Answer
The birds that will not follow migration tend to save a lot of energy as well as time and at the same time obtain food without much efforts. With time, these birds might lose their migratory nature, the genes of which would be passed on to their offspring.
Work Step by Step
Few bird populations not follow migration if they get a continuous supply of food. The hypothesis that explain this unusual behavior can be that this gives them maximum fitness. People feed birds and this provides a continuous supply of food for them.
The natural behavior of migration tends to get lost when the birds get ample amount of food for survival. Thus, they remain in the place where people feed them. The birds get benefit out of this and become fit.
