Answer:
Natural selection.
Step-by-step explanation:
Imagine in the beginning there was a single species of bird. Naturally, this bird had to expand to the rest of the world, and was distanced as the continents drifted off. But this bird had to cope with various climates and environments. So the bird would gradually mutate over time. The bird in the jungles of central america would develop different colors and a shaped beak, and would better help it survive. This led to being the new normal for birds of this area. It would evolve to cover all the surviving birds in the region, until it is so drastically changed then it is a new species. There are so many different environments, each with its own challenges to overcome, so so many different bird evolve to survive them.