(Changed for 1.8.0)
Income comes from three sources: Trade, Population ( taxes on both Imperial Citizens and Civilians ) and Interest:
Income = (Imperial Population/70000 + Civilian Population/94275 + TradeOutput ) * TRADEVALUE + 1%*Savings Morrigi get only 80% of Income from Imperials, and 59% (?) from Civilians.
也就是说鸟族靠人口产钱真是不行的