Correct - you need to touch two separate wires, or a wire and the ground, to make a complete circuit - when this happens, the electricity runs through the circuit, including your body, and you're electrocuted. But if you just touch one wire, as squirrels and birds do when they run/sit on the single wire, they're not earthing the electricity, not making a complete circuit, and so are not harmed. This works in exactly the same way for all animals, whether human or non-human - squirrels, birds, cats, humans, it works the same for all of us.