Ask the Expert Sam Fried Sam Fried has seen and photographed almost all North American birds. Your question is short, but embraces the entirety of the evolutionary process.
Animals gradually change during the course of millions of years.
The climate, geography and geology also shift slowly over time. Some animals develop adaptations that particularly suit them for survival in their new environment, while others that do not adapt may not be able to survive. In nature, these changes occur very slowly, but humanity has caused many changes to happen at a much faster rate, thereby causing some species to adapt and expand while others to become extinct or greatly diminished. For example, Eastern and Western Screech-owls have a common ancestor, but there populations were gradually separated by geological and climatic changes in North America, leading to the evolution of two new species that were particularly well adapted for their environments. This resulted in the near extinction of some species like Kirtland's Warbler, which could not quickly adapt to this new threat to their breeding success.