Scientists disagree about many aspects of the evolution of birds. Many paleontologists (scientists who study fossils to learn about prehistoric life) believe that birds evolved from small, predatory dinosaurs called theropods. These scientists say that many skeletal features of birds, such as light, hollow bones and a furculum, were present in theropod dinosaurs prior to the evolution of birds. Others, however, think that birds evolved from an earlier type of reptile called thecodonts-a group that ultimately gave rise to dinosaurs, crocodiles, and the flying reptiles known as pterosaurs. These scientists assert that similarities between birds and theropod dinosaurs are due to a phenomenon called convergent evolution-the evolution of similar traits among groups of organisms that are not necessarily related.

Scientists also disagree about how flight evolved. Some scientists believe that flight first occurred when the ancestors of birds climbed trees and glided down from branches. Others theorize that bird flight began from the ground up, when dinosaurs or reptiles ran along the ground and leaped into the air to catch insects or to avoid predators. Continued discovery and analysis of fossils will help clarify the origins of birds. Despite uncertainties about bird evolution, scientists do know that numerous types of birds lived during the Cretaceous Period, which dates to about 138 million to 65 million years ago. Among these birds were Ichthyornis victor, which resembled a gull and had vertebrae similar to those of a fish, and Hesperonis regalis, which was nearly wingless and had vertebrae like those of today's birds. Most birds of the Cretaceous Period are thought to have died out in the mass extinctions-deaths of large numbers of animal species-that took place at the end of the Cretaceous Period.

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During the Pleistocene Epoch, from 1.6 million to 10,000 years ago, also known as the Ice Age, glacier ice spread over more than one-fourth of the land surfaces of the earth. These glaciers isolated many groups of birds from other groups with which they had previously interbred. Scientists have long assumed that the resulting isolated breeding groups evolved into the species of birds that exist today. This assumption has been modified as a result of studies involving bird DNA within cellular components called mitochondria. Pairs of species that only recently diverged from a shared ancestry are expected to have more similar mitochondrial DNA than are pairs that diverged in the more distant past. Because mutations in mitochondrial DNA are thought to occur at a fixed rate, some scientists believe that this DNA can be interpreted as a molecular clock that reveals the approximate amount of time that has elapsed since two species diverged from one another. Studies of North American songbirds based on this approach suggest that only the earliest glaciers of the Pleistocene are likely to have played a role in shaping bird species.

The evolution of birds has not ended with the birds that we know today. Some bird species are dying out. In addition, the process of speciation-evolutionary changes that result in new species-continues all the time.