Birds have contributed to studies in genetics, behavior, migration, and evolution. Darwin’s finches helped formulate natural selection, and birds are key in studying animal behavior and the effects of hormones on physiology. Ecological studies have shown that bird biodiversity varies by region, with factors such as habitat diversity and migration routes influencing the number of species. For example, Texas and California, two of the largest U.S. states, have about 620 species each, while more than 800 species can be counted in just a small part of Costa Rica.