Mockingbirds are extremely versatile singers, and not only do they mimic the songs of other birds, but also animals such as frogs and crickets as well as sounds that come from machinery. Each mockingbird's song is different, and may contain some or all of these elements and may also include some other improvised sounds. Their songs may also last for quite a long time. As Jim mentions, female mockingbirds are attracted to males with the most versatile songs. Terry probably gets all the girls.
Of course, none of that is what this strip is really about, but I figured the real message was obvious.