Birds
Introduction
Birds are a group of warm-blooded vertebrates constituting 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. Birds live worldwide and range in size from the 5.5 cm (2.2 in) bee hummingbird to the 2.8 m (9 ft 2 in) ostrich. There are about ten thousand living species, more than half of which are passerine, or "perching" birds.
Evolution and Classification
Birds are dinosaurs, specifically theropods, and are the only group of dinosaurs to have survived until the present day. The fossil record indicates that birds evolved from feathered ancestors within the theropod group, which are traditionally placed into the "avian dinosaurs". The classification of birds is a contentious issue, with new discoveries and theories often causing a reevaluation of relationships between various species.
Anatomy and Physiology
Birds exhibit a range of physical characteristics, many of which are adaptations tied to their environments and lifestyles. They have a lightweight skeleton achieved through a number of modifications, including the fusion and elimination of several bones. Birds also possess a highly efficient respiratory system to meet the high oxygen demand for flight. Their digestive and urinary systems are also uniquely adapted to their diet and mode of life.
Behavior
Birds exhibit a wide array of behaviors, including various methods of flight, breeding practices, and a diversity of feeding strategies. Many species undertake long distance annual migrations, and many more perform shorter irregular movements. Birds are social, communicating with visual signals, calls, and songs, and participating in social behaviors including cooperative breeding and hunting, flocking, and mobbing of predators.
Conservation
Many bird populations are declining worldwide due to habitat destruction, overhunting, pollution, and other anthropogenic factors. Conservation efforts are being made globally, with various organizations working to protect and restore bird habitats, enact laws that protect birds, and create programs that encourage sustainable practices.