There are a few different theories as to why cage birds sing. One popular theory is that they sing to express their emotions, similar to how humans sing when they're happy or sad. Another theory is that they sing to attract mates, since they can't do so in the wild. And finally, some people believe that cage birds sing out of boredom, since they don't have much else to do in their small cages. Whatever the reason, it's clear that cage birds enjoy singing, and it's one of the few things they can do to express themselves.