Birds are defined by several key characteristics that distinguish them from other animals:
- Feathers : Birds are the only living creatures with feathers, which are adaptations of reptilian scales. Feathers serve multiple functions including flight, insulation to regulate body temperature, protection, mate attraction, and species identification
- Warm-blooded vertebrates : Birds maintain a constant internal body temperature regardless of the environment, allowing them to sustain the high energy levels needed for flight. This endothermy differentiates them from cold-blooded reptiles and amphibians
- Hard-shelled eggs : Birds lay eggs with hard shells, which provide protection and support embryo development outside the mother's body
- Hollow bones : Their bones are lightweight and hollow, reducing body weight to facilitate flight
- Beaked jaws without teeth : Birds have toothless beaks made of keratin, adapted to their feeding habits
- Wings : Most birds have wings adapted for flight, although some species like penguins use wings for swimming, and others like ostriches have wings but do not fly
- Specialized anatomy and physiology : Birds have highly efficient respiratory and circulatory systems, strong flight muscles, excellent eyesight, and a sense of balance, all supporting flight and complex behaviors such as long-distance migration and vocal communication
In summary, a bird is a warm-blooded vertebrate characterized primarily by feathers, hard-shelled eggs, hollow bones, and adaptations for flight, including wings and specialized physiology
