All birds lay eggs and most can fly.
There are about 8,600 different species of birds alive today. Harder - Birds make up the scientific class Aves. All birds today have descended from their flying ancestors, but a few such as ostriches, emus, some grebes, and cormorants have lost their capacity for aerial flight. Others, such as penguins, have become adapted to flying in a much denser medium, water. Birds are found in all habitats, from the icy shores of Antarctica to the hottest parts of the tropics, and from mountains, deserts, plains, and forests to open oceans and densely urbanized areas.