Earth Science: Vertebrates – Aves
The ability to fly sets birds apart from other vertebrates. They are remarkably adapted for flight and display impressive capabilities in the air, often flying at speeds of 15-50 mph (24-80 km/h)
Birds are distiguished by being the only vertebrates with feathers. Furthermore, not only can birds fly, but they also have excellent vision. Sight is especially highly developed in birds of prey, which must spot small animals from great heights.
Beaks come in many forms and are adapted to eating habits. Raptors, for example, have sharp-edged beaks, woodpeckers have heavily reinforced beaks, and hummingbirds have tube- shaped beaks. Birds are warm-blooded with efficient hearts, lungs, and circulation. Their feathers reflect the lifestyle and behavior of the bird. In cold climates, birds have thick layers of feathers and insulating down.
Many male birds have colorful plum-age to attract mates, whereas those with dull-colored feathers use them as camouflage. When food supplies dwindle in the fall, many birds migrate to warmer areas, with some species traveling up to 6,200 miles (10,000 km) to reach their winter homes.
About 8,000 species of birds are known to exist. A few, such as the African ostrich and the New Zealand kiwi, have lost the ability to fly. About 60 percent of all birds are passeriformes, or perching birds, including sparrows, swallows, and finches.
Mating and reproduction
Birds are territorial. They maintain and defend an area to mate and nest. Songbirds usually attract mates with vocal signals while others use visual signals, such as attractive plumage. Some birds, such as raptors, owls, nuthatches, and penguins, mate for life, whereas others have partner relationships only for the period of egg incubation.
Some species use a nest for multiple years and can make them highly elaborate. A few-such as Antarctic penguins, which incubate their eggs standing up-do not build nests at all. All birds lay eggs but the number varies from one to about 20. Most eggs are white, though brown, spotted, and pastel colors are not unusual.
Parent birds take turns sitting on the eggs to maintain a warm temperature for the incubation period (11 days to 20 weeks). Once hatched, many chicks are fed by the parents until they learn to fly. Parasitic breeders (such as the cuckoo) lay their eggs in the nests of other birds, who consequently become unwitting foster parents.
FLYING
Humans have been intent upon imitating the flight of birds for centuries, but have been unable to duplicate their efficiency. Birds use their powerful chest muscles to flap their wings to gain aerodynamic lift and hold their wings out to ride air currents.
Birds weigh very little for their size-most weighing no more than 33 pounds (15 kg)—due to their extremely light, hollow bones, which also play a role in breathing.