Birds are
two legged, warm-blooded,
egg laying, vertebrate
(have a backbone), animals characterized primarily by feathers, forelimbs modified as wings,
and hollow bones.
Birds range in size from the tiny hummingbirds to the huge Ostrich and Emu.
Depending on the taxonomic
viewpoint, there are about 8,800–10,200 living bird species (and
about 120–130 that have become extinct in the span of human
history) in the world, making
them the most diverse class of terrestrial
(land) vertebrates.
Birds feed on nectar, plants, seeds, insects, fish, mammals, carrion, or other birds.
Most birds are diurnal, or active during the day, but some birds, such as
the owls
and nightjars, are nocturnal
(active at night) or crepuscular (active during twilight hours), and many coastal waders feed when the tides are appropriate, by day or night.
Many birds
migrate long distances to
utilize optimum habitats (e.g.,
Arctic Tern) while others spend almost all their time at sea
(e.g. the
Wandering Albatross). Some, such as
Common Swifts, stay aloft for days at a time, even sleeping
on the wing.
Common characteristics of birds include a bony beak
with no teeth, the laying of hard-shelled eggs, high metabolic rate, a 4-chambered heart, and a light but strong skeleton. Most birds are characterized by flight, though the ratites are flightless, and several other species,
particularly on islands, have also lost this ability. Flightless
birds include the penguins, ostrich, kiwi,
and the extinct Dodo.
Flightless species are vulnerable to extinction when humans or
the mammals they introduce arrive in their habitat. The Great Auk, flightless rails, and the moa
of New Zealand, for example, all became extinct due to human
influence.
Birds are among the most extensively studied of all animal
groups. Hundreds of academic journals and thousands of
scientists are devoted to bird research, while amateur
enthusiasts (called birdwatchers or, more commonly, birders) probably number in the millions.
Boxes may either have an entrance
hole or be open fronted. Some nest boxes can be highly decorated
and complex. Designs mimicking human house design are common in
some large gardens.
The size of a nest box should be suited for the animal for
which it is intended. Quite large boxes are used for ducks, owls
and other raptors.
Nest boxes for wild birds should be cleaned annually to remove
old nests and kill parasites.
Eggs
All birds lay eggs with hard shells made mostly of calcium carbonate.
Most passerines, sometimes known as
perching birds or, less accurately, as songbirds, lay
colored eggs.
Non-passerines typically have white eggs.
The brown or red protoporphyrin markings on passerine eggs reduce brittleness
and are a substitute for calcium when that element is in short
supply. The color of individual eggs is genetically influenced,
and appears to be inherited through the mother.
The eggs are laid in a nest,
which may be anything from a bare cliff ledge or ground scrape
to elaborately decorated structures such as those of the oropendolas.
Social systems and parental care
The three mating systems that predominate among birds are
polyandry, polygyny, and monogamy.
Polyandry is a type of breeding adaptation in which one
female mates with many males, and is seen in less than 1% of
birds.
Polygyny is when one male mates with many females
and constitutes 2% of all birds.
Monogamy is the custom or condition of having only one mate
during a period of time
Monogamy is seen in approximately 91% of all bird species and
is the custom or condition of having only one mate during the
breeding season. In some cases,
the individuals may pair for life.
One reason for the high rate of monogamy among birds is the
fact that male birds are just as adept at parental care as
females. In most groups of animals, male parental care is rare,
but in birds it is quite common; in fact, it is more extensive
in birds than in any other vertebrate class. In birds, male care
can be seen as important or essential to female fitness. "In one
form of monogamy such as with obligate monogamy a female cannot
rear a litter without the aid of a male".
The parental behavior most closely associated with monogamy
is male incubation. Interestingly, male incubation is the most
confining male parental behavior. It takes time and also may
require physiological changes that interfere with continued
mating. This extreme loss of mating opportunities leads to a
reduction in reproductive success among incubating males. "This
information then suggests that sexual selection may be less
intense in taxa where males incubate, hypothetically because
males allocate more effort to parental care and less to mating".
In other words, in bird species in which male incubation is
common, females tend to select mates on the basis of parental
behaviors rather than physical appearance.
Birds and humans
Birds are an important food source for humans. The most commonly eaten species is the domestic chicken and its eggs, although geese, pheasants, turkeys, and ducks
are also widely eaten. Other birds that have been utilized for
food include emus, ostriches, pigeons, grouse, quails, doves, woodcocks, songbirds, and others, including small passerines such as finches. Birds grown for human consumption are referred to
as poultry.
At one time swans
and flamingos were delicacies of the rich and powerful, although
these are generally protected now.
Besides meat and eggs, birds provide other items useful to
humans, including feathers for bedding and decoration, guano-derived
phosphorus and nitrogen used in fertilizer and gunpowder, and
the central ingredient of bird's nest soup.
Many species have become extinct through over-hunting, such
as the Passenger Pigeon, and many others have become endangered or
extinct through habitat destruction, deforestation and intensive agriculture being common causes for declines.
Numerous species have come to depend on human activities for
food and are widespread to the point of being pests. For
example, the common pigeon or Rock Pigeon (Columba livia) thrives in urban areas
around the world. In North America, introduced House Sparrows, European Starlings, and House Finches are similarly widespread.
Other birds have long been used by humans to perform tasks.
For example, homing pigeons were used to carry messages before the advent
of modern instant communications methods (many are still kept
for sport). Falcons are still used for hunting, while cormorants are employed by fishermen. Chickens and pigeons are popular as experimental subjects, and are often
used in biology and comparative psychology research. As birds are very sensitive
to toxins, the Canary was used in coal mines to indicate the presence of poisonous gases,
allowing miners sufficient time to escape without injury.
Colorful, particularly tropical, birds (e.g. parrots, and mynas)
are often kept as pets
although this practice has led to the illegal trafficking of some endangered species; CITES,
an international agreement adopted in 1963, has considerably
reduced trafficking in the bird species it protects.
Threats to birds
According to Worldwatch Institute, bird populations are
declining worldwide, with 1,200 species facing extinction in the
next century.
Among the biggest cited reasons are habitat loss,
predation by nonnative species,oil spills and pesticide use, hunting and fishing, and climate
change.
Trivia
