Population shifts will continue adding House seats to Sun Belt states such as California and Texas while districts now represented by blacks and Democrats suffer declines, a private study said. California could pick up as many as six House seats, with Texas adding four and Florida three, Election Data Services said, while New York could lose three seats and two apiece could be lost by Illinois, Michigan, Ohio and Pennsylvania. Kimball Brace, president of the political consultant group Election Data Services, said the shifts were based on 1990 population estimates prepared by Temple University. They update estimates released just two weeks ago, which showed slightly less dramatic changes based on 1987 population estimates. Of the 50 districts with the largest population losses since the 1980 census, 44 are represented by Democrats, Brace said.