Table schema:
CREATE TABLE table_name_14 (births__000s_ INTEGER, total_fertility_rate VARCHAR, natural_growth VARCHAR)

Question: 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