Why the arc when bonding-on during high voltage repairs?
This question is somewhat related to yours.

The phenomenon has nothing to do with magnetic fields. There is an intense electric field around any high-voltage wire (AC or DC), and the intensity is inversely proportional to the distance from the wire. At very close distances, the intensity exceeds the breakdown threshold for the air itself.

The presence of the conductive helicopter (and the repairman with his metal mesh suit) distorts this field, concentrating it even further in the space between the wire and the helicopter. When the bonding strap gets close enough, an arc forms. The current in this arc is limited by the overall capacitance of the helicopter.

Once the bonding connection has been made, the helicopter and the mesh suit are all at the same potential as the line itself. The electric field is still distorted, but now the concentrated areas are moved away from the repairman and the connection between the wire and the helicopter.

And regarding the birds, when have you ever seen a bird sitting on one of these high-tension transmission lines?