H.J. Ruiz – Avian101

Very interesting article for people interested in avians:

Birds are vertebrate animals in the class Aves. There are approximately 8,800 species of birds, divided among 28 living orders. Of these, slightly more than 900 species are found in North America. There has been considerable disagreement among ornithologists about the appropriate level for differentiating species, leading to multiple classification schemes. But however one distinguishes between species, each species belongs to a larger group, called a genus; each genus belongs to a family; and each family belongs to an order. One order, the Passeriformes or perching birds, accounts for more than one-half of the living species of birds.

Birds are believed to have evolved from saurischian dinosaurs, about 150 million years ago. The first truly bird-like animal was Archaeopteryx lithographica, which lived during the Jurassic period, about 130 million years ago. This 3 ft (1 m) long animal is considered to be an evolutionary link between the birds and the dinosaurs. Archaeopteryx had teeth and other dinosaurian characters, but it also had a feathered body and could fly.

Birds can often be characterized by their habitats. Species are more likely to be found in a single habitat in the tropics, than in more temperate regions (with variable climates) where they may have to accommodate several environments. During times of migration, birds may show up in very non-traditional places. Major habitats that serve as homes to birds include polar regions; tundra; alpine regions; coniferous forests; deciduous forests; tropical rainforests; grasslands; deserts; freshwater lakes, ponds, and streams; shore and marshes; and seas.

Most species of birds will defend their territories, particularly in nesting season, against other birds that happen to stray into the area, and are perceived to be a threat to the nest. Territories generally fall into the following categories: mating, nesting, and feeding; mating and nesting; mating; narrowly restricted nesting; feeding; wintering; roosting; and group territory.

Some birds prefer to live apart from other birds, while others are more social. Certain mated woodpeckers, for example, seem to constantly resent each other, and go out of their way to avoid each other’s presence. The Australian wood swallows, on the other hand, feed, bathe, roost, attack predators, breed, preen, perform aerial acrobatics, and feed each other in a communal fashion. Assembly into a flock (consisting of either a single species or several different species) offers the advantage of more eyes and ears for spotting predators and finding new food sources.

Most birds have distinctive calls. Males may sing to warn off rivals, or to attract a mate. Some species have separate calls for courtship and for communication. Bird calls serve several functions including: reproductive functions (to proclaim the sex of an individual, advertise for a mate, or establish territorial sovereignty); social functions (space out birds in a given environment, teach the young their species’ song, or convey information about food and enemies); or individual functions (provide emotional release, identify individuals to each other, or simply to perfect a song).

All birds have feathers, and in this way are unlike all other types of animals. Most birds loose their old feathers each year, these being replaced by new ones (molting). Usually, molting occurs when the bird is neither nesting nor migrating. Some birds will undergo a complete molt in the summer or fall, and then a partial molt in the spring (replacing feathers of the body and head). The chief functions of feathers are to protect the body and to promote flight.

Because birds have four limbs, they are referred to as tetrapods. However, the fore limbs are highly modified into structures known as wings, which are primarily used for flying or gliding. The hind limbs are mostly used for walking or hopping, so these animals are bipedal when moving on the ground. Some species of birds are flightless, but most can fly or soar well.

The bones of all flying birds are modified for flight, and are relatively light with many hollow regions. The flying birds have a strongly keeled breastbone or sternum, to which the flight muscles are attached. Their pelvic bones are fused into a structure known as a synsacrum. Birds have a relatively long neck and their mandibles are modified into a keratinous beak. They do not have teeth. Birds have a four-chambered heart and a double circulation of the blood, with complete separation of oxygenated and de-oxygenated blood.

Approximately 90% of all birds are monogamous, which is to say that one male bonds with one female. Bonds may last for one nesting period (e.g., house wrens), an entire breeding season (most species), several seasons (e.g., American robins), or life (e.g., geese, swans, hawks, albatrosses, and loons). Current thinking is that most monogamous birds probably mate outside of the primary pair bond, but the pair contributes substantially only to the raising of young in their own nest. Most birds whose young are reared in a nest form monogamous pairs. But monogamous versus polygamous relationships are not rigidly fixed in some species. Swans, hawks, doves, finches, and thrushes, for example, normally show monogamous behavior but occasionally form polygamous bonds.

Situations in which one male mates with more than one female, while each female mates with only one male, are referred to as polygyny. Scientists have noted that when male birds hold territories that vary greatly in the quality of resources, females tend to choose males with high-quality territories as mates. Given the choice between a poor nesting territory and becoming the second mate of a male with a high quality territory, many females will opt for the latter course. In other cases, where territoriality does not appear to be a factor, an overabundance of resources may also lead to polygyny. In North America, red-winged blackbirds and yellow-headed blackbirds sometimes form polygynous relationships.

There are also rare mating systems in which one female mates with more than one male, while each male mates with only one female (known as polyandry). Two forms of polyandry are known. In one case, the female holds a large territory that includes the nesting territories of two or more males who care for the eggs and the young, such as the spotted sandpipers and red phalaropes of North America. In an even rarer type of polyandry, more than one male may mate with a single female with the resultant offspring reared collectively by the female and her mates. The North American acorn woodpecker occasionally exhibits this type of cooperative polyandry.

Read more: Birds – Characterization http://science.jrank.org/pages/921/Birds.html#ixzz0rbdJBsEZ