Some scientists believe that there were huge fires and the skies were dark and full of pollution due to a meteor exploding and killing off the dinosaurs, and if it's true that this occurred, the Ostrich evolved to be a land animal due to it being too dangerous in the air for them. The long neck may have developed to help them get food that was higher up which is the same speculation for the Giraffe. To stay on land they had to be stronger, bigger, and faster than they were in flight.
