If you want to know how song changes the shape of a finch's brain, science can help. If you want to know how learning a song alters genetic patterns, affects mate choice and ultimately influences populations, you can learn that too. But what if you want to know how a singing bird feels?

That, it turns out, is a scientifically uncertain and even controversial question. It's difficult to study animal emotions with formal rigor, and the notion that animals might have rich inner lives was disregarded for much of the 20th century. From the behaviorist perspective pioneered by psychologists like...