What Makes An Animal Smart?

Many birds are capable of complex problem solving, and even language, to a degree that seems too advanced if we just look at brain size. After all, a crow brain and a chimp brain aren’t the same size, yet some birds and great apes have been documented performing cognitive tasks at similar levels. So how do birds match wits with animals whose brains are tens of times bigger than theirs? And what does it even mean to be “intelligent?”