In Animal Farm, Chapter 1, what interrupts thier singing?

The animals woke Mr. Jones up with their revelry and were interrupted when he shot his gun at the wall of the barn.

Unfortunately, the uproar awoke Mr. Jones, who sprang out of bed, making sure that there was a fox in the yard. He seized the gun which always stood in a corner of his bedroom, and let fly a charge of number 6 shot into the darkness. The pellets buried themselves in the wall of the barn and the meeting broke up hurriedly.