Every organism at one time or another has to learn to adapt to a new environment. This is why evolution of organism is needed in order to survive changes to its environment, such as the case of the flightless birds. For one reason or another over a period of 100, 000 years the need for wings was unnecessary to their survival. There could be many reasons why the birds evolved to be more adaptable to their environment. A small population of birds would have to replenish and start a new population. In all populations there are great reproductive potential to replace numbers and expand the number of population.
After their population grew there would be a leveling off of the population once it reached a stable size. The birds' population would become stable due a lack of food to support every bird in the population, along with predators helped to contribute to keeping the population stable. After being on the island a while the birds' population would start showing a variation in different traits. Such things as color, wing size and overall size may vary, due to predators and the terrain of the environment. Eventually the birds evolved to the point where the use of their wings was no longer needed, as a necessity to survive.
The adaptation could have been because flying itself took to much energy, so to conserve food they didn't fly too much. The bird's food also could have been close to the ground and there was no need to really fly. Another thing could have been that it was hard for predators to catch the birds if they stayed in think under brush or traveled along the ground. For whatever reason the birds didn't need their wings this trait was passed of from generation to generation. The reason this trait was passed on was because it helped the survival of these individual birds.
They were able to reproduce and pass these traits through heredity. lycos. com.