Nature Video - Hummingbirds: The most commonly distributed UV hues, however, are structural colors produced by light scattering from the spongy medullary layer of feather barbs. In birds, refers to unusually large numbers of non-migratory birds moving out of their typical nesting range. Also used on a Magic Halo. Oviparous animals are animals that lay eggs, with little or no other development within the mother. Sometimes migration is moving between elevations between mountains and adjacent lowlands as in Western Bluebirds.
Domestic fowl require access to grit. Stones swallowed by ostriches can exceed a length of 10 centimetres 3. Amphibians such as the Axolotl are also known to deliberately ingest rocks that are presumed to be gastroliths. Some extinct animals such as sauropod dinosaurs appear to have used stones to grind tough plant matter. A rare example of this is the Early Cretaceous theropod Caudipteryx zoui from northeastern China, which was discovered with a series of small stones, interpreted as gastroliths, in the area of its skeleton that would have corresponded with its abdominal region.
Aquatic animals, such as plesiosaurs , may have used them as ballast, to help balance themselves or to decrease their buoyancy, as crocodiles do. Gastroliths associated with dinosaur fossils can weigh several kilograms. In , George Reber Weiland reported the presence of worn and polished quartz pebbles associated with the remains of plesiosaurs and sauropod dinosaurs and interpreted these stones as gastroliths.
Brown was among the first paleontologist to recognize that dinosaurs used gastroliths in their digestive systems to aid in the grinding of food. In , Friedrich von Huene found stones in Late Triassic sediments, in association with the fossil remains of the prosauropod Sellosaurus and interpreted them as gastroliths.
In , William Lee Stokes recognized the presence of gastroliths in the remains of sauropod dinosaurs recovered from Late Jurassic strata. Geologists usually require several pieces of evidence before they will accept that a rock was used by a dinosaur to aid its digestion. First, the stone must be unlike the rock found in its geological vicinity. Secondly, it should be rounded and polished, because inside a dinosaur's gizzard any genuine gastrolith would have been acted upon by other stones and fibrous materials in a process similar to the action of a rock tumbler.
Lastly, the stone must be found with the fossils of the dinosaur which ingested it. It is this last criterion that causes trouble in identification, as smooth stones found without context can possibly erroneously in some cases be dismissed as having been polished by water or wind.
Whittle ,9 pioneered scanning electron microscope analysis of wear patterns on gastroliths. Wings found that ostrich gastroliths would be deposited outside the skeleton if the carcass was deposited in an aquatic environment for as little as a few days following death. The species was described in after having been found in rocks of the Huajiying Formation of northeastern China that date to The other major group of toothed Cretaceous birds, the Odontornithes, included one of the best-known groups of fossil birds, Hesperornis and its relatives.
These birds were highly specialized foot-propelled divers of the Late Cretaceous. Hesperornis was up to 1. The sternum lacked a keel, the humerus was small and weak, and other elements of the wing were missing entirely. The pelvis and hind limb had a strong but superficial resemblance to those of modern loons and grebes.
However, two major features and several less obvious ones indicate that the resemblance was the result of convergent evolution rather than common ancestry. Hesperornis was remarkable for three features. It had 1 teeth set in grooves, not sockets, 2 a stout fourth toe with a unique rotary ball-and-flange type of articulation, and 3 tail caudal vertebrae with limited vertical motion, making the tail somewhat beaverlike in its action.
Baptornis , a contemporary relative of Hesperornis , was smaller and less strongly modified. Though flightless, its wings were less reduced than Hesperornis , and it lacked the peculiar modifications of the fourth toe and caudal vertebrae.
In the Late Cretaceous also appeared the first modern birds, assigned to the infraclass Neornithes , or Carinata.
Living alongside Hesperornis and other Odontornithes was a group of flying birds that included Ichthyornis and Apatornis. Although not related to gulls, these birds resembled them superficially and may well have been their ecological counterparts. It was long believed that Ichthyornis had teeth, like Hesperornis , but it is now thought that the toothed jaws formerly thought to belong to Ichthyornis were really those of a small mosasaur , a marine reptile.
A genetic study of bird lineages estimated that birds began to lose the outer covering of enamel from their teeth, and possibly their teeth as well, some million years ago during the Early Cretaceous. The study also showed that this change was accompanied by an increase in beak development that assisted in the diversification of living birds. In the evolution of modern birds from an Archaeopteryx -like form, the development of active flight must have occurred early.
This meant an increase in size of the muscles moving the wing and the development of a keel on the sternum as an added area of attachment for these muscles. As the tail took on more of a steering function and less of a supportive one, it became shorter and more readily moved as a unit. Feathers became specialized for different functions, and at the same time the eyes, brain, and respiratory and circulatory systems continued to develop in a manner associated with the evolution of homeothermic, arboreal, gliding animals.
By the time birds became strong fliers, they were ready to exploit many new environments , and by the Cretaceous Period they had begun to do so, producing the wide array of adaptive types known today.
The major diversification of modern birds probably took place in the Cretaceous, and it must have started early in that period because fragmentary fossil evidence of foot-propelled divers Enaliornis and of an early relative of the flamingos Gallornis are known from Lower Cretaceous deposits in Europe. Upper Cretaceous deposits have yielded, besides Hesperornis and Ichthyornis and their relatives, diving birds similar to Enaliornis , other early flamingo-like birds, and species in the same suborders as gannets , ibises , rails , and shorebirds.
Deposits from the Paleocene Epoch In addition, large, flightless predatory birds culminating in Diatryma made their appearance during this period. From the far richer Eocene Epoch Almost certainly all living orders and most living families of birds were in existence by the end of the Eocene.
One of the most interesting finds from this period was fossils of Neocathartes, a long-legged bird allied to the New World vultures. There are several anatomical similarities between this group of vultures and the storks , and the existence of this fossil lends support to the idea that the storks and New World vultures are more closely related to each other than each family is to the birds with which it is usually grouped.
By Pliocene times 5. After the extinction of the dinosaurs and before large carnivorous mammals evolved, two groups of large flightless birds evolved to fill a similar niche. From the late Paleocene to the middle Eocene, Diatryma and its relatives were major predators in the Northern Hemisphere. The largest species stood over 2. They are of uncertain relationships but may have been distantly related to the cranes and rails order Gruiformes. The second group, that of Phororhacos and related genera, had a long history from the lower Oligocene to the middle Pliocene in South America , which was without large carnivores until relatively recent times.
The Phororhacos line evidently evolved from seriema -like stock and radiated into numerous genera and species, the largest of them Onactornis standing 2. Large grazing or browsing birds appear to have evolved several times.
On continents where there are large predators, these birds have always been rapid runners ostriches , rheas , emus , but on islands lacking such predators, they were slow-moving, heavy-bodied birds. Two such groups were the elephant birds of Madagascar and the moas of New Zealand , the largest in each group approaching 3 metres 10 feet in height. Fragmentary fossil material from Eocene and Oligocene deposits in Egypt indicates that similarly adapted birds occurred there before the advent of large carnivores.
In classifying birds, most systematists have historically relied upon structural characteristics to infer evolutionary relationships. Plumage characteristics include the number of various feather types; the presence or absence of down on the feather tracts and on the preen gland; and the presence or absence of an aftershaft.
Characteristics of the bill and feet are also useful, as is the arrangement of bones in the palate and around the nostrils. The presence or absence of certain thigh muscles is considered, as are the arrangement of the carotid arteries, the syrinx , and the deep flexor tendons of the toes as well as the condition of the young when hatched. Advances in the study of DNA sequences and computerized construction of phylogenetic trees have provided new means of testing hypotheses of taxonomic relationships.
It has frequently been stated that birds are one of the best known of animal groups. This is true in the sense that most of the living species and subspecies in the world have probably been described; but because of inadequacies in the fossil record and repeated cases of convergent evolution within the group, our knowledge of the phylogenetic relationships between orders, suborders, and families of birds is inferior to that of mammals and reptiles.
Most, if not all, of the major lineages of modern birds arose rapidly in the Late Cretaceous and the Paleogene Period about million to 23 million years ago. DNA data continue to resolve the relationships among major groups of birds. The penguins Sphenisciformes , tube-nosed seabirds Procellariiformes , and pelicans Pelecaniformes form a triad of related lineages. Waterfowl Anseriformes and chickenlike birds Galliformes are linked and together may be the oldest assemblage of modern birds.
Some caprimulgiforms owlet frogmouths seem clearly related to swifts Apodiformes through a link between owlet frogmouths and treeswifts. The taxonomic positions of several bird groups remain open to question. The hoatzin , included below in the Cuculiformes, is often given its own order, Opisthocomiformes. The sandgrouse are listed separately in order Pteroclidiformes. The turacos, sometimes included in the Cuculiformes, are considered by many authors to warrant separation and are listed here as Musophagiformes.
Diatryma and several related genera of extinct flightless predators are often placed in a distinct order, Diatrymiformes, near Gruiformes.
The flamingos , which constitute the order Phoenicopteriformes in some classifications, are placed in the Ciconiiformes in this classification , but their relationships are still unknown. One area particularly in need of study is the relationships among the various groups of ratites ostriches, rheas, emus, moas , and others. Formerly, some authorities argued that these birds and the penguins arose independently from cursorial reptiles, but it is now generally agreed that all of them passed through a flying stage in the course of their evolution.
The ratite groups differ greatly in morphology and yet show remarkable similarities in palate and bill characters. The principal unanswered questions are how many different flightless lines evolved from flying ancestors and from how many different groups the flying ancestors evolved.
On zoogeographic grounds, it is likely that the isolated kiwi-moa, elephant bird , and emu-cassowary lines arose independently from each other and from ratites on the other continents.
But the ostriches and rheas could be descended from a common flightless ancestor because of the known former land connections from Asia to North and South America. Kiwis, ostriches, rheas, emus, and cassowaries are contained within order Struthioniformes in this classification.
All species of birds with the exception of the penguin, have a small region of their lungs devoted to "neopulmonic parabronchi". This unorganized network of microscopic tubes branches off from the posterior air sacs, and open haphazardly into both the dorso- and ventrobronchi, as well as directly into the intrapulmonary bronchi. Unlike the parabronchi, in which the air moves unidirectionally, the air flow in the neopulmonic parabronchi is bidirectional.
The syrinx is the sound-producing vocal organ of birds, located at the base of a bird's trachea. As with the mammalian larynx , sound is produced by the vibration of air flowing across the organ. The syrinx enables some species of birds to produce extremely complex vocalizations, even mimicking human speech. In some songbirds, the syrinx can produce more than one sound at a time. Birds have a four-chambered heart ,  in common with mammals, and some reptiles mainly the crocodilia.
This adaptation allows for an efficient nutrient and oxygen transport throughout the body, providing birds with energy to fly and maintain high levels of activity. A ruby-throated hummingbird 's heart beats up to times per minute about 20 beats per second.
Many birds possess a muscular pouch along the esophagus called a crop. The crop functions to both soften food and regulate its flow through the system by storing it temporarily. The size and shape of the crop is quite variable among the birds. The avian stomach is composed of two organs, the proventriculus and the gizzard that work together during digestion. The proventriculus is a rod shaped tube, which is found between the esophagus and the gizzard, that secretes hydrochloric acid and pepsinogen into the digestive tract.
The gizzard is composed of four muscular bands that rotate and crush food by shifting the food from one area to the next within the gizzard. The gizzard of some species of herbivorous birds, like turkey and quails,  contains small pieces of grit or stone called gastroliths that are swallowed by the bird to aid in the grinding process, serving the function of teeth. The use of gizzard stones is a similarity found between birds and dinosaurs , which left gastroliths as trace fossils.
The partially digested and pulverized gizzard contents, now called a bolus, are passed into the intestine , where pancreatic and intestinal enzymes complete the digestion of the digestible food. The digestion products are then absorbed through the intestinal mucosa into the blood.
The intestine ends via the large intestine in the vent or cloaca which serves as the common exit for renal and intestinal excrements as well as for the laying of eggs. There are three general ways in which birds drink: Fluid is also obtained from food.
Most birds are unable to swallow by the "sucking" or "pumping" action of peristalsis in their esophagus as humans do , and drink by repeatedly raising their heads after filling their mouths to allow the liquid to flow by gravity, a method usually described as "sipping" or "tipping up". The only other group, however, which shows the same behavior, the Pteroclidae , is placed near the doves just by this doubtlessly very old characteristic.
Although this general rule still stands, since that time, observations have been made of a few exceptions in both directions. In addition, specialized nectar feeders like sunbirds Nectariniidae and hummingbirds Trochilidae drink by using protrusible grooved or trough-like tongues, and parrots Psittacidae lap up water. Many seabirds have glands near the eyes that allow them to drink seawater.
Excess salt is eliminated from the nostrils. Many desert birds get the water that they need entirely from their food. The elimination of nitrogenous wastes as uric acid reduces the physiological demand for water,  as uric acid is not very toxic and thus does not need to be diluted in as much water. Male birds have two testes which become hundreds of times larger during the breeding season to produce sperm. Some species of birds have two functional ovaries, and the order Apterygiformes always retain both ovaries.
Most male birds have no phallus. In the males of species without a phallus, sperm is stored in the seminal glomera within the cloacal protuberance prior to copulation. During copulation , the female moves her tail to the side and the male either mounts the female from behind or in front as in the stitchbird , or moves very close to her. The cloacae then touch, so that the sperm can enter the female's reproductive tract. This can happen very fast, sometimes in less than half a second.
The sperm is stored in the female's sperm storage tubules for a period varying from a week to more than days,  depending on the species. Then, eggs will be fertilized individually as they leave the ovaries, before the shell is calcified in the oviduct.
After the egg is laid by the female, the embryo continues to develop in the egg outside the female body. Many waterfowl and some other birds, such as the ostrich and turkey , possess a phallus.
This appears to be the primitive condition among birds, most birds have lost the phallus. These vaginal structures may be used to prevent penetration by the male phallus which coils counter-clockwise. In these species, copulation is often violent and female co-operation is not required; the female ability to prevent fertilization may allow the female to choose the father for her offspring.
After the eggs hatch, parents 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 though, are often totally incapable of unassisted survival. The process whereby a chick acquires feathers until it can fly is called "fledging". Some birds, such as pigeons, geese, and red-crowned cranes , remain with their mates for life and may produce offspring on a regular basis.
Avian kidneys function in almost the same way as the more extensively studied mammalian kidney, but with a few important adaptations; while much of the anatomy remains unchanged in design, some important modifications have occurred during their evolution. A bird has paired kidneys which are connected to the lower gastrointestinal tract through the ureters. Blood vessels and other tubes make up the remaining mass. Unique to birds is the presence of two different types of nephrons the functional unit of the kidney both reptilian-like nephrons located in the cortex and mammalian-like nephrons located in the medulla.
Reptilian nephrons are more abundant but lack the distinctive loops of Henle seen in mammals. The urine collected by the kidney is emptied into the cloaca through the ureters and then to the colon by reverse peristalsis. Birds have acute eyesight—raptors birds of prey have vision eight times sharper than humans—thanks to higher densities of photoreceptors in the retina up to 1,, per square mm in Buteos , compared to , for humans , a high number of neurons in the optic nerves , a second set of eye muscles not found in other animals, and, in some cases, an indented fovea which magnifies the central part of the visual field.
Many species, including hummingbirds and albatrosses , have two foveas in each eye. Many birds can detect polarised light. The avian ear is adapted to pick up on slight and rapid changes of pitch found in bird song.
General avian tympanic membrane form is ovular and slightly conical. Morphological differences in the middle ear are observed between species. Ossicles within green finches, blackbirds, song thrushes, and house sparrows are proportionately shorter to those found in pheasants, Mallard ducks, and sea birds.
In song birds, a syrinx allows the respective possessors to create intricate melodies and tones. The middle avian ear is made up of three semicircular canals, each ending in an ampulla and joining to connect with the macula sacculus and lagena, of which the cochlea, a straight short tube to the external ear, branches from. Birds have a large brain to body mass ratio.
This is reflected in the advanced and complex bird intelligence. The immune system of birds resembles that of other animals. Birds have both innate and adaptive immune systems. Birds are susceptible to tumours , immune deficiency and autoimmune diseases.
The bursa of fabricius , also known as the cloacal bursa, is a lymphoid organ which aids in the production of B lymphocytes during humoral immunity. The bursa of fabricius is present during juvenile stages but curls up, and in the sparrow is not visible after the sparrow reaches sexual maturity. The bursa of fabricius is a circular pouch connected to the superior dorsal side of the cloaca.
The bursa is composed of many folds, known as plica, which are lined by more than 10, follicles encompassed by connective tissue and surrounded by mesenchyme. Each follicle consists of a cortex that surrounds a medulla. The cortex houses the highly compacted B lymphocytes , whereas the medulla houses lymphocytes loosely.
There are , B lymphocytes located around each follicle. From Wikipedia, the free encyclopedia. This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.
September Learn how and when to remove this template message. Bird feet and legs. Comb anatomy , Lore anatomy , and Gular skin. Bird vision and Avian pallium. The Evolution and Extinction of the Dinosaurs second ed. Onderstepoort Journal of Veterinary Research. The Onderstepoort Veterinary Institute. Fun Science Facts from the Library of Congress". How stable is the hold phase?