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A Bird, warm-blooded, egg-laying, vertebrate animal having its body
covered with feathers and its forelimbs modified into wings,
which are used by most birds for flight.

Birds compose the class Aves.

There are an estimated 9,000 living species.



History of Birds

Birds are believed to be extant members of a group of dinosaurs
called maniraptors (other maniraptors include Velociraptor and
Oviraptor). They share with dinosaurs such characteristics as a
foot with three primary toes and one accessory toe held high in
back. Early avians include such primitive birds as
Arachaeopteryx, the rooster-sized Patagopteryx, and the
ichthyornithiforms, skillful flyers with toothed beaks. The
fossil remains of the Archaeopteryx, which date to the Jurassic
period, show reptilian tails, jaws with teeth, and clawed wings,
but feathers were well developed. Pterosaurs, another group of
flying reptiles, did not share the common characteristics of
birds and dinosaurs and are not considered birds. Whether the
capacity for flight arose in tree-living dinosaurs that glided
from branch to branch (the “trees-down” hypothesis) or in
fast-running terrestrial dinosaurs (the “ground-up” hypothesis)
continues to be debated. Indeed, the inclusion of birds in the
dinosaur family tree, although accepted by most paleontologists,
is debated by some, and the identification (2000) of the oldest
known feathers on 220-million-year-old, four-legged reptile
fossil, Longisquama insignis, raised questions concerning the
theory.


See: Prehistoric Birds


The Value of Birds

Birds are of enormous value to humanity because of their
destruction of insect pests and weed seeds. Many are useful as
scavengers. The game birds hunted for food and sport include
grouse, pheasant, quail, duck, and plover. The chief domestic
birds are the chicken (see poultry), duck, goose, turkey, and
guinea fowl. Parrots and many members of the finch family are
kept as pets.


Characteristic Features and Behaviors of Birds

Like mammals, they have a four-chambered heart, and there is a
complete separation of oxygenated and deoxygenated blood. The
body temperature is from 2° to 14° higher than that of mammals.

Birds have a relatively large brain, keen sight, and acute
hearing, but little sense of smell. Birds are highly adapted for
flight. Their structure combines lightness and strength. Body
weight is reduced by the presence of a horny bill instead of
heavy jaws and teeth and by the air sacs in the hollow bones as
well as in other parts of the body. Compactness and firmness are
achieved by the fusion of bones in the pelvic region and in other
parts of the skeleton. The heavier parts of the body—the gizzard,
intestines, flight muscles, and thigh muscles—are all
strategically located for maintaining balance in flight.


See how Birds Fly


Feathers, despite their lightness, are highly protective against
cold and wet. The flight feathers, especially, have great
strength. Feathers are renewed in the process of molting. Some
birds, such as the ostrich, the penguin, and the kiwi, lack the
power of flight and have a flat sternum, or breastbone, without
the prominent keel to which the well-developed flight muscles of
other birds are attached. The bills of birds are well adapted to
their food habits. Specialized bills are found in the crossbill,
hummingbird, spoonbill, pelican, and woodpecker.



Bird Courtship and Gender

In the majority of species there are differences between male and
female in plumage coloring. In these birds the male (except in
the phalarope) is usually the more brilliant or the more
distinctly marked and is the aggressor in courtship. Unusual
courtship displays are performed by several species, particularly
by the ruffed grouse, the bird of paradise, the crane, the
pheasant, and the peafowl. Birdsong reaches its highest
development during the breeding season, and singing ability is
usually either restricted to or superior in the male. Most birds
build a nest in which to lay their eggs. Some birds, such as the
oriole, weave an intricate structure, while others lay their eggs
directly on the ground or among a few seemingly carelessly
assembled twigs. Eggs vary in size, number, color, and shape. In
spring and fall many birds migrate. Not all of the factors
motivating this behavior are fully understood. These trips often
involve flights of hundreds and even thousands of miles over
mountains and oceans.