Complex sentence: During the Chinese Civil War, a large number of Chinese mainlanders migrated to Hong Kong as well as a surge in birth rates owing to the prospering economy, causing the population to dramatically increase from 2,015,300 in 1951 to 3,129,648 in 1961.
Split into simpler sentences: During the Chinese Civil War, a large number of Chinese mainlanders migrated to Hong Kong. There was at this time a surge in birth rates owing to the prospering economy, causing the population to dramatically increase from 2,015,300 in 1951 to 3,129,648 in 1961.