I cant find the original link to the Autoglym blog where it shows that the 'etching' is due to the paint cooling at different rates.
"Motor researchers have revealed that when the sun warms up the cars paint, it causes the lacquer to soften and expands, at the same time the bird droppings dry and harden on the surface. As soon as the paint lacquer cools down again it contracts and hardens itself around the texture of the bird dropping. Thus, causing a dulled or etched patch on the paintwork."
Anyhow, heat is the answer (like above re the sun). There are loads of threads about it, for example
https://www.detailingworld.co.uk/for...d.php?t=380428