Stranger things

Travelling home today, and my first walk was through the garden to see what has changed. I spotted a strange leaf on the birch tree!
Not sure I should remove these visitors in case they eat all the leaves of my new tree that is still settling into the garden and suffering from the drought or if it can deal with this!
They stopped munching as soon as I approached and made this formation, keeping perfectly still. They didn’t want to be caught out!