it out?![]()
More to the point, why not make sure it was out before you left the water.
Sorry, but any prank when something is set alight doesn't sit well with me.