Answer:
Ground.
Step-by-step explanation:
Basically, the reason why birds don't get electrically shocked, it's because they only reach a single path of the current. However, if they are able to toch something connected to the groun while they are on a cable, they will be shocked, becuase they are completeling the electrical cicle needed to flow the electricity.
Having said that, even a person can do that. Someone can grab a single power line without touching the ground or other cable, and that person will be fine, there can be thousands of volts and amperes, but that person will be totally fine.
Therefore, the word missing here is ground.
Birds sitting on a single power line don't get shocked because there is no path for the current to flow to the ground.