Why do power lines sag more in summer?

Answer: Heat makes the metal wires expand and lengthen, so they hang lower.

Hot ground makes the poles lean apart, loosening the wires. — Hot ground can soften soil and let poles move slightly, but that is not the usual cause. Poles are set deep and kept upright. The clear summer change is the metal wire lengthening.

Heat makes the metal wires expand and lengthen, so they hang lower. ✓ — Heat makes the metal in the wire expand, and the wire becomes longer. The poles stay roughly in place, so the extra length has to hang down as sag. Aluminum and copper both expand as they warm, which is why the same line sits lower on a hot day than a cold one. Engineers leave some slack so winter contraction does not snap the wire, and in extreme heat the low wire can touch trees or the ground.

Birds sitting on the wires add weight that pulls them down. — Birds do add a little load, but not enough to change sag noticeably. The wires are under high tension and weigh far more than a few birds. The summer change comes mainly from the wire itself getting longer.