What are the reasons for the manifestation of the flight instinct in birds?

Some birds have an instinct to fly. These are some species that have all the adaptations for a long flight. Basically, small birds cannot be in flight for a long time, since they simply do not have enough energy to fly. But some species can. Therefore, they are able to change their location at a time of unfavorable natural conditions. They go to warmer places and come back at a certain moment. This has been developed in birds for a long evolutionary period.