Why do birds not get electrocuted when they sit on telegraph poles?

The danger to you ... or to the bird, or a squirrel on the line ... has really nothing

to do with the potential on the line, or its voltage with respect to ground. Next

time there is a line crew in a bucket truck in your neighborhood, you may see a

lineman go up in the bucket, connect the bucket to the line, and then proceed to

work on the line with his hands.

The danger is the result of current through the body, and to accomplish that,

two points of the body have to be in contact with two points at different potential,

i.e. with voltage between them. That's why you don't want to touch a live circuit

and a grounded point at the same time.

If the lineman's bucket is well insulated from the boom and the rest of the truck,

then once the bucket is clamped to the line, the lineman's body is at the same

potential as the line, and he has no problem handling it. The bird is only touching

the line, nothing else, and there's no voltage difference between two parts of

the bird that can drive current through him.

The squirrel is OK if he jumps from one line to another, but he may be in trouble if he

stands on one line (neutral or primary e.g.) and reaches across to another one.