About birds.

Birds are a class of two-legged, warm-blooded vertebrates with beaks, wings and feathers that lay eggs. Most birds can fly, but not all. There are currently between 9000 and 10 000 different bird species in the world, depending on how you count, making them the most diverse land vertebrates. They inhabit ecosystems across the globe, from the Arctic to Antarctica. Birds range in size from 5 inches high hummingbird to 2.7 meters big ostrich. Fossil finds suggest that birds evolved from dinosaurs during the Jurassic period, about 150-200 million years ago, and the earliest known bird is Archaeopteryx from the Late Jurassic, for around 155-150 million years ago. Most paleontologists regard birds as the only species of dinosaurs that survived the Cretaceous / Tertiary extinction event for approximately 65.5 million years ago.
What distinguishes birds from all other animals is their plumage. Characteristic of modern birds are also advised to have a beak without teeth, they lay eggs with hard shells, has a high metabolism, a heart with four chambers and a lightweight but strong skeleton. All birds have front limbs evolved into wings and most can fly, with some exceptions including ratites, penguins, and a number of endemic species on islands. Birds also have unique digestive and respiratory systems that are highly adapted to flight. Some birds, especially corvids and parrots, are among the most intelligent of animals. A number of bird species have been observed making and using tools, and many social species exhibit cultural transmission of knowledge across generations.
Many species carry out long distance migrations, and many perform shorter irregular movements. Normally, it is estimated that an adult bird energetically three annual main items: the breeding, moulting and migration. For resident birds are wintering in cold or extremely dry climate one of the main items instead of moving. Birds are social, they communicate using visual signals and through singing, and participating in social behaviors that cooperative breeding and hunting, flocking, and mobbing against predators. The vast majority of bird species are socially monogamous, usually for one breeding season at a time, sometimes for years, but rarely for life. Other species have breeding systems that are polygynous ("many females") or, rarely, polyandrous ("many males"). Eggs are usually in a nest and incubated by the parents. Most birds have a long period after hatching when the parents take care of the kids.
Many species of economic importance, mostly as sources of food obtained by hunting or farming. Some species, particularly songbirds and parrots, are popular as pets.Another use is the harvesting of guano (droppings) for use as fertilizer. Birds have prominent roles in all aspects of human culture from religion to poetry to popular music.About 120-130 species have become extinct as a result of human activity since the 1600s, and hundreds more before then. There are currently about 1200 bird species threatened with extinction by human activity, although efforts are made to protect them.