Alright, sunlight doesn't incinerate trolls, but it does threaten them.
Except in winter.
I like that effect on the last panel! The sun sickness is setting in! :O
Ah, so this is the side effect of heat on Trolls, nice details!
Indeed, trolls come from northern lands, temperate climates are hard on them.
They can manage it, but it involves seeking out coolness, water and shade. So yeah, they do hang out under bridges, when available.