I know why the caged bird owner sings

There is something awfully endearing about an animal in a cage. Not so much a tightly packed cage with no room to roam, but a spacious and comfortable enclosed area. In this way, the animal is like a toy that can be played with. It is especially endearing when one toy interacts with another toy of a different type. In the wild, though, an animal is observed. It because a visual phenomenon - one that can be appreciated for its beauty, but not as fun to play with as a toy.