"There are six birds in the tree."
Why did I need 上 if they were in, and not specifically on top, of the tree?
I think that's just how it's expressed in Chinese. In the German language, we also say birds are "on" a tree. But it doesn't mean they're on top of it, just means they're "in" it, as you English speakers put it. ;)
Why doesn 在……上 not work here? Why cannot I use 在 anywhere in this sentence?