Bird
Classification and Evolution
Birds, or Aves, are a group of endothermic vertebrates constituting the class Aves. They are 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 cm (2 in) bee hummingbird to the 2.75 m (9 ft) ostrich. There are about ten thousand living species, more than half of which are passerine, or "perching" birds.
Birds have wings whose development varies according to species; the only known groups without wings are the extinct moa and elephant birds. Wings, which evolved from forelimbs, gave birds the ability to fly, although further evolution has led to the loss of flight in some birds, including ratites, penguins, and diverse endemic island species. The digestive and respiratory systems of birds are also uniquely adapted for flight.
Anatomy and Physiology
Bird anatomy, or the physiological structure of birds' bodies, shows many unique adaptations, mostly aiding flight. Birds have a light skeletal system and light but powerful musculature which, along with circulatory and respiratory systems capable of very high metabolic rates and oxygen supply, permit the bird to fly. The development of a beak has led to evolution of a specially adapted digestive system.
Birds also have more cervical (neck) vertebrae than many other animals; most have a highly flexible neck consisting of 13-25 vertebrae. Birds are the only vertebrate animals to have a fused collarbone (the furcula or wishbone) or a keeled sternum or breastbone. The keel of the sternum serves as an attachment site for the muscles used for flight, or similarly for swimming in penguins.
Behavior and Ecology
Bird behavior refers to the actions of a bird in response to environmental situations. Some bird behavior is instinctive, whilst other behavior is learned. Birds that are closely related may exhibit similar behaviors. Understanding bird behavior is an important and often overlooked aspect of birding. Birds have a range of behaviors including foraging, breeding, navigation, migration, social behavior and grooming.
Bird ecology is the relationship between birds and their environments. This includes the interactions of birds with both biotic (living) and abiotic (non-living) components of their environments. These interactions can affect bird behavior, population dynamics, community structure, and the distribution and abundance of species.
Conservation
Bird conservation is a field in the science of conservation biology related to threatened birds. Humans have had a profound effect on many bird species. Over one hundred species have gone extinct in historical times, although the most dramatic human-caused extinctions occurred in the Pacific Ocean as humans colonised the islands of Melanesia, Polynesia and Micronesia, during which an estimated 750-1800 species of bird became extinct.
Conservation efforts have led to noticeable improvements for several species of birds that were previously threatened or endangered. This includes species such as the peregrine falcon, bald eagle and whooping crane.