Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
What is the average birth rate that has a total fertility rate of na, and a natural growth smaller than 2.5?

### Inputs:
table_name: table, columns: ['Year', 'Births (000s)', 'Deaths', 'Natural Growth', 'Total Fertility Rate']

### Response:
SELECT AVG Births (000s) FROM table WHERE Total Fertility Rate = na AND Natural Growth < 2.5