Bird Class: characteristics, habitat, feeding, reproduction

 Birds are vertebrate animals, have a feathered body and can control their body temperature independently of the environment.

They are distributed all over the planet and so far there are 0. more known species. Most have the ability to fly, but there are those that are completely adapted to the terrestrial and aquatic environment.

The characteristics common to all birds are:

  • Feather coated body
  • Wings
  • Nozzle
Before you go into detail about the bird study, please find a summary below.

What are birds?


Birds rank second in the number of species among vertebrates. There are 10,000 species worldwide. It is estimated that there are 100 billion individuals in total.

What are birds like?

Birds have an exceptional aerodynamic body that is covered with feathers.

Many have very colourful feathers, the others are hardly noticeable in nature due to their modest appearance. The front limbs are modified in wings.

Apart from a few exceptions, all can fly. The hind limbs have four fingers in most cases. Jaws are covered by a beak.

The weight varies a lot. The ostrich is the largest among the birds (it weighs up to 150 kg), the hummingbird is the smallest (it weighs 1.6 g).

Where do they live

They live in several habitats: deserts, mountains, forests, tundras, near water bodies, etc.

Many never leave their places of life, others migrate to warmer countries when the weather changes. Migrations can be tens of thousands of kilometres long.

How do birds move

Most can fly on feathery wings, powerful chest muscles move their wings. The tail helps direct the flight.

Rigid skeleton light and fast digestion of the ones favors the flight. Hind limbs - legs - are for walking on the ground. Waterfowl have swimming feet with membranes.

Several of them can swim, walk and fly, but most of them are even better adapted for one or two of these.

Bird Respiratory System


The loss of energy is enormous in flying and requires a great deal of oxygen to the bird's organism. Therefore, birds have special organs - air pockets - in addition to their lungs.

The air always passes through the lungs in one direction, allowing maximum oxygenation of the blood.

Bird's circulatory system

Bird's circulatory system

The birds have four heart cavities. Thus, the systemic circulation and the pulmonary circulation are completely separated, consequently the arterial and venous blood cannot mix.

This is one of the reasons why birds are warm-blooded, i.e. their body temperature does not depend on the ambient temperature. The average body temperature of birds is 41 to 42°C.

What do they eat?

They have several diets: they eat plants and animals from the air, water or earth. They have no teeth - the food enters the stomach of two chambers.

In the first chamber the food is mixed with digestive enzymes, the second chamber is the gizzard that acts as a substitute for the teeth.

Birds must ingest large amounts of food to generate the energy needed both for flight and temperature regulation, especially in winter and during migration.

Some small birds have to eat larger amounts of food daily than their own weight.

What are the main organs of the senses?

The sense organs are unequally developed. They depend more on sight than any other sense.

Birds have an acute sense of sight and hearing, but they have a bad smell and taste. Most do not distinguish between smells.

How do birds breed and develop?

All birds have separate sex, fertilization is internal. Most birds build nests. These are diverse and are situated on land, in trees, in tree pits, in burrows, or on the sides of cliffs, etc.

Many begin to nestle in complicated rituals - dating. Males and females of most species form pairs - that is, they remain with each other throughout the breeding season. Laying eggs is followed by incubation and hatching.

Young puppies are able to follow their mother almost immediately after being incubated for several species.

General characteristics of birds

Birds, like all terrestrial vertebrates (Tetrapoda), have two pairs of extremities, the former of which are transformed into wings.

The plumage essentially determines the general appearance of the birds: the body is covered with feathers.

These keratin structures serve as a wing and control surface for flying, an aerodynamically favourable coating of the body and as insulation, which is uniform, usually depending on temperature and wind, changeable.

In addition, the plumage has colours and is often used for sexual advertising. In water birds, it is water repellent and provides buoyancy.

The plumage is altered at certain times. In addition, in all birds, the legs are covered with hair.

All recent birds have a beak without real teeth.

Some extinct species were toothless. The beak of birds consists of horned bone substance, only in pigeons (Columbiformes) and geese (Anseriformes) the bone mass of the beak is covered with soft skin.

All known species have a relatively high and constant body temperature (endothermia) which is higher than all other live animals today and about 42° C. Some species of birds, z. Like hummingbirds and large breasts, they decrease body temperature by about 10° C during the night.

Most birds are navigable. All non-flying species evolved from species originally capable of flying. These include, for example, ratites, penguins and many forms of islands such as kiwis (New Zealand) and stubble cormorants.

The skeleton of birds is slightly built. It has hollow bones (pneumatized) for weight reduction.

The proportion of bone mass accounts for only 8 to 9 per cent of total body mass, while it can be up to 0 per cent in some mammals.

The very large sternum has a raised keel (Carina), which serves as an approach for very large flying muscles.

The heartbeat frequency is high: the maximum heartbeat frequency of an ostrich is 178 beats per minute, that of a domestic sparrow 900 and finally that of a blue throat star hummingbird 1260 beats per minute.

The central nervous system (CNS) is highly developed, among the sense organs, especially the ocular performance is remarkable.

Some birds are probably pentachromatic, so they have five types of colour sensory cells ( cones ). The area of the brain responsible for processing optical stimuli is greatly increased.

In birds, vocalization does not occur in the larynx because the vocal cords are absent. In the tracheal bifurcation, there is a separate organ called the head of the voice ( siringe ), also called the lower larynx.

The lungs of the birds with their air sacs are more complicated than those of all other vertebrates.

The birds have a cloaca, i.e. eggs, urine and faeces go through a uniform execution that opens outwards.

The excretion of electrolytes (sea salts) occurs in many birds as well as reptiles through the nasal gland.

Birds as well as bats and reptiles excrete nitrogen compounds through their urine as guanine as well as uric acid.

Although guanine is more energetic than urea, it hardly needs water to excrete, so the animals don't need much drinking water like mammals and the water in the body doesn't need to be carried.

This mass saving to be moved supports the ability to fly. Bird droppings can reach degradable widths.

The so-called island guano, which consists of sea bird droppings, is thus (usually bat dung) extracted and used as a phosphate-rich natural fertilizer.

Like reptiles, birds lack a urinary bladder as well.

Most bird species have special glandular septicaemia, the pest. In some species, its function is supported or completely replaced by the so-called powdered dunes (cockatoos, pigeons, herons).

Some types are losing vomited and powdered dunes.

Of the above characteristics, none are exclusive to birds. So there are (or existed) forms flying on mammals (bats) and reptiles (pterosaurs), through springs possessed the dinosaurs nonviated feathers, amniotic eggs are also of reptiles and monotremados established and also a bill is not limited to birds.

Classification and taxonomy

Birds, amphibians, reptiles and mammals are traditionally considered as a class of terrestrial vertebrates ( tetrapoda ).

However, this is wrongly considered cladistically, as they are the only surviving group of dinosaurs, a subgroup of reptiles.

In order to preserve a uniform terminology that is a monophyletic group consisting of reptiles and birds, as mentioned sauropsids. Thus, the three classes of terrestrial vertebrates are called:

  • Amphibians
  • Sauropsides (Sauropsida)
  • Mammals (Mammalia)

Orders and families

The bird class is the richest in terrestrial vertebrate species. It comprises about 0. recent and about extinct species in historical time.

Of its two subclasses, the first is small and extinct for families. This includes, in particular, ratites unable to fly due to a lack of sternal ridge.

Among the other about 0 requests of these Neognathae including passerines about 0 percent of all species. Again, the subordination of birds (Passeri) is the largest in this group.

Under Pygostylia there is a different classification of birds, which also includes extinct groups.


The bird feeder is a common feature of all birds.

There are viviparous reptiles and mammals that lay eggs - the same goes for fish and amphibians - but not for live birds.

Probably a longer gestation period and the associated weight change would be very unfavourable for birds as aviators, although bats, on the other hand, and probably pterosaurs are quite viviparous or were.

The males of some groups of birds, such as ratites and geese, have well-developed copulation organs, while others do not have or have simply built a bird's penis.

In general only one, the left ovary is developed in female birds, while two gonads are present in males.

For the ability to fly the lowest possible body weight is of great importance.

Thus the sex organs are very small in birds outside the mating period, but they generally do not shrink immediately after oviposition in order to produce a second clutch if necessary.

At mating the male climbs onto the female's back and both press their capes together. The male's seminal fluid flows into the female's cloaca.

The mating usually takes only a few seconds, but is repeated frequently.

The follicular jump - the release of the egg - can be triggered by various stimuli (such as the vision of a sexual partner).

At the opening of the uterine tube, the egg remains for a few minutes and is fertilized by the sperm that migrates along the fallopian tubes.

Afterwards, the muscular movements of the uterine tube carry the still unfinished egg towards the genital opening.

In the fallopian tubes are the yolk (consisting of one third protein and two thirds fat-like substances as well as vitamins and mineral salts) and the jaundiced embryo to the egg white (protein, salts and water), the coifae finally the shell of the stuck egg.

In contrast to soft-shelled reptile eggs, bird's egg shells are strongly calcified (up to % calcium carbonate), yet they allow the exchange of gases to breathe.

Due to the muscular movements of the fallopian tube (the last part of which is called the uterus), birds' eggs obtain their typical shape.

In female birds, induced by estrogens, before oviposition, a calcium-rich bone substance grows in the leg bones between the outer hard bone and the bone marrow, in which calcium is stored for the formation of the eggshells.

This bone substance, which regresses after oviposition, becomes the marrow bone referred to. marrow bone is also in known non-avarian dinosaurs and living fossils and is used in paleontology to determine female animals. 0

Eggs contain all the nutrients, vitamins and trace elements that the embryo needs to develop. The essential for oxygen metabolism is absorbed through the solid shell.

In one case, the chicks are largely independent soon after hatching, while the nest faeces are completely defenceless, without food and usually blind that come out of the egg and are fed by the parents for a long time.

Many birds breed only one egg, while the largest 0 to 0 eggs in hens occur.

In some species, two or more females lay their eggs in a common nest (see the large ratites ). Many bird species raise several chicks per year in a single breeding season.

Incubation and hatching of eggs

Eggs birds

Some species of birds (big feet) use external heat to hatch their eggs. Most birds, however, heat their eggs in the thoracic and abdominal plumes.

In some species, the sexes involved in the litter have litter spots (areas of skin without hair on the thorax and abdomen), where body heat may improve for the eggs than by insulating plumage.

In many species both partners hatch, in others only the female or, more rarely, only the male, for example the large ratites or the emperor penguin.

The raw temperature is around 34° C. The eggs are often processed during breeding to ensure uniform warming.

Many other factors, such as sufficient humidity, excessive solar heat, absence or just a few weak vibrations etc. are important for successful reproduction. P

ouco before hatching, the limestone shell became thinner due to the calcification of the embryo. Even before leaving the egg, juvenile birds often scream, which often serve to synchronise the incubation process or are of crucial importance for the relationship between the adult bird and the chick.

The young bird rubs and grabs the eggshell from inside until a small hole is formed. At the top of the upper beak - and in some bird species at the bottom of the beak - there is a small, hard cusp, the tooth of the egg, which falls or recedes a few days after hatching.

The whole incubation process takes - depending on the bird - a few minutes or even four days (noses).

Evolution of Birds

Descendants of dinosaurs

The evolution of birds begins in Jura. The ancestors of the birds were small, according to most researchers predatory dinosaurs (teropods) of the Maniraptora group which, however, according to the fossil record were for a long time considered pure land corridors.

The representative of a non-dinosaurus origin of birds long ago pointed out that flying animals or gleitfliegende always descended from arboreal ancestors that evolved from trees.

Birds accordingly would be since they live in reptile trees are descendants. Only in the year 000 it became Microraptor discovered, probably a species of feathered teropod tree.

This discovery supports the thesis of birds belonging to the maniraptors (and therefore to the dinosaurs). The debate on whether the birds evolved from bottom runners or tree jumpers has not yet been decided.

Forerunner of modern birds

The best known evolutionary link between reptiles and birds is the genus Archaeopteryx (literally, "wing"). The wings of Archaeopteryx are very similar to the wings of modern birds. Fossils of this genus were found in the Solnhofen limestone of the Upper Jurassic period.

De Archaeopteryx is long took a middle position between the two classes, because it looked like a mosaic to show characteristics of both reptiles and birds.

For this reason, incomplete and badly preserved specimens - like the "Haarlem" copy in Teylers Museum not long ago as fossils of this animal - recognized Service.

However, with the discovery of more and more feathered dinosaurs since the 0's, it was shown that many of the characteristics of Archaeopteryx, once considered typical of birds, were also found in many other teropod species.

Birds therefore increasingly appear as dinosaurs adapted to flies, but on the other hand typical. It is not clear, therefore, whether Archaeopteryx was a true "original bird", that is, a direct ancestor of modern birds. Many researchers believe that it belonged to a line of development that ends blindly.

The Upper Jurassic Archaeopteryx still had jaws with teeth, a long tail column and moving mitral bones.

He probably had, as is also believed, for his relatives of non-bird dinosaurs, a constant and actively regulated body temperature ( homoiothermia ).

The fossil water birds (Ichthyornis and relatives) found in Upper Cretaceous sedimentary rocks were also toothed. The current groups with their toothless pines did not develop until the Cenozoic.

The loss of hard enamel needs genetic analysis already indicate more than 00 million years ago in the Cretaceous in the evolutionary line that leads to modern birds (Neornithes, crown bird group leads), have taken place and that after the division of the line to Ichthyornis leads.

In the most recent common ancestor of the two main modern bird lines ( Palaeognathae and Neognathae ), eventually all the genes responsible for the development of the teeth should have been disconnected.  Although the oldest unquestionable fossil record of birds in the Upper Cretaceous crown group ( Maastrichtian ) comes, they are already known from the last birds of the Lower Cretaceous, whose anatomy is very close to modern birds.

An example of such a representative is Gansus yumenensis from the Xiagou Formation (-0 mya ) of the Chinese province of Gansu. In general, the birds had been in the rich Cretaceous in biodiversity. A group of cretaceous birds, anatomically less advanced, relatively close to Gansus, are the Enantiornithes, whose fossil remains have been found, among other places, in northeast China.

Development in the Cenozoic

Mass extinction, millions of years ago, killed most dinosaurs. Birds and many other groups of animals also suffered a significant loss of species and higher rates.

At the beginning of the Cenozoic, a large number of new groups, which formed the basis of today's avifauna ( avifauna ), developed in a very short period of probably surviving species.

Some of these groups died again. The Eocene bird fossils (e.g. from the Messel well) prove the existence of a diverse bird fauna, where not all species should yet be attributed to living groups.

An example of extinct species are large, carnivorous, non-flying birds such as Gastornis, which in the Eocene could have taken on the ecological role of predators that have not yet been developed.

Evolutionary explosion

The Lower Cretaceous fossils found in the People's Republic of China resembled Archaeopteryx in terms of clawed hands, abdominal ribs (Gastralia) and pelvic architecture.

But some fossils showed feathers and a sternum breast like the birds of today, besides a beak without teeth and only a small caudal spine.

The first published description of the small feathered dinosaur Caudipteryx contributed significantly to the understanding of evolution, the development of plumage and part of flight.

Consequently, the birds' ancestors initially developed feathers at the front and rear ends, thus being able to slide from tree to tree.

The wing formations on the hind limbs were reduced in the course of evolution, leaving only the wings of the arm and hand to fly.

According to another thesis, the feathers first formed to protect against heat loss in two-legged dinosaurs, dormant at the bottom.

Even birds today have thousands of relatively simple built feathers, but only about 0 feathers.

Bird senses

Sensory performances are not fundamentally different from those of mammals. However, due to the different ways of life, there are differences in the construction and weighting of individual senses, which often makes it difficult to imagine how they perceive their environment.


Birds' eyes are larger than mammals' in relation to their size.

Most species can distinguish more images per second than humans.

Depending on the ecology, birds' eyes have many special adaptations. For example, birds looking for food under water can flex their extra-soft eye lenses through the muscles to adjust their refractive power.

Photosensitivity and visual acuity

Although owls can see more at night than birds of the day or humans, their vision is less focused on visual acuity than on light output.

This works because of the same physics as high speed lenses, which also do well in low light, but deal with sharpness problems, especially with depth of field.

Pilgrims, however, are optimized for daily hunting, they can detect and track small objects such as birds of prey at distances of more than one kilometre.

At least small birds are able to see UV light from large birds, the UV light is also filtered out of the eye.

Many species have not only three colour receptors as humans, but four colour receptors (humans according to recent findings from the receptor to the red and projected to the green light of a common one, so that birds actually have two more mammalian receptors), one of them to the violet light, which has evolved to a UV receptor (about nanometers) in some groups of birds.

Evolution requires only the mutation of three opsin amino acids.

The benefits of UV light vary greatly:

  • Urine derate glows in the UV strip, mouse capture griffon can evaluate a landscape from above in your mouse environment.
  • For fruit, the degree of ripeness can be best assessed with the help of UV light, Some moulds have different colours in the UV band and are therefore more visible.
  • There are some bird species where the sexes do not differ in visible light but in UV light. Stareou single species of breasts (blue breasts ) are an example of this.

Spatial vision

Depending on the ecological adaptation, the ability to see from one species to another is very different.

In groups of species that are under strong pressure from predators (e.g. pigeons and chickens) the eyes are located on the side of the head. This allows an almost complete 360° view, but the overlapping fields of vision and therefore the ability to see in space is relatively low.

The other extreme is represented by owls and with them the eyes are arranged side by side in front of the head (as in humans).

The fields of vision of the eyes overlap very strongly, so the spatial vision is correspondingly good. The small lateral extent of the field of vision is characterised by a very strong mobility of the balanced cervical spine. Owls can turn the head up to 270°.

Many birds swing their heads back and forth as they walk.

Here, backward movement is used to keep the head for a moment in relation to the environment in peace, so that the image on the retina does not move, in favor of better recognizability of moving objects - like a predator.

Magnetic direction

In some species, especially migratory birds, a notion of the Earth's magnetic field has been demonstrated. This magnetic sense is probably located in both eyes of the bird and / or the beak.

The magnetic sense in the eye works with the help of so-called radical pair formation. Here, attractive light disintegrates certain molecules into radicals.

This reaction can be influenced by the Earth's magnetic field. The magnetic direction in the nozzle works by embedded magnetic particles, which align with the Earth's magnetic field and exert a stimulus on the surrounding nervous tissue.

In contrast to the technical compass, the magnetic direction of the birds is not based on the polarity of the Earth's magnetic field, but is based on the recognition of the inclination of the magnetic field.

Listening to birds

The birds have no auricle, the outer ear opening is surrounded by a crown of small feathers. For sound location, they need to make intensive head movements.

The middle ear has only one auditory ossicle, the columella, which corresponds to the stapes of mammals. The snail is relatively short and only a little crooked and is called the basic papillae.

The sense of hearing is still relatively well developed in birds and is, for example, very important for hunting.

The smallest perceptible frequency range is similar to mammals, but the high shades above 1000 hertz are not perceived by most.

The temporal resolution of sounds is higher than that of humans. Ornithologists have to manage many bird vocalizations by recording and slowing down the speed to hear the details.

Sense of balance

They have several independent bodies of balance. Besides an organ of balance in the ear, a second organ stays in the pelvis, which analyses the position of the body, for example, when sitting on branches.

Birds, in which this organ is destroyed, can no longer react to disturbances such as the rotation of the spike without any sense of sight.


For a long time it was assumed that birds have only a slightly developed sense of smell.

The exception was the New Zealand kiwi, which has its nostril at the tip of its beak and is mainly based on smell.

But also the New American Vulture group has a proven ability to smell. This is attributed, inter alia, to the lack of nasal septum and has been proven in field observations and attempts.


The taste buds of birds are not on the tongue, as in mammals, but in the area of the base of the tongue and throat.

The number of taste buds is significantly lower than in mammals (duck about 200, man 9000), but the taste of birds is not as subordinate to birds as has long been assumed. Depending on the nature of the taste, but against the feel of the beak and tongue, but in food selection it plays a minor role.


Research since the 1990s has shown that birds, or at least representatives of certain large groups, are more intelligent than previously thought.

The traditional nomenclature of the bird's brain anatomy has been based on the assumption that the bird's brain structures are equivalent to the mammalian brain.

Since this assumption turned out to be wrong, a new nomenclature for the bird brain has since been developed (see also bird brain and cognition ).

The orbital organelle seems to be associated with a very high number and density of neurons in relation to brain volume and mass.

For example, the brains of sparrow and parrot birds, considered the most intelligent, have significantly more neurons than the brains of mammals (including primates) with the same mass.

Furthermore, brain mass and the number of neurons in these birds is higher in relation to body mass than in mammals (except primates).

So the winter golden chicken has a body weight nine times lower than a mouse, but its brain contains more than twice as many neurons.

Here, neurons are concentrated in the brain and especially in the cerebral cortex, which in the brains of birds and psittaciformes has a density of neurons three to four times higher than in primates.

The comparatively low weight and volume of these bird brains results from a reduction in individual neurons, while the cerebral cortex, like most mammalian brains, is composed of relatively few but relatively large nerve cells.

Differences in the degree of encephalisation also exist within birds.

Economic importance of birds

Man holds numerous species as pets or pets.

Poultry consumption plays an important role in the human diet because meat is low in fat and provides high quality protein.

However, the use of native bird species as pets has decreased significantly in Germany since the 19th century.

Some species, if already as young birds and cages patiently encouraged to learn well, short words and phrases to repeat, especially - ordered by descendant docility - parrots and mynah, starlings and various types of corvids.

Coexistence with birds, in particular intensive contact, contains for humans, but sometimes serious dangers in the form of communicable diseases ( Zoonosen ).

In individual cases, for example, the causative agent of avian influenza or HN and HN has been transmitted to humans, the disease usually ends fatally.


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