Birds are warm-blooded vertebrates classified in the class Aves, characterized by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. They range widely in size from the tiny bee hummingbird to the large common ostrich, with over 11,000 living species split into 44 orders. Birds have wings—modified forelimbs—that enable most species to fly, although some have lost this ability through evolution. Their digestive and respiratory systems are uniquely adapted for flight, and many aquatic birds have evolved adaptations for swimming as well.
Birds are descendants of theropod dinosaurs, making them the only living dinosaurs. Modern birds evolved around the Late Cretaceous period and diversified after the mass extinction event 66 million years ago. The group is divided mainly into two subclasses: Palaeognathae (flightless birds like ostriches and the weak-flying tinamous) and Neognathae (all other birds, including perching birds). Birds engage in various social behaviors, communicate through songs and calls, and exhibit diverse breeding systems. They produce offspring by laying fertilized eggs and generally provide extended parental care.
Birds’ roles extend to ecological, economic, and cultural importance: they are sources of food (eggs, meat), raw materials (feathers), and fertilization (guano), as well as popular pets. Humans impact many bird species through habitat change and other activities, with around 120-130 species having gone extinct due to human activity since the 17th century, and about 1,200 species currently threatened with extinction.
For those interested in birdwatching and specific bird species, resources like the Cornell Lab of Ornithology’s "All About Birds" provide extensive guides on bird identification, behavior, sounds, and conservation, including detailed regional field guides for areas like California and the southeastern United States.
This summary offers a comprehensive overview of what birds are, their evolutionary background, diversity, characteristics, behavior, and significance. Further detailed study can be accessed through specialized ornithological resources.