Why does the author describe them this way?

On summer evenings at Bracken Cave, near San Antonio, a dark cloud rises out of the ground. It spirals upward and stretches across the sky. The cloud is made of bats: millions of Mexican free-tailed bats leaving the cave to feed. Bracken Cave is home to one of the largest bat colonies in the world.

Mexican free-tailed bats spend the winter in Mexico and fly north to Texas in the spring. The females gather in caves to give birth, usually to a single pup each. In the fall, the bats return south. Texas has even named the Mexican free-tailed bat its official state flying mammal.

These bats hunt at night using echolocation. A bat makes high-pitched calls, most of them too high for people to hear, and listens for the echoes that bounce back from objects around it. From those echoes, it can tell where an insect is and how it is moving, even in complete darkness. Free-tailed bats often fly high above the ground and catch insects in midair.

Those insects matter to farmers. Many of the moths the bats eat lay their eggs on crops such as corn and cotton, and the caterpillars that hatch feed on the plants. A colony of millions of bats eats an enormous number of insects in a single night. Scientists who study bats say that by eating these pests, bats protect crops and save farmers money they would otherwise spend on pesticides.

Bats live in cities, too. In Austin, more than a million Mexican free-tailed bats spend the summer in the narrow gaps under the Congress Avenue Bridge. When the bridge was rebuilt in 1980, its new design accidentally created perfect places for bats to roost. At first, many residents were afraid and wanted the bats removed. Over time, as people learned that the bats would not bother them if they were left alone, attitudes changed. Today, crowds gather on summer evenings to watch the bats fly out, although visitors are still warned never to touch a bat.

Bats face real threats, including the loss of the caves and other places where they live. Protecting those places means protecting animals that work for Texas every night, mostly without anyone noticing.