The bird egg is one of the yolk- rich (polylecithalen) eggs and corresponds in structure to the eggs of reptiles and egg-laying mammals ( monotons ). That is why these three classes together form the natural kinship group of the Amniota (this group also includes the viviparous mammals whose germinal vesicle is homologous to the membrane ). The term amniota was first introduced into the scientific discussion in 1866 by Ernst Haeckel ("Amnionthiere") and stands for the higher vertebrate classes of the ( paraphyletic ) "reptiles", birds and mammals, as opposed to the anamnia , ie fish and amphibians . In the phylogenetic system, the amphibians are the sister group of the amniotes.
A feature of the ice Archosauria - birds and crocodiles - is the rigid shell of lime ( calcium carbonate ), while in most Schuppenkriechtieren (except some gekkota ) and monotremes is parchment-like shell ( amniotes egg ), with some turtles come kalkschalige Eggs before.
The smallest bird eggs are laid by the hummingbird type bee elf .
The development of the egg
The development is described here using the chicken egg as an example , but it proceeds in a comparable way in other birds (apart from the size and time information).
The formation begins in the hen ovary (in female birds the genital tract is only developed on the left). There are thousands of egg cells that grow into large yolk balls over the course of several days through the accumulation of yellow yolk . These get into the fallopian tubes ( oviductus , also known as the laying bowel ). In the fallopian tube funnel ( infundibulum , approx. 9 cm long), a loose membrane forms around the yolk skin, from which the hail cords ( chalazae ) later emerge. In the infundibulum, the egg can be fertilized by sperm from a rooster .
If this happens, a germinal disc forms on the yolk ball . In the subsequent, approximately 32 cm long Magnum be of glands , the precursors of the albumen (protein, 5, 6) is formed and attached to the egg. Turning around its own axis on the way out ensures even deposition. In this phase, the hail cord is also created (4, 13). It later holds the yolk (egg yolk) in the middle of the lime shell and prevents it from being damaged by hitting the lime shell in the event of vibrations. The shell skin is formed in the adjacent isthmus (2, 3).
In the back of the fallopian tube is the around 10 cm long uterus ( egg container ) in which the porous calcareous shell (1) is built up. The color of brown-shelled eggs is only deposited in the last 4–5 hours of the shell formation. Occasionally the egg travels too quickly through the egg container, resulting in shell-less eggs, the crab eggs . They are only held together by the shell skin (2, 3). In this phase of egg development, the air chamber (14) is also created. Why this is always at the blunt end of the egg is an unresolved mystery.
In the last section, the vagina, the upper membrane (cuticle) is formed (15). It seals the porous lime shell and prevents the penetration of bacteria . Eggs that are not eaten immediately should not be washed, as the cuticle is destroyed and bacteria can get into the egg and multiply there. A bit of feces or dirt on an egg is much less of a problem as long as the cuticle is intact.
Occasionally, two yolk globules move from the ovary into the fallopian tube at the same time. Egg white then accumulates on both of them, resulting in an egg with two yolks. This is also known as the double yolk. Since there is not enough space in the egg for two chicks, both die in the course of development.
The shell of a chicken egg is permeable to air to supply the chick with oxygen. It is stable in order to be able to withstand the weight of the chicken when it is brooding . At the same time, the shell is very thin, about 0.3 to 0.4 millimeters, so that the chicks can pick it up from the inside.
- Main article Brut
The vast majority of birds incubate their own eggs; only a few (e.g. the big foot fowl ) let them hatch through rotting heat or place them (like the cuckoo ) in foreign nests as brood parasitism .
A bird gradually lays so many eggs until the clutch size typical for the respective species is reached. In waders like the lapwing this is usually four eggs, in hens it is about a dozen. If the eggs are lost, there may be, usually smaller, additional clutches. However, this differs from species to species. Chicken birds like the quail or the bankiva chicken (the wild form of the domestic chicken ) are particularly persistent when it comes to reloading and can thus produce several hundred eggs a year. Humans have been making use of this since ancient times by domesticated the chicken and practically harvesting the eggs. The predominant forms of animal husbandry carried out for egg production are the battery and free range .
When using eggs as food , the bird's eggs make up the main part. The most common avian egg in the world for human consumption is the hen's egg . It is not only an ingredient in a wide variety of dishes , but is also used in painting (e.g. in some tempera paints ) and was part of historical mortars ; See chicken egg for details.
The oology is the bird's eggs customer and a branch of Ornithology . In contrast to embryology , it deals with the outer shell of the eggs and not with their contents. Another neighboring area is nest science ( Kaliology ).
- Tim Birkhead: The Most Perfect Thing: Inside (and Outside) a Bird's Egg . Bloomsbury, London 2016, ISBN 9781408851258 .
- Why bird eggs are so often spotted. If there is a calcium deficiency, the mother birds incorporate a dye to stabilize the shells.
- ↑ badische-zeitung.de: The bee elf lays the smallest eggs . Badische Zeitung , April 15, 2014 (April 20, 2014).