Birds are unharmed on high voltage lines, because they touch only one wire and don't experience voltage as the difference of potentials. However, birds also seem quite comfortable there. Could this be, because the wire is warmer than the air? The wire obviously has some resistance and the current generates some heat, so the wire is indeed warmer than the air. However, is it warmer enough for the birds to feel (say, by a degree)?
I hope someone with a knowledge of the typical magnitude of the current and wire cross section in high voltage lines (likely aluminum) would post an estimate of how much warmer the wire may be compared to the air around the freezing point.