Define birds. Give two examples of flightless birds.

Birds (Class Aves) are warm-blooded, egg-laying vertebrates characterized by:

Defining features:

• Feathers - Unique to birds, providing insulation and enabling flight
• Wings - Modified forelimbs (though not all birds fly)
• Beaks/Bills - Without teeth
• Four-chambered heart - Efficient circulatory system
• Hollow bones - Lightweight skeleton adapted for flight
• Oviparous - Lay hard-shelled eggs
• High metabolism - Maintaining constant body temperature (endothermic)

Two examples of flightless birds:

1. Ostrich (Struthio camelus): The world's largest living bird, native to Africa, with powerful legs for running at speeds up to 70 km/h
2. Penguin (Family Spheniscidae): Aquatic birds of the Southern Hemisphere with wings modified into flippers for swimming, e.g., Emperor penguin (Aptenodytes forsteri)

Other flightless birds: Emu, kiwi, cassowary, and the extinct dodo. Flightlessness typically evolved in environments lacking terrestrial predators or where other adaptations (swimming, running) proved more advantageous