Small birds often perch on a power line with both feet. At this time, their entire body is in contact with only one wire, so they cannot form an electric circuit, there will be no electric current passing through their bodies, so they will not be electrocuted.
However, with large carnivorous birds such as hawks, eagles, falcons and owls, when perched on a power line, if their wings and feet touch both wires at the same time, it will create a circulating electric circuit, and they will still be electrocuted to death.