Bird eggs are laid by the females and range in quantity from one (as in condors) to up to seventeen (the grey partridge). Clutch size may vary latitudinally within a species. Some birds lay eggs even when the eggs have not been fertilized; it is not uncommon for pet owners to find their lone bird nesting on a clutch of infertile eggs, which are sometimes called wind-eggs.
All bird eggs contain the following components: [1]
The default color of vertebrate eggs is the white of the calcium carbonate from which the shells are made, but some birds, mainly passerines, produce colored eggs. The pigments biliverdin and its zinc chelate give a green or blue ground color, and protoporphyrin produces reds and browns as a ground color or as spotting. Non-passerines typically have white eggs, except in some ground-nesting groups, such as the Charadriiformes, sandgrouse and nightjars, where camouflage is necessary, and some parasitic cuckoos, which lay eggs that often closely match the passerine host's egg. To varying degrees of success, host passerine species have combined speckles, blotches and other egg markings [2] with visual pattern-recognition abilities to help them reject cuckoos' counterfeit eggs. Most other passerines, in contrast, lay colored eggs, even if there is no need of cryptic colors. [3]
However, the photographic markings on passerine eggs have been suggested to reduce brittleness by acting as a solid state lubricant. [4] If insufficient calcium is available in the local soil, the egg shell may be thin, especially in a circle around the broad end. Protoporphyrin speckling compensates for this, and increases inversely to the amount of calcium in the soil. [5] To understand the functional significance of eggshell spotting (or maculation) it is important to quantify this pigmentation [6] For the same reason, later eggs in a clutch are more spotted than early ones, as the female's store of calcium is depleted.
Birds which build in trees generally have blue or greenish eggs, either spotted or unspotted, while birds that build in bushes or near or on the ground are likely to lay speckled eggs. The color of individual eggs is also genetically influenced, and appears to be inherited through the mother only, suggesting the gene responsible for pigmentation is on the female-specific W chromosome (female birds are WZ, males are ZZ). Color was once thought to be applied to the shell immediately before laying, but this research shows coloration is an integral part of the development of the shell, with the same protein responsible for depositing calcium carbonate, or protoporphyrins when there is a lack of that mineral. [7]
In species such as the common guillemot, which nest in large groups, each female's eggs have very different markings, making it easier for females to identify their own eggs on the crowded cliff ledges on which they breed. [8]
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Bird eggshells are diverse. For example:
Tiny pores in a bird eggshell allow the embryo to breathe. The domestic hen's egg has around 7500 pores. [9]
The shape of eggs varies considerably across bird species, ranging from near-spherical (such as those of the little bee-eater) to highly pyriform or conical (such as those of the common murre) with the familiar shape of the chicken egg lying in between. Early scientific investigators of egg shape [10] suggested that the oval shape of eggs was caused by the egg being forced through the oviduct by peristalsis. In this often-repeated [11] [12] [13] but incorrect theory of egg shape formation, the contraction and relaxation of the muscles which push the egg down the oviduct cause the spherical egg membrane to distort slightly into an ovoid shape, with the blunt end caudal (i.e. furthest down the oviduct and closest to the cloaca). The calcification of the egg in the shell gland/uterus then fixes it in this shape, and the egg is laid with the blunt end appearing first (for a diagram showing the different sections of the bird oviduct, see here). However, this theory has been refuted by studies of egg shell formation in a number of bird species using techniques such as X-ray photography [14] [15] which have demonstrated that egg shape is determined in the oviduct isthmus (before shell calcification) with the pointed end caudal (furthest down the oviduct). These observations cannot be explained by peristalsis. It has been proposed [14] that the egg acquires its shape (with the pointed egg caudal) as it is forced through the narrow isthmus, but this assertion has not been thoroughly verified.[ citation needed ]
Cliff-nesting birds often have highly conical eggs. They are less likely to roll off, tending instead to roll around in a tight circle; this trait is likely to have arisen due to evolution via natural selection. In contrast, many hole-nesting birds have nearly spherical eggs.[ citation needed ]
The shape has biological significance. A pointed egg will tend to sit on its side, with the big end tipped upward. The big end contains the air sac and its shell is pierced by a higher density of pores than the more pointed end. Tipping the big end upwards improves oxygen flow to the large head, with the physiologically demanding eyes and brain, that develops in the big end while the tail develops at the more pointed end.[ citation needed ]
In a 2017 publication in the journal Science , mathematical modeling of 50,000 bird eggs data showed that bird egg shape is a product of flight adaptations and not the outcome of nesting conditions or a bird's life history. [16] A strong correlation was found between egg shape and flight ability on broad taxonomic scales, such that birds engaging in high powered flights usually maximize egg size by having elliptical shaped eggs while maintaining a streamlined body plan. [16] Selection acting during incubation, such as breeding site and incubation posture, are also thought to influence egg-shape variation across birds. [17]
Egg size tends to be proportional to the size of the adult bird,[ citation needed ] from the half gram egg of the bee hummingbird to the 1.5 kg egg of the ostrich. Kiwis have disproportionately large eggs, up to 20% of the female's body weight. [18] This evolutionary trait results in kiwi chicks that can emerge from the nest days after hatching, ready to start foraging for food.[ citation needed ]
The number of eggs laid in a single brood is referred to as the clutch. Clutch size is usually within a small range of variation. Some birds respond to the accidental loss of eggs by laying a replacement egg. Others will stop laying based on the apparent size of the clutch. According to whether they respond to addition, removal, or both addition and removal of eggs, birds are classified as determinate layers (number of eggs laid is predetermined and do not respond to change) or as indeterminate layers. In general, birds with small clutch size tended to be indeterminate. [19]
Many animals feed on eggs. For example, the principal predators of the black oystercatcher's eggs include raccoons, skunks, mink, river and sea otters, gulls, crows and foxes. The stoat (Mustela erminea) and long-tailed weasel (Neogale frenata) steal ducks' eggs. Ostrich eggs are vulnerable to many predators ranging from mongooses and Egyptian vultures, to hyenas. [20] Snakes of the genera Dasypeltis and Elachistodon specialize in eating eggs. Humans have a long history of both eating wild bird eggs and raising birds for farmed eggs for consumption.[ citation needed ]
Brood parasitism occurs in birds when one species lays its eggs in the nest of another. In some cases, the host's eggs are removed or eaten by the female, or expelled by her chick. Brood parasites include cowbirds, honeyguides, and many Old World cuckoos.[ citation needed ]
Today, the hard shells of bird eggs are unique among animals. However, this trait evolved in the larger group to which birds belong: the dinosaurs. The bird egg is thus a dinosaur egg. Eggs of the modern type, with an inner mammillary, more outer prismatic and an outer crystalline layer, appear during the Jurassic and are associated with fossils of the Maniraptora. [21]
The oviduct in vertebrates is the passageway from an ovary. In human females, this is more usually known as the fallopian tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, or will degenerate in the body. Normally, these are paired structures, but in birds and some cartilaginous fishes, one or the other side fails to develop, and only one functional oviduct can be found.
The black-headed gull is a small gull that breeds in much of the Palearctic including Europe and also in coastal eastern Canada. Most of the population is migratory and winters further south, but some birds reside in the milder westernmost areas of Europe. The species also occurs in smaller numbers in northeastern North America, where it was formerly known as the common black-headed gull.
An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the animal hatches.
Egg incubation is the process by which an egg, of oviparous (egg-laying) animals, develops an embryo within the egg, after the egg's formation and ovipositional release. Egg incubation is done under favorable environmental conditions, possibly by brooding and hatching the egg.
The welcome swallow is a small passerine bird in the swallow family Hirundinidae that is native to Australia and nearby islands. It has self-introduced into New Zealand in the middle of the twentieth century. It is very similar to the Pacific swallow with which it is often considered conspecific.
Oviraptoridae is a group of bird-like, herbivorous and omnivorous maniraptoran dinosaurs. Oviraptorids are characterized by their toothless, parrot-like beaks and, in some cases, elaborate crests. They were generally small, measuring between one and two metres long in most cases, though some possible oviraptorids were enormous. Oviraptorids are currently known only from the Late Cretaceous of Asia, with the most well-known species and complete specimens found only in the Gobi Desert of Mongolia and northwestern China.
A bird nest is the spot in which a bird lays and incubates its eggs and raises its young. Although the term popularly refers to a specific structure made by the bird itself—such as the grassy cup nest of the American robin or Eurasian blackbird, or the elaborately woven hanging nest of the Montezuma oropendola or the village weaver—that is too restrictive a definition. For some species, a nest is simply a shallow depression made in sand; for others, it is the knot-hole left by a broken branch, a burrow dug into the ground, a chamber drilled into a tree, an enormous rotting pile of vegetation and earth, a shelf made of dried saliva or a mud dome with an entrance tunnel. The smallest bird nests are those of some hummingbirds, tiny cups which can be a mere 2 cm (0.8 in) across and 2–3 cm (0.8–1.2 in) high. At the other extreme, some nest mounds built by the dusky scrubfowl measure more than 11 m (36 ft) in diameter and stand nearly 5 m (16 ft) tall. The study of birds' nests is known as caliology.
An eggshell is the outer covering of a hard-shelled egg and of some forms of eggs with soft outer coats.
The king quail, also known as the blue-breasted quail, Asian blue quail, Chinese painted quail, or Chung-Chi, is a species of Old World quail in the family Phasianidae. This species is the smallest "true quail", ranging in the wild from southern China, South and Southeast Asia to Oceania, south to southeastern Australia, with 9 different subspecies. A failed attempt was made to introduce this species to New Zealand by the Otago Acclimatisation Society in the late 1890s. It is quite common in aviculture worldwide, where it is sometimes misleadingly known as the "button quail", which is the name of an only very distantly related family of birds, the buttonquails.
The white-fronted plover or white-fronted sandplover is a small shorebird of the family Charadriidae that inhabits sandy beaches, dunes, mudflats and the shores of rivers and lakes in sub-saharan Africa and Madagascar. It nests in small shallow scrapes in the ground and lays clutches of one to three eggs. The species is monogamous and long-lived, with a life expectancy of approximately 12 years. The vast majority of pairs that mate together stay together during the following years of breeding and retain the same territory. The white-fronted plover has a similar appearance to the Kentish plover, with a white fore crown and dark bands connecting the eyes to the bill.
The black honeyeater is a species of bird in the honeyeater family Meliphagidae. The black honeyeater exhibits sexual dimorphism, with the male being black and white while the female is a speckled grey-brown; immature birds look like the female. The species is endemic to Australia, and ranges widely across the arid areas of the continent, through open woodland and shrubland, particularly in areas where the emu bush and related species occur.
Dinosaur eggs are the organic vessels in which a dinosaur embryo develops. When the first scientifically documented remains of non-avian dinosaurs were being described in England during the 1820s, it was presumed that dinosaurs had laid eggs because they were reptiles. In 1859, the first scientifically documented dinosaur egg fossils were discovered in France by Jean-Jacques Poech, although they were mistaken for giant bird eggs.
Dinosaur reproduction shows correlation with archosaur physiology, with newborns hatching from eggs that were laid in nests. Dinosaurs did not nurture their offspring as mammals typically do, and because dinosaurs did not nurse, it is likely that most dinosaurs were capable of surviving on their own after hatching. Although, parental care may have been required for some dinosaur species, as shown by fossil evidence. Dinosaur reproduction also required a mate; evidence of sexual dimorphism and courting displays have been found from fossil scrapings in sandstone and feathers on dinosaurs that lacked flight.
Elongatoolithus is an oogenus of dinosaur eggs found in the Late Cretaceous formations of China and Mongolia. Like other elongatoolithids, they were laid by small theropods, and were cared for and incubated by their parents until hatching. They are often found in nests arranged in multiple layers of concentric rings. As its name suggests, Elongatoolithus was a highly elongated form of egg. It is historically significant for being among the first fossil eggs given a parataxonomic name.
Macroelongatoolithus is an oogenus of large theropod dinosaur eggs, representing the eggs of giant caenagnathid oviraptorosaurs. They are known from Asia and from North America. Historically, several oospecies have been assigned to Macroelongatoolithus, however they are all now considered to be a single oospecies: M. carlylensis.
Continuoolithus is an oogenus of dinosaur egg found in the late Cretaceous of North America. It is most commonly known from the late Campanian of Alberta and Montana, but specimens have also been found dating to the older Santonian and the younger Maastrichtian. It was laid by an unknown type of theropod. These small eggs are similar to the eggs of oviraptorid dinosaurs, but have a distinctive type of ornamentation.
Clutch size refers to the number of eggs laid in a single brood by a nesting pair of birds. The numbers laid by a particular species in a given location are usually well defined by evolutionary trade-offs with many factors involved, including resource availability and energetic constraints. Several patterns of variation have been noted and the relationship between latitude and clutch size has been a topic of interest in avian reproduction and evolution. David Lack and R.E. Moreau were among the first to investigate the effect of latitude on the number of eggs per nest. Since Lack's first paper in the mid-1940s there has been extensive research on the pattern of increasing clutch size with increasing latitude. The proximate and ultimate causes for this pattern have been a subject of intense debate involving the development of ideas on group, individual, and gene-centric views of selection.
Elongatoolithidae is an oofamily of fossil eggs, representing the eggs of oviraptorosaurs. They are known for their highly elongated shape. Elongatoolithids have been found in Europe, Asia, and both North and South America.
The Masai ostrich, also known as the East African ostrich is a red-necked subspecies variety of the common ostrich and is endemic to East Africa. It is one of the largest birds in the world, second only to its sister subspecies Struthio camelus camelus. Today it is farmed for eggs, meat, and feathers.
The egg of the ostrich is the largest of any living bird. The shell has a long history of use by humans as a container and for decorative artwork, including beads. The eggs are not commonly eaten.
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