> This theory posits that the formic acid produced by ants might act as a sort of stimulant for birds, and might even be addictive.

I like that idea. I also remember growing up we used to put straw on top of ant hills waiting for ants to "attack it" with formic acid, then we'd lick the straw. It was a fun sour taste. Since birds groom their feathers, maybe they just enjoy it like kids might enjoy sour candy.