Birds are a large group of animals that inhabit almost all environments on the planet. The abilities of some species to travel large distances allows them to exploit different habitats at different times, usually according to season. However, many species of birds are largely sedentary. Birds are found in both hemispheres, and can range from the South Polar pack ice to the Tropics. They can be a few cm long, such as the smallest hummingbird, to several metres tall, such as the largest living bird, the ostrich. The methods birds use to feed include herbivory, carnivory and omnivory. Although the diversity of birds is enormous, they all have these characteristics in common:

• Feathers, whichFeathers, which are modified scales (for insulation and flight); and a modified front pair of limbs (wings or flippers).
• An amniote egg which protects against water loss and provides the embryo with all its developmental necessities.
• A unique respiratory system in which large lungs supply oxygen during activity such as flight.
• Hollow bones which lower your weight, decreasing your energy needs during activity.
• To be warm-blooded (endothermic).
• To be a marine bird (eg terns, penguins and sea eagles) you need to spend some time in marine habitats, or depend on them for some resource (eg food).