The reason birds can safely sit on electric power lines is the birds are just sitting on one wire and not making contact with the ground or neutral wire or grounding themselves out.
To be electrocuted from an electrical wire or other power line you have to complete a circuit through your body which means you either have to be touching the neutral wire along with the hot wire or be touching the ground while touching the live electrical wire.
Birds do not get grounded out or touch the other wire when sitting on top of power lines so they're safe.
It's the same reason you can see squirrels walking across power lines without being electrocuted.