Birds (Class Aves) arose in the fossil record during the Mesozoic era, about 150 to 200 million years ago. The reptilian origin of birds is evident in similarities in their skeleton; a single occipital condyle, single ear bone, and a lower jaw composed of five fused bones. Today, birds can be seen in a variety of sizes; from the small ruby-throated hummingbird, northern cardinal and the yellow-billed magpie to the large golden eagle, great blue heron and the griffon vulture. All modern birds have feathers, no true teeth, are bipedal, have pneumatic bones (bones that are hollow) and nearly all have forelimbs specialized for flight. Ornithologists believe there are about 10,000 bird species in the world with new species discovered all the time. The migratory nature of birds is evident in the various landscapes they can be found; from arid deserts and mountain tops to arctic ice fields and in your own backyard.

The early 20th century saw a sharp decline in bird populations due to the widespread use of DDT, a chemical insecticide. Environmental scientist Rachel Carson meticulously documented the effects its use in Silent Spring (1962), which led to public outcry, helped set the stage for DDT's ban in 1972, and popularized the modern environmental movement. Further scientific study confirmed that birds, fish and other forms of wildlife were killed by either directly ingesting the chemical poison, by eating poisoned insects or their predators, or, in the case of certain birds, by egg-shell thinning that prevented chicks from hatching. Since DDT's ban, some bird species (notably the bald eagle and peregrine falcon) have recovered. This kind of chemical, however, persists in the environment, and continues to poison insects and birds.

Current threats to bird populations include climate change, chemicals like pesticides and herbicides, low output of offspring, poaching, lead poisoning, habitat destruction, and consuming plastic or trash as food.

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