Why does the Cag-ed Bird Sing?

Here are some more pics from the China. The one with the bird cage was kind of interesting. I got lost in some byways in the Xi'an, which is an easy thing to do, and there on a brick wall was this cage--no one around. And as I stood and looked on, the bird broke into song, and went on and on. I stood and watched it for about 5 minutes, then walked away--with the bird continuing happily, or at least, loudly. So, to answer my own question, I guess it sings only for me.