Birds are bipedal, constant-temperature, egg-laying vertebrates, covered with feathers. Birds' feathers are divided into normal feathers (mainly used for flying) and down feathers (mainly used for keeping warm). The forelimbs evolved into wings with hard beaks (bird mouths). Birds vary in size, from tiny hummingbirds to giant ostriches and emus (a big flightless bird from Australia).
There are many kinds of birds, which are distributed all over the world and have diverse ecology. Now birds can be divided into three major families. Flat-chested suborder includes a bird that is good at walking but can't fly, such as ostrich. Penguins include a class of birds that are good at swimming and diving but can't fly, such as penguins. The order Convex Chest includes birds with developed wings that can fly, and most birds belong to this order.
Most birds can fly, but a few flat-breasted birds can't, especially the birds living on the island, which have basically lost their ability to fly. Birds that can't fly include penguins, ostriches, kiwis (a wingless bird from New Zealand) and extinct dodos. When humans or other mammals invade their habitats, these flightless birds will be more likely to become extinct, such as the fear birds in great auk and New Zealand.
There are many kinds of birds, second only to fish among vertebrates. At present, there are more than 9,000 known species of birds in the world, among which there are more than 1300 recorded in China alone, and many of them are endemic to China (see the list of endemic birds in China). About 120~ 130 species of birds have become extinct. Compared with other terrestrial vertebrates, birds have many unique physiological characteristics. These birds are quite different in size, shape, color and living habits. Among so many birds, the largest is the ostrich, which is the "giant" among birds. The African ostrich is 2.75 meters tall and weighs up to 165.5 kg. The smallest hummingbird in South America is only 50 mm long and weighs the same as a coin. Birds can fly, but not all birds can fly. For example, the ostrich's wings have degenerated, its sternum is small and flat, and there is no keel protrusion, so it can't fly. Penguins are specialized seabirds, whose wings have become fins and lost the ability to fly. Although some birds can fly, they don't fly very far. For example, a chicken has short wings and can't fly high, but it can fly at least tens of meters. However, domestic ducks have completely lost the ability to fly. Among the birds, the vulture flies the highest, which can fly over 9000 meters. The fastest flight is the goshawk, and the fastest speed of short-distance flight can reach more than 600 kilometers per hour. The longest flight distance is tern, which can fly from the South Pole to the far North Pole, with a journey of about 1.76 thousand kilometers. Birds have strong metabolism and digestion, so they eat more. Hummingbirds, for example, consume twice their body weight in a day. Some birds eat 10% ~ 30% of their body weight every day. Most birds are omnivorous and not very picky. Every spring and autumn, birds fly in groups in the sky, covering the sun. This seasonal phenomenon of changing habitats in different seasons, or moving from nesting sites to wintering sites, or returning from wintering sites to nesting sites is called bird migration. Every spring, birds begin a series of activities, such as courtship, reproduction, nesting, hatching and brooding.
Birds eat all kinds of food, including nectar, seeds, insects, fish, carrion or other birds. Most birds move during the day, and some birds (such as owls) move at night or at dusk. Many birds migrate long distances to find the best habitat (such as Arctic terns), and some birds spend most of their time at sea (such as albatrosses). Birds feed on beaks, which are generally long and narrow and have no teeth in their mouths.
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