High voltage itself is of little consequence so far as there is no flow of electricity from one point to another. Instead of high voltage it is the high voltage difference that matters. When a bird, hopefully a small bird, lands on a transmission cable it acquires the same voltage as the cable, so there is no potential difference. Even a large bird can sit there safe and relaxed. But when it spreads wings and touches something like another cable or some point connected to earth, electricity would flow through its body due to voltage difference and the bird would be electrocuted instantly. It is for this reason that electricity companies maintain adequate spacing between two cables. Even so, large birds of prey do sometimes fall prey to electrocution.
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