An individual bird cannot change the special structures that it is born with

North Island brown kiwi, close up of head and bill from 1974.

It takes thousands or millions of years for physical adaptations to become distinct within a species. Individuals cannot adapt in this way. For example, although a long beak is useful to exploit a good food source, individual birds do not suddenly grow long beaks. Assuming that the good food source remains constant, the birds with longer beaks in the population survive best over generations, and so beaks in general get longer.