Why a person is shocked

Change one thing about the bird's situation — put the second contact point on the ground instead of on the wire — and the outcome is completely different.

Figure to be added

Figure 19.32 — a person touching a live overhead wire with one hand while standing barefoot on the ground, with the current shown passing down through the body to the feet.

What makes the circuit

If a person touches a live wire at one point while standing on the ground without footwear, they are shocked. The reason is that there is a difference in potential between the two places the electricity touches the body. Building a connection between the wire and the ground creates a closed electric circuit, and the electricity entering from the live wire is earthed through the person's feet.

Enters at the hand → Through the body → Out through the feet to earth

The one difference from the bird

The bird's two contact points were both on the wire, at the same potential. The person's two contact points are the wire and the earth, which are at very different potentials. Everything else about the two situations is the same.

Touching one wire is not automatically safe

It is wrong to think that touching a single electric wire cannot harm you. What decides it is whether your body completes a path between two points at different potentials — and standing on the ground usually means it does.