Dear WW: As far as I can find out, mockingbirds do not eat seeds. I have a URL which says they will eat a variety of berries. A friend from South Carolina told me many years ago that they will eat raisins strung on a thread. I never had any luck with it, though. The recorded song on the URL is LOUSY.
http://birds.cornell.edu/BOW/NORMOC/

P.S. The only thing I have ever seen them eat is live insects.