would you mind telling me the mistake I made?Yes, but the sentence has other errors.
The sparrow was on his tummy, not its own tummyA sparrow lay on its tummy staring at him and pecked his hand.![]()
Yes."A sparrow stood on his tummy and pecked his hand." would be okay too.
How does the following sentence sound?:
A sparrow lay on his tummy and pecked his hand.