I live across the street from a park too and I can relate to the "shifts." My park has the toddler shift in the morning, school aged kids in the late afternoon (by far the noisiest crew!), and the teens after dinner. The teens usually are listening to music at the picnic table or on the swings, just like you mentioned. I had never thought of it as "shifts", but that is such a great word. I don't spend much time at the park anymore, but maybe I will change that! : )