Small patches of lymphatic tissue are located throughout the corium. Some species of birds have two functional ovaries, and the order Apterygiformes always retain both ovaries. They are constant visitors to the feeder. The avian stomach is divided into 2 parts:. Birds do not possess sweat glands. Gastrointestinal tracts of a carnivorous hawk, an omnivorous chicken, and 4 herbivorous birds. Vegetables raised for seed production.
There are more than 10, extant species of birds which makes them the most speciose tetrapod vertebrate class in the world. All the living species of birds belong to the Neornithes subclass, inhabiting all types of ecosystems from the Arctic region to the Antarctic region.
The extant species can vary greatly in size. The smallest living species is the Bee Hummingbird which grows no more than 2 inches 5 cm in size while Ostrich is the largest extant species growing up to 9 feet 2. The Condors are regarded as the largest birds of prey. It is indicated by the fossil records that birds first emerged during Jurassic period, approximately million years ago within the theropod dinosaurs.
As mentioned above, it is accepted by most scientists that birds originated as a specialized subgroup in the suborder theropod dinosaurs. Birds belong to the Maniraptora group of theropods that also includes oviraptorids and dromaeosaurs, among others. Many non-avian theropods have been discovered that are closely related to modern day birds. This has given birth to arguments regarding the distinction between birds and non-birds. Modern paleontology shows that avialans or birds are very closely related to the deinonychosaurs, including troodontids, dromaeosaurids and possibly archaeopterygids.
These three families form a group known as Paraves. Some animals in this group, including Archaeopteryx and Microraptor, have special physical features for flying or gliding. The small size of the most primary deinonychosaurs gives rise to the theory that the ancestor of all paravians were arboreal, having adaptive features that enables them to glide or fly. Recent studies show that unlike the carnivorous feathered dinosaurs and the Archaeopteryx, the primitive birds were herbivores.
The evolution of birds led to a great diversity in their form and appearance during Cretaceous Period. The primitive characteristics like teeth and clawed wings remained in many groups, though most groups, including the modern birds, gradually evolved to lose the teeth. The earliest groups like the Jeholornis and Archaeopteryx retained their characteristic long bony tails while the tails became shorter in advanced birds.
All modern birds developed better olfactory senses than that in the primitive groups during the late Cretaceous period. Various groups started developing special adaptive features. For example, the Mesozoic seabird order Hesperornithiformes became well adapted for the marine environments with special adaptive features for hunting fishes. Birds in this order became flightless and primarily aquatic. Despite these extreme specializations, these seabirds represent some very close relatives of the modern birds.
The classification given below only includes the taxonomic orders belonging to the extant subclass Neornithes. The traditional classification method known as Clements order revised by Sibley-Monroe classification has been used in the following list.
Its name has been derived from the Greek word 'paleognath', meaning 'old jaws'. The name refers to the comparatively primitive skeletal system of the species in the superorder, which is more reptilian than that of other birds. Palaeognathae superorder is divided into two orders that include 49 living species in total. This superorder comprises of 27 orders that collectively includes around ten thousand different species.
The species in the Neognathae superorder have undergone different adaptive changes which produced the major diversity in their appearance especially of their feet and bills , behavior and function.
The bird skeleton comprises of lightweight bones, which keeps them light enough to fly easily. The pneumatic cavities large cavities filled with air in the bones connect them to the respiratory system. The skull bones in adult birds are fused and do not have visible cranial sutures. They have large orbits separated by one bony septum. The spinal column is divided into cervical, lumbar, thoracic and caudal regions.
The number of the cervical vertebras may vary significantly from one species to another. The neck is highly flexible; however, movement is restricted in anterior thoracic vertebras and absent in later vertebras. The last few vertebras fuse with the pelvis and form the synsacrum. Birds have flattened ribs and a keeled sternum that attaches the flight muscles. But, this arrangement is not present in flightless birds.
Like animals from the reptile class, most birds are uricotelic. This means nitrogenous wastes are extracted from their bloodstream by the kidney and are excreted as uric acid, rather than ammonia or urea, through their ureters into their intestine. These creatures do not have any external urethral opening or urinary bladder apart from Ostrich , so the uric acid and the feces are eliminated as semisolid waste materials.
However some species, such as hummingbirds, excrete majority of the nitrogenous waste material as ammonia. Like many mammals, birds excrete creatine instead of creatinine. These waste materials along with the intestinal output emerge from the cloacal opening. Additionally, many bird species are known to regurgitate pellets. The digestive systems of most species belonging to this class are very efficient and allow rapid digestion, which produces high amounts of energy to help in flying.
The muscular crop plays a vital role in the digestion process by softening the foods in the stomach and storing them temporarily to regulate their flow through the digestive system. The shape and size of the crop often varies among different birds. Their respiratory systems are among the most complex ones in all animal groups. This way, the bones get filled with fresh air. During exhalation, the used air is eliminated from the lungs and is replaced simultaneously by the fresh air stored in the posterior air sacs.
Thus, the lungs of birds are constantly supplied with fresh air both during inhalation and exhalation. The heart of a bird has four chambers like that of humans and most other mammal species.
The origin of their main arteries carrying the blood from the heart lies in right aortic arch or the pharyngeal arch. The blood reaches the postcava from the bird's limbs via its renal portal system. Unlike in mammals, the nuclei are retained by a bird's circulating RBCs red blood cells. The nervous systems of birds are quite large compared to their body size.
This explains their complex and advanced intelligence. The flight-related functions are controlled by the most developed region of their brains while their movements are coordinated by the cerebellum. The cerebrum part of their brains control their behavior patterns, mating, nest-building and navigation. Birds have eyelids, but they do not use it for blinking. Their eyes are lubricated by a third eyelid or membrane named the nictitating membrane which moves horizontally.
This membrane also protects the eye and allows aquatic birds to see under water by working as contact lenses. The retina of these creatures has a fan-shaped blood supply system called the Great Cormorant. In some species, the eyes are located at the sides of the heads which provides a very wide visual field.
On the other hand, birds like owls have eyes in front of their heads which provides them with a binocular and enables them to estimate the depths of fields. Barn Owls at this nest site have laid an average clutch size of 4. This year was not without its tribulations, however, as we are sad to report that the adult male unexpectedly died while in the nesting box after the breeding season in early December.
He was last seen leaving the nest box in apparently healthy condition the day before he returned and eventually succumbed to what was ailing him. Even though this successful year ended on a bittersweet note, the events of the breeding season give plenty of hope that there will be many new suitors for the female in the lead up to spring.
In addition to the adult male Barn Owls who may be looking for a new mate, if other owls in the area had a booming reproductive year in , there may be a surplus of new breeders searching for a nest site to call their own in Barn Owls weigh from just under a pound to about 1.
They are 12—16 inches long and have a wingspan of 39—50 inches. Females are larger than males. Barn Owls are usually monogamous and stay together for life. If one of the pair dies, the other will find a new mate. There have been reports of polygamy, with pairs raising second broods during a given year with a different mate.
There is even one report of polygyny, with two females and one male nesting and raising young together in one nest box. Many details about mate selection are not known. Somehow, a higher number of spots is correlated with offspring parasite resistance, and so males that pair with heavily spotted females gain substantial reproductive benefits.
No, just the female. This patch has lots of blood vessels just beneath the skin that transfer heat to the eggs. Incubation begins after the first egg is laid and the female only rarely leaves the nest during this time, and only for brief periods. The male delivers prey to the female during the time she is incubating the eggs and brooding the young chicks.
In many cases the male will bring excess prey that is stored in the nest for later consumption. Home ranges of the Barn Owl are highly variable and vary in relation to prey density and habitat characteristics.
Hunting areas may be located outside of this home range, but usually only by a few miles. When lots of nest sites are available and prey populations are abundant, many pairs may nest in close proximity and share a hunting area. Barn Owls eat mostly small mammals, particularly rats, mice, voles, lemmings, and other rodents; also shrews, bats, and rabbits.
Most of the prey they eat are active at night, so squirrels and chipmunks are relatively safe from Barn Owls. They occasionally eat birds such as starlings, blackbirds, and meadowlarks. Amphibians, reptiles, fish, insects, scorpions, and crayfish are rarely taken.
Nesting Barn Owls sometimes store dozens of prey items at the nest site while they are incubating to feed the young once they hatch. It has not been determined whether Barn Owls are specialist or opportunistic predators.
Though the owls appear to be specialized in hunting small mammals, this actually may only reflect the fact that these prey items are what the owls are most likely to encounter in their nocturnal habitat.
The amount of food required for a Barn Owl depends on the size of the owl and the time of the year. They consume more food in the winter and when they are very active. One captive female consumed about Barn Owls hunt primarily at night, beginning about one hour after sunset and ending about one hour before sunrise. They occasionally hunt during the day.
Owls typically swallow small prey whole, bones and all. Bones are broken down in the stomach to provide important nutrients such as calcium and phosphorus. Any indigestible parts of prey such as fur and undigested bones are regurgitated as a pellet.
Barn Owls usually cough up pellets once or twice a day. When a prey item is swallowed whole, indigestible parts of prey, such as fur, bone, and tough insect parts, will form a pellet in a muscular area of the stomach called the gizzard.
In North America, Barn Owls have been found to produce one to two pellets per day on average. The minimum interval between eating and casting is about 6. Pellet regurgitation appears to be stimulated by the sight of a potential meal. It is transparent and can close and protect the eye when hunting.
Like all birds, owls do not have teeth. Owls swallow food whole or rip it apart in their beak and swallow pieces. Bird poop is actually brown.
Mammals excrete waste as urea dissolved in urine; birds excrete it as uric acid, which has a low solubility in water, and so it comes out as a white paste. Barn Owls get most of the water that they need from eating their prey. They have almost never been observed drinking water in the wild. However, recent research reveals that some species of birds have a high number of active genes that are associated with smell. Scientists have also discovered that some species of birds can tell each other apart by smell.
Barn Owls have excellent vision. Like other owls, the shape of their eyes limits their ability to move them in the eye sockets, but their necks can turn up to degrees. This is accomplished by the 14 vertebrae in their necks, twice as many as in mammals. Barn Owls have superb hearing. Their ability to locate prey by sound alone is the most accurate of any animal that has ever been tested.
These owls can catch mice in complete darkness in the lab, or prey hidden by vegetation or snow out in the real world. Like other owls, their ears are placed unevenly on their head and point in slightly different directions, giving the ability to hear where a sound is coming from without moving their heads.
Owls can also funnel sound toward their ears by manipulating different types of feathers around the ears and face. Females give the call infrequently. These owls also use a variety of other sounds, with a larger repertoire during the breeding season. Distress and warning calls usually consist of a series of drawn-out harsh screams, and both adult and nestling birds will repeatedly hiss to intimidate predators.
Twitters and chirrups are uttered by adults and chicks when delivering food, feeding, begging for food, in discomfort, or greeting each other. When agitated, Barn Owls snap their bill mandibles together. As part of a display flight, males sometimes clap their wings together once or twice. Mobbing calls are an explosive yell directed usually at a mammalian predator. Various other sounds are associated with the breeding season and can be heard during copulation, food deliveries, and greeting.
Young begin to use adult calls while still in the nest. They do not usually bring in nesting material, though most females contain the eggs by arranging a cup of their own shredded pellets. Barn Owls lay 2—18 eggs, usually 5—7. There is little correlation between clutch size and latitude. They have 1—2 clutches per year in most areas, but in some climates or in captivity, Barn Owls can breed year-round.
In years with abundant food, subsequent clutches may overlap, with the female incubating a second set of eggs and the male continuing to feed the fledged young. Barn Owls may lay eggs throughout the year depending on location. The severity of weather during the preceding winter has a significant effect on the date of first clutches; longer winters with a high number of days with deep snow cover will cause clutches to come later in the year.
Another way to predict when eggs will be laid is that egg laying usually happens about one month after courtship begins. In Texas, Barn Owls may lay their first clutch as early as November. The eggs in this nest were laid March 23, 25, and Eggs are usually laid every 2—3 days until the clutch is complete. The eggs are about 1. They are dull white, though are often dirtied in the nest, appearing darker.
This may allow them to lay more eggs using the same amount of body reserves. It is normal for parents to leave the eggs and nestlings exposed now and then. Barn Owls have evolved over millions of years to cope with variables such as harsh weather. When the oldest chick is about 25 days old, the female will usually spend less time with the nestlings and increase her hunting efforts.
In these cases, if only one egg is damaged, the parents generally continue to incubate the other ones. If something happens to the entire first clutch of eggs, most birds will often lay a second clutch if it is still early in the breeding season.
Oxygen gets into the egg through pores in the shell. Owl chicks may take more than 12 hours to make their way out of the egg after pipping. They get their first big gulp of air when they pierce the membrane of the egg under the shell.
Only the female Barn Owl incubates the eggs and broods the young. Barn Owls typically lay an egg once every 2—3 days until their clutch is complete.
The eggs laid first have a head start and hatch sooner than the ones laid last. Eggs do not successfully hatch in many Barn Owl nests. For example, Marti conducted a year study in Utah and found that only 63 percent of eggs hatched.
The male delivers prey to the female on and off the nest and she feeds it to the chicks by tearing it into small pieces. Often before two weeks of age, the young are able to consume whole prey on their own. At this time, the male will still deliver prey and the female may also begin to start hunting. As they get older and achieve their adult size and color, we may be able to guess the sex; females are usually larger than males and have a heavily spotted chest with more color on the head and body.
Banding birds with an individually numbered ring on their leg is a common practice in ornithology to mark and study individual birds. Special permits are required to band birds for scientific study.
If the owls were needed in a study, then we would consider banding them, but presently the birds are not part of a study and we do not plan to band them. In order to avoid unnecessary disturbance at the nest, banding nestlings is done only when scientifically warranted. Hatchling Barn Owls may weigh 12—21 grams depending on the subspecies. Barn Owls in the U. Growth is rapid, especially during the first month, and the chicks are usually heavier than the adults at the time they fledge, at about 50—55 days after hatching.
As the young grow, they can eat and digest bigger meals and the parents may stay away from the nest for longer periods of time. In cases of severe food shortages, the smallest perish and are sometimes even consumed by their siblings. Though nest failures such as this are difficult to watch, this strategy enables the parents to produce as many young as conditions allow. The chicks can breathe even when their parents are brooding them.
This is a natural, well-documented behavior for nestlings of some bird species, including Barn Owls. In general, there is little strife between Barn Owl nestlings other than some squabbling over food.
Unlike other species, older Barn Owls chicks may even feed their younger siblings. However, in some cases siblicide and even cannibalism may occur.
This is more likely to happen in nests where there is a lack of food. Sadly, it is rare for all Barn Owl hatchlings to survive to fledging. One year study in Utah found that, on average, only 63 percent of eggs laid hatched and 87 percent of hatchlings survived to fledging.
Similar observations have been made on Barn Owl nests in other parts of the world. Here at the Cornell Lab of Ornithology, we have been closely monitoring and actively discussing the developments in the Barn Owl nest box of Italy, Texas, where today May 30, just one of the original six chicks survives.
Others have been consumed by their nest mates after perishing, or have been injured and killed by what we assume to be an intruding adult Barn Owl. These circumstances were brought on by a several day period of intensely bad weather, including record rainfall, which limited or curtailed altogether the opportunities for hunting and provisioning the nest. Such events are difficult to watch, and understandably we have been besieged with requests to intervene and offers to help, either by providing supplemental food or by rescuing the individuals and caring for them in captivity.
As lovers of birds and nature, all of us share the feelings of sorrow and empathy for these individuals as they endure mortal hardships in the nest. Like everyone, we always hope for the best as we monitor the breeding efforts of wild birds that we are lucky enough to get to know personally through live-streaming cameras.
A reduction in the adductor chambers has also occurred  These are all conditions seen in the juvenile form of their ancestors. The premaxillary bone has also hypertrophied to form the beak while the maxilla has become diminished, as suggested by both developmental  and paleontological  studies. This expansion into the beak has occurred in tandem with the loss of a functional hand and the developmental of a point at the front of the beak that resembles a "finger".
The structure of the avian skull has important implications for their feeding behaviours. Birds show independent movement of the skull bones known as cranial kinesis. Cranial kinesis in birds occurs in several forms, but all of the different varieties are all made possible by the anatomy of the skull. Animals with large, overlapping bones including the ancestors of modern birds  have akinetic non-kinetic skulls.
Birds have a diapsid skull, as in reptiles, with a pre-lachrymal fossa present in some reptiles. The skull has a single occipital condyle. The shoulder consists of the scapula shoulder blade , coracoid , and humerus upper arm.
The humerus joins the radius and ulna forearm to form the elbow. The carpus and metacarpus form the "wrist" and "hand" of the bird, and the digits are fused together. The bones in the wing are extremely light so that the bird can fly more easily. The hips consist of the pelvis, which includes three major bones: These are fused into one the innominate bone.
Innominate bones are evolutionary significant in that they allow birds to lay eggs. They meet at the acetabulum hip socket and articulate with the femur, which is the first bone of the hind limb. The upper leg consists of the femur. At the knee joint, the femur connects to the tibiotarsus shin and fibula side of lower leg. The tarsometatarsus forms the upper part of the foot, digits make up the toes. The leg bones of birds are the heaviest, contributing to a low center of gravity, which aids in flight.
They have a greatly elongate tetradiate pelvis , similar to some reptiles. The hind limb has an intra-tarsal joint found also in some reptiles. There is extensive fusion of the trunk vertebrae as well as fusion with the pectoral girdle. Birds' feet are classified as anisodactyl , zygodactyl , heterodactyl , syndactyl or pamprodactyl.
This is common in songbirds and other perching birds , as well as hunting birds like eagles , hawks , and falcons. Syndactyly, as it occurs in birds, is like anisodactyly, except that the third and fourth toes the outer and middle forward-pointing toes , or three toes, are fused together, as in the belted kingfisher Ceryle alcyon.
This is characteristic of Coraciiformes kingfishers , bee-eaters , rollers , etc. This arrangement is most common in arboreal species, particularly those that climb tree trunks or clamber through foliage. Zygodactyly occurs in the parrots , woodpeckers including flickers , cuckoos including roadrunners , and some owls.
Zygodactyl tracks have been found dating to — Ma early Cretaceous , 50 million years before the first identified zygodactyl fossils. Heterodactyly is like zygodactyly, except that digits three and four point forward and digits one and two point back. This is found only in trogons , while pamprodactyl is an arrangement in which all four toes may point forward, or birds may rotate the outer two toes backward.
It is a characteristic of swifts Apodidae. Most birds have approximately different muscles, mainly controlling the wings, skin, and legs. They provide the powerful wing stroke essential for flight.
The muscle medial to underneath the pectorals is the supracoracoideus. It raises the wing between wingbeats. Both muscle groups attach to the keel of the sternum. This is remarkable, because other vertebrates have the muscles to raise the upper limbs generally attached to areas on the back of the spine. The skin muscles help a bird in its flight by adjusting the feathers, which are attached to the skin muscle and help the bird in its flight maneuvers.
There are only a few muscles in the trunk and the tail, but they are very strong and are essential for the bird. The pygostyle controls all the movement in the tail and controls the feathers in the tail.
This gives the tail a larger surface area which helps keep the bird in the air. The scales of birds are composed of keratin, like beaks, claws, and spurs. They are found mainly on the toes and tarsi lower leg of birds , usually up to the tibio-tarsal joint, but may be found further up the legs in some birds.
In many of the eagles and owls the legs are feathered down to but not including their toes. The scales and scutes of birds were originally thought to be homologous to those of reptiles;  however, more recent research suggests that scales in birds re-evolved after the evolution of feathers.
Bird embryos begin development with smooth skin. On the feet, the corneum , or outermost layer, of this skin may keratinize, thicken and form scales. These scales can be organized into;. The rows of scutes on the anterior of the metatarsus can be called an "acrometatarsium" or "acrotarsium".
Reticula are located on the lateral and medial surfaces sides of the foot and were originally thought to be separate scales. However, histological and evolutionary developmental work in this area revealed that these structures lack beta-keratin a hallmark of reptilian scales and are entirely composed of alpha-keratin.
The bills of many waders have Herbst corpuscles which help them find prey hidden under wet sand, by detecting minute pressure differences in the water. However this is more prominent in some birds and can be readily detected in parrots. The region between the eye and bill on the side of a bird's head is called the lore. This region is sometimes featherless, and the skin may be tinted, as in many species of the cormorant family. The beak, bill, or rostrum is an external anatomical structure of birds which is used for eating and for grooming , manipulating objects, killing prey, fighting, probing for food, courtship and feeding young.
Although beaks vary significantly in size, shape and color, they share a similar underlying structure. Two bony projections—the upper and lower mandibles—covered with a thin keratinized layer of epidermis known as the rhamphotheca.
In most species, two holes known as nares lead to the respiratory system. Due to the high metabolic rate required for flight, birds have a high oxygen demand. Their highly effective respiratory system helps them meet that demand. Although birds have lungs, these are fairly rigid structures, which do not expand and contract as they do in mammals, reptiles and many amphibians.
The structures that act as the bellows which ventilate the lungs, are the air sacs distributed throughout much of the birds' bodies. The walls of these air sacs do not have a good blood supply and so do not play a direct role in gas exchange. They act like a set of bellows  which move air unidirectionally through the parabronchi of the rigid lungs. Birds lack a diaphragm , and therefore use their intercostal and abdominal muscles to expand and contract their entire thoraco-abdominal cavities, thus rhythmically changing the volumes of all their air sacs in unison illustration on the right.
The active phase of respiration in birds is exhalation, requiring contraction of their muscles of respiration. Three distinct sets of organs perform respiration — the anterior air sacs interclavicular, cervicals, and anterior thoracics , the lungs , and the posterior air sacs posterior thoracics and abdominals.
Typically there are nine air sacs within the system;  however, that number can range between seven and twelve, depending on the species of bird. Passerines possess seven air sacs, as the clavicular air sacs may interconnect or be fused with the anterior thoracic sacs.
During inhalation, environmental air initially enters the bird through the nostrils from where it is heated, humidified, and filtered in the nasal passages and upper parts of the trachea.
The primary bronchi enter the lungs to become the intrapulmonary bronchi, which give off a set of parallel branches called ventrobronchi and, a little further on, an equivalent set of dorsobronchi. Each pair of dorso-ventrobronchi is connected by a large number of parallel microscopic air capillaries or parabronchi where gas exchange occurs.
From the dorsobronchi the air flows through the parabronchi and therefore the gas exchanger to the ventrobronchi from where the air can only escape into the expanding anterior air sacs. So, during inhalation, both the posterior and anterior air sacs expand,  the posterior air sacs filling with fresh inhaled air, while the anterior air sacs fill with "spent" oxygen-poor air that has just passed through the lungs.
During exhalation the intrapulmonary bronchi were believed to be tightly constricted between the region where the ventrobronchi branch off and the region where the dorsobronchi branch off.
From there the fresh air from the posterior air sacs flows through the parabronchi in the same direction as occurred during inhalation into ventrobronchi. The air passages connecting the ventrobronchi and anterior air sacs to the intrapulmonary bronchi open up during exhalation, thus allowing oxygen-poor air from these two organs to escape via the trachea to the exterior.
The blood flow through the bird lung is at right angles to the flow of air through the parabronchi, forming a cross-current flow exchange system see illustration on the left. The blood capillaries leaving the exchanger near the entrance of airflow take up more O 2 than do the capillaries leaving near the exit end of the parabronchi. When the contents of all capillaries mix, the final partial pressure of oxygen of the mixed pulmonary venous blood is higher than that of the exhaled air,   but is nevertheless less than half that of the inhaled air,  thus achieving roughly the same systemic arterial blood partial pressure of oxygen as mammals do with their bellows-type lungs.
The trachea is an area of dead space: In comparison to the mammalian respiratory tract , the dead space volume in a bird is, on average, 4. In some birds e.
Air passes unidirectionally through the lungs during both exhalation and inspiration, causing, except for the oxygen-poor dead space air left in the trachea after exhalation and breathed in at the beginning of inhalation, little to no mixing of new oxygen-rich air with spent oxygen-poor air as occurs in mammalian lungs , changing only from oxygen-rich to oxygen-poor as it moves unidirectionally through the parabronchi. Avian lungs do not have alveoli as mammalian lungs do. Instead they contain millions of narrow passages known as parabronchi, connecting the dorsobronchi to the ventrobronchi at either ends of the lungs.
Air flows anteriorly caudal to cranial through the parallel parabronchi.