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Bird

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For other meanings of bird, see bird (disambiguation).

Birds

<tr><td>Kingdom:<td>Animalia <tr><td>Phylum:<td>Chordata <tr><td>Class:<td>Aves </table> <tr><th bgcolor=pink>Orders <tr><td> Many - see text </table> Birds are bipedal, warm-blooded, egg-laying vertebrates characterized primarily by feathers, forelimbs modified as wings, and hollow bones. There are almost 9000 known species of birds in the world, making them the most diverse class of terrestrial vertebrates.

Contents

Introduction

Birds range in size from the tiny hummingbirds to the huge Ostrich and Emu.

Although most birds are characterised by flight, 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, for example the Great Auk, flightless rails, and the moa of New Zealand.

Birds are a very differentiated class, with some feeding on nectar, seeds, insects, rodents, fish, carrion, or other birds. Most birds are diurnal, or active during the day. Some birds, such as the owls and nightjars, are nocturnal or crepuscular (active during twilight hours). Many birds migrate long distances to utilise optimum habitats (e.g., Arctic Tern) while others spend almost all their time at sea (e.g. the Wandering Albatross).

Common characteristics of birds are the ability to fly using feathered wings, a bony beak with no teeth, the laying of hard-shelled eggs, high metabolic rate, and a light but strong skeleton. Birds are among the most extensively studied animal groups, with hundreds of academic journals devoted to their study.

To preen or groom their feathers, birds use their bills to brush away foreign particles.

The birds of a region are called the avifauna.

Reproduction

Although most male birds have no external sex organs, the male does have two testes which become hundreds of times larger during the breeding season to produce sperm. The female's ovaries also become larger, although only the left ovary actually functions.

In the males of species without a phallus (see below), sperm is stored within the proctodeum compartment within the cloaca prior to copulation. During copulation, the female moves her tail to the side and the male either mounts the female from behind or moves very close to her. He moves the opening of his cloaca, or vent, close to hers, so that the sperm can enter the female's cloaca, in what is referred to as a cloacal kiss. This can happen very fast, sometimes in less than one second.

The sperm is stored in the female's cloaca for anywhere from a week to a year, depending on the species of bird. Then, one by one, eggs will descend from the female's ovaries and become fertilized by the male's sperm, before being subsequently laid by the female. The eggs will then continue their development in the nest.

Many waterfowl and some other birds, such as the ostrich and turkey, do possess a phallus. Except during copulation, it is hidden within the proctodeum compartment within the cloaca, just inside the vent. The avian phallus differs from the mammalian penis in several ways, most importantly in that it is purely a copulatory organ and is not used for dispelling urine.

After the eggs hatch, parent birds provide varying degrees of care in terms of food and protection. Precocial birds can care for themselves independently within minutes of hatching; altricial hatchlings are helpless, blind, and naked, and require extended parental care. The chicks of many ground-nesting birds such as partridges and waders are often able to run virtually immediately after hatching; such birds are referred to as nidifugous. The young of hole-nesters, on the other hand, are often totally incapable of unassisted survival. "Fledging" is the process of a chick acquiring feathers until it can fly.

Some birds, such as pigeons, geese, and Japanese cranes, remain 'married' for life. The male and female remain together for life or for a long period after mating and may produce offspring on a regular basis.

Mating Systems

There are many different mating systems that have evolved over time. There are essentially three systems that include polyandry, polygyny, and monogamy. For avian mating systems, monogamy is seen in approximately 91% of all bird species. Polygyny constitutes 2% of all birds and polyandry is seen in less than 1%. Monogamous species of males and females pair for the breeding season. In some cases, the individuals may pair for life. Sexual selection theory and the relatively low typical parental investment that males make in offspring would suggest that polygynous mating systems should predominate in the animal kingdom. The real puzzle then is why do males become monogamous at all, especially so often in bird species.

In most groups of animals, male parental care is rare, but in birds it is quite common. Actually parental care by males is more extensive in birds than in any other vertebrate class. This seems very strange in relation to other species. One reason for this large percentage of monogamy with birds is due to the fact that male birds are just as adept at parental care as females. In fact, 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.” (Gowaty, 1983) It is assumed that when such male care is necessary, monogamy will evolve directly with this. It seems strange then that a male’s ability to defend mates and resources relies on his ability to separate himself from parental care

The parental behavior most associated with monogamy was male incubation. This is very interesting, because male incubation is the most confining male parental behavior. It not only consumes time, but also may require physiological changes that interfere with usual mating. With the extreme loss of mating opportunities, there is a reduction in the reproductive success among 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.” (Ketterson and Nolan, 1994) It is understood then that the females associated with these males base their choice of mate on parental behaviors rather than physical appearance.

It is surprising that with the amount of time and energy needed, that paternal care is selected for at all. As stated earlier, there does appear to be some obvious benefits to male parental care. Theory on mating system evolution suggests that a female should choose mates that confer maximum fitness. Therefore, if males contribute to the care of the offspring both male and female fitness may be maximized. “High paternal care has been seen among many species of monogamous mammals and several authors suggest that male investment is one of the primary advantages of monogamous mating.” (Zeveloff and Boyce, 1980) This suggests then that this mating system whether costly or not must be advantageous in some way or these females would not be selecting males with this behavior.

It seems likely that fitness is increased in some way by this behavior of male parental care, but not so extreme that it is adopted by most other species. Surprisingly, there appears to be a lot of research to show that monogamy and paternal behavior are actually maladaptive due to its cost to fitness. However, is this due to the use of male care in this mating system? While the answer may be yes, it also seems to prevail in bird populations for some reason. What this reason is though may never be pin pointed to an exact answer. Some observations are thrown out by a couple of researchers but with no conclusive data or results. The only thing concluded seems to be that it is unnecessary. For now, until more research is done, we must instead be content to just know it persists.

Gowaty, Patricia Adair: Male Parental Care and Apparent Monogamy among Eastern Bluebirds (Sialia Sialis). The American Naturalist 121(2):149-160 (1983)

Ketterson, Ellen D. and Nolan, Val: Male Parental Behavior in Birds. Annual Review of Ecology and Systematics 25:601-28 (1994)

Zeveloff, Samuel and Boyce, Mark: Parental Investment and Mating Systems in Mammals. Evolution 34(5):973-982 (1980)

Respiration

A juvenile Laughing Gull on the beach at Atlantic City

Birds respire by a method of crosscurrent flow, ie: flow at a 90° angle. There are three sections involved in respiration. These are the anterior air sacs (interclavicular, cervicals, and anterior thoracics), the lungs, and the posterior air sacs (posterior thoracics & abdominals). It takes a bird two full breaths (inhaling and exhaling), to cycle air through.

The air flow through air sacs and lungs is as follows:

First inhalation: air flows through the trachea and bronchi into the posterior air sacs.
First exhalation: air flows from the posterior air sacs to the lungs
Second inhalation: air flows from the lungs to the anterior air sacs
Second exhalation: air flows from the anterior sacs back through the trachea and out of the body.

In birds, air flows in only one direction. Because of this, birds are able to diffuse more oxygen into their blood. Unlike humans and other mammals, there is no mixing of oxygen rich air and carbon dioxide rich air. Thus, the partial pressure of oxygen in a bird's lungs is the same as the environment. This is also why you would more likely see a bird on Mount Everest, than say a mouse.

Evolution

Birds are generally considered to have evolved from theropod dinosaurs. Specifically, birds are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids. The exact boundary between dinosaurs and birds is unclear, especially with the recent discoveries in North-east China (Liaoning Province) demonstrating that many small theropod dinosaurs had feathers.

The recently discovered dromaeosaur, Cryptovolans, was capable of powered flight, contained a keel and had ribs with an uncinate process. In fact, Cryptovolans makes a better "bird" than Archaeopteryx who is missing some of these modern bird features. Because of this, some paleontologists have suggested that dromaeosaurs are actually basal birds whose larger members are secondarily flightless. Evidence for this theory is currently inconclusive.

It should be noted that although ornithischian (bird-hipped) dinosaurs share the same hip structure as birds, birds actually originated from the saurischian (lizard-hipped) dinosaurs, and thus arrived at their hip structure condition independently. In fact, the bird-like hip structure developed a third time among a peculiar group of theropods, the Therizinosauridae.

The early bird Archaeopteryx, from the Jurassic, is well-known as one of the first "missing links" to be found in support of evolution in the late 19th century. It remains the most primitive known bird. Other early birds now extinct include the Confuciusornithidae, Enantiornithes, Ichthyornis, and Hesperornithiformes, a group of flightless divers resembling grebes.

Modern birds are classified in Neornithes. The flightless Palaeognaths, the Ostrich group, were first to diverge from the avian lineage, and it is now thought that the basal divergence from the remaining Neognathes was that of the Galloanseri, the superorder containing the Anseriformes (ducks, geese and swans), and the Galliformes (the pheasants, grouse and their allies). See the chart.

Sibley & Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds (although frequently debated and constantly revised).

Bird orders

This is a list of the taxonomic orders in the class Aves. List of birds gives a more detailed summary, including families.

Struthioniformes, Ostrich, emus, kiwis, and allies
Tinamiformes, tinamous
Anseriformes, waterfowl
Galliformes, fowl
Sphenisciformes, penguins
Gaviiformes, loons
Podicipediformes, grebes
Procellariiformes, albatrosses, petrels, and allies
Pelecaniformes, pelicans and allies
Ciconiiformes, storks and allies
Phoenicopteriformes, flamingos
Accipitriformes, eagles, hawks and allies
Falconiformes, falcons
Turniciformes, button-quail
Gruiformes, cranes and allies
Charadriiformes, plovers and allies
Pteroclidiformes, sandgrouse
Columbiformes, doves and pigeons
Psittaciformes, parrots and allies
Cuculiformes, cuckoos
Strigiformes, owls
Caprimulgiformes, nightjars and allies
Apodiformes, swifts
Coraciiformes, kingfishers
Piciformes, woodpeckers and allies
Trogoniformes, trogons
Coliiformes, mousebirds
Passeriformes, passerines

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..

At one time swans and flamingos were delicacies of the rich and powerful, although these are generally protected now.

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.

Chinstrap Penguin

Numerous species have come to depend on human activies for food and are widespread to the point of being pests. For example the common pigeon or Rock Dove (Columba livia) thrives in urban areas around the world. In North America, introduced House Sparrows (Passer domesticus), Common Starlings (Sturnus vulgaris), and House Finches (Carpodacus mexicanus) are similarly widespread.

Other birds have been used by humans: for example Homing pigeons to carry messages (many are still kept for sport), falcons for hunting, cormorants for fishing. Chickens and pigeons are popular subjects in experimental research in biology and comparative psychology. As birds are extra-sensitive to toxins, the Canary was often used in coal mines to indicate the presence of poisonous gases, so that the miners could escape.

Colorful, particularly tropical, birds (e.g., parrots, and mynahs) are often kept as pets although this has led to smuggling of some endangered species; CITES does considerable work to deter this.

Bird diseases that can be contracted by humans include: psittacosis, salmonellosis, campylobacteriosis, Newcastle's disease, mycobacteriosis (avian tuberculosis), influenza, giardiasis, and cryptosporiadiosis.

Few birds use chemical defences against predators. Tubenoses can eject an unpleasant slime against an aggressor, and some species of pitohui, found in New Guinea secrete a powerful neurotoxin in their feathers.

External links

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