In substitution type of questions, some particular objects are assigned code names. Then a question is asked that is to be answered in the code language.
If cloud is called white, white is called rain, rain is called green, green is called air, air is called blue and blue is called water, where will the birds fly ?
Clearly, the birds fly in the 'air' and 'air' is called 'blue'.
so, the birds fly in the 'blue'.