No sound but still fascinating viewing. The Rook here is emulating the crow in the Aesop fable:
THE CROW AND THE WATER JAR
A thirsty crow noticed a huge jar and saw that at the very bottom there was a little bit of water. For a long time the crow tried to spill the water out so that it would run over the ground and allow her to satisfy her tremendous thirst. After exerting herself for some time in vain, the crow grew frustrated and applied all her cunning with unexpected ingenuity: as she tossed little stones into the jar, the water rose of its own accord until she was able to take a drink.
This fable shows us that thoughtfulness is superior to brute strength, since this is the way that the crow was able to carry her task to its conclusion.