I read an ammusing article talking about what shit disturbers crows are. Here is an excerpt:

Why do the crows put stones on railroad tracks? Since the practice doesn't seem to have much use in survival, some wild bird researchers think the crows' rock-placing behavior could a form of play. Crows are well-known for playful and mischievous behavior. One of their games involves snatching balls off golf courses and tussling among themselves over the balls. They display more interest in colored balls than white ones, and they keep taking the balls if people make a fuss but lose interest if the golfers ignore them. In Japan, where many houses are roofed with metal or tile, they also like to drop pebbles onto roofs. Apparently, they get a kick out of the pinging or plonking sounds the pebbles make when they hit the roofs. So maybe the crows like the clinking sound made when they place a rock on a rail.