The western United States contains a series of tall mountain ranges that also extend far down into Mexico. The largest of these ranges are the Rockies and the Sierra Madres, but they are all part of the same mountain-building event that occurred many millions of years ago, and they all have similar features. However, the biodiversity of the southernmost mountains has more connection to that of Central America, while the biodiversity of the Rockies shares more affinity with temperate North American species. Still, much of the forest vegetation on these mountaintops is the same, or very similar, and they are often referred to as the "Sky Islands" because of their similarity to each other and their isolation from the lower, often drier, areas surrounding them. With global climate change producing increasingly visible effects, conservation biologists fear that these mountaintop ecosystems will undergo major changes in temperature and humidity and that the many endemic species that live there may literally have no place to go.There are groups of tropical birds, such as trogons and parrots that are usually considered to live in Central America and the mountains of southern and central Mexico. Small populations of these birds migrate into the United States each year. It is probably true that ________.

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