The heating comes from having a resistance greater than zero and having an electrical current pass through it (I²R). By pushing more current through the same cross-section, the ohmic heating increases.
If the conductor maintains the same conductivity at higher temperature, then it's no less efficient than using several conductors to keep the temperature low. However, metals tend to exhibit decreased conductivity; i.e. increased resistance with higher temperature. So the specific losses increase. It doesn't usually matter in small quantities but when you're talking about thousands of gigawatt-hours being transmitted every day, and 10's of them not being available at the billable-consumer end, then minimization of those losses becomes important to be competitive.
The additional insolation (incident sunlight) heating pushes from 80⁰C up to a maximum allowed of 120⁰C. Hang some metal in the summer sun, away from a convective breeze and see how much it heats up all by itself.
Doing a quick bit of Googling it does look like these high temperature power lines do exist to a limited extent in Germany. This news story (date not given, but must be 2012 at the earliest) from RWE's website talks about the first ever high temperature power line, of about 12km length, to be installed in Germany:
The operating temperature is given as 175ºC rather than 210ºC.
With normal temperature power lines birds have no trouble, as far as I'm aware,in perching on them and may even be attracted by the warmth. It would be interesting to see the reaction of the German green movement if birds tend to get injured in trying to perch on these power lines that are operating at oven-like temperatures.