CREATE TABLE table_name_14 (births__000s_ INTEGER, total_fertility_rate VARCHAR(255), natural_growth VARCHAR(255))
What is the average birth rate that has a total fertility rate of na, and a natural growth smaller than 2.5?
    
SQL: SELECT AVG(births__000s_) FROM table_name_14 WHERE total_fertility_rate = "na" AND natural_growth < 2.5