ABSTRACT
It is generally accepted that birds descended directly from dinosaurs, although there is some debate about their precise placement on phylogenetic trees with some evidence suggesting that they are best considered a subgroup of reptiles instead of a class unto themselves. Regardless of those details, birds clearly share distinct characteristics with their reptilian ancestors such as the laying of amniote eggs, the presence of claws, and scales. In contrast to those similarities, birds evolved further to develop feathers, changes in bone structure and the respiratory system to enable flight, became endothermic, and replaced teeth with a hard beak. Subsequent to these early evolutionary steps, birds continued to develop traits that have enabled them to occupy a variety of niches today.
