Why do flightless birds still have wings?

Some species of birds are flightless because they are too heavy, and/or their wings are too small.

Birds become flightless due to their environment. Flying requires a lot of energy, so if there is no need to fly, they won't. For example, lack of predators or lack of trees means these birds don't need to fly as much. Over time, they will lose their flight ability altogether.

For example, penguins live in Antarctica. There are no trees there to perch on, and there are also no land predators. Instead penguins became adapted to swimming in the seas where there is food. Their wings then became more like flippers for swimming, and they became fatter to keep warm.

Ostriches and emus live in grasslands where there are few trees, so the birds have to stick to the ground. If they don't want to be eaten by predators they have to develop another way to move, so ostriches and emus became adapted for running quickly rather than flying.

Many Flightless Birds belong to the group known as ratites. Other ratites include the Southern cassowary of Australia and New Guinea, the kiwi from New Zealand, the ostrich from Africa and the rhea from South America. The reason these birds are flightless is that ratites have wings but the bones in their chests do not have the capacity for flight muscles, which is what a bird needs to fly. Although the emu does not actually weigh as much as it appears (30-45kg), its wings are also too small by comparison to lift it.