The birds sitting on an overhead transmission line suffer no harmful effects because:

Explanation

Birds perched on an overhead transmission line are not harmed because both of their feet are at the same electrical potential, eliminating the potential difference required for current to flow through their bodies. Hence, no current passes through them. Option A is incorrect because the resistance of the bird's body is not the determining factor. Option B is incorrect because the bird's feet are not insulators, but rather, they maintain the same potential on a single wire. Option D is incorrect as transmission lines are not typically insulated, but rather, the lack of potential difference keeps the birds safe.