You might find that some flightless birds cannot fly because they used to live in places where there were no predators to hunt them. They evolved to have no wings. For example, the dodo lived on an island with no predators so it didn't need wings to escape from them.
One of the most fascinating flightless birds include the cassowary who lays green eggs and is the closest relative to dinosaurs we know today. It is also known as the most dangerous bird alive on the planet. Another interesting flightless bird is the emu. They blink sideways and their feathers are extremely dense and look like fur.
My personal favourite is the penguin. They are amazing swimmers and huddle together to keep warm. The parents take shifts to incubate the egg. The chick hatches with fluffy down instead of feathers but as they grow up the fluff will change into water-proof feathers. When the parents come back from hunting, they must learn to recognise their chicks' calls so they can find them in the colony.