If you're interested:
"Why the Mockingbird Sings : And Why at Night, When Most Birds Sleep?"
http://articles.latimes.com/1987-01-25/magazine/tm-5613_1_mockingbird-sings
Recording of same. May not be exact since they mimic different birds and other things in different areas:
http://www.youtube.com/watch?v=5pUxOaN_rec