Brood Parasitism

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The reed warbler feeds the almost full-fledged cuckoo

Brood parasitism is the behavior of some animal species not to incubate their clutch themselves, but to have it hatched by surrogate parents ( hosts ) who then take on the subsequent feeding and management of the mostly alien young animals. The best-known example of a European brood parasite is the cuckoo , which does not raise its own young birds. Most bird parasites, however, are found in Africa: 50 species do not care for their broods there, while in Southeast Asia this applies to 34 bird species.

Brood parasitism is primarily an ornithological term, but it is also used in other areas of zoology, such as entomology . In most cases, brood parasitism leads to a reproductive disadvantage of the host parents. The brood parasites thus reduce their expenses for brood care or brood care . This saves them the hassle of searching for food for their offspring, which enables them to find more food for themselves and thereby lay more eggs. The disadvantage of this reproduction strategy is that some hosts recognize the foreign clutch and either remove it or start a completely new clutch.

In German, parasitic animals often have the suffix Kuckucks ~ , for example the cuckoo duck , cuckoo bees or the cuckoo whiskered catfish .

A distinction is made between intraspecific and interspecific brood parasitism.

Intraspecific brood parasitism

Clutch of a grouse ( Falcipennis canadensis ) The average clutch size of this species is 5 eggs; a 14 clutch indicates that several females are involved

In this variant of parasitization, a female of one species lays her eggs in the nest of the same species. Whether it also incubates a clutch of eggs itself differs both individually and species-specifically. Intraspecific brood parasitism is difficult to detect, so that it may occur more frequently than previously assumed. Many ducks and some colony breeders, such as the rook or some species of swallow, show this behavior at least occasionally. Whenever clutches are unusually large, this form of parasitizing must also be considered. However, the boundaries to community breeding are sometimes blurred, so that one cannot always speak of a parasitic behavior that reduces the host's chances of reproduction.

Interspecific brood parasitism

Female of the brown-headed cowbird - Brown-headed cowbird are among the obligatory breeding parasites
Egg of the brown-headed cowbird in the nest of the
white-bellied phoebetyrann ( Sayornis phoebe )
Clutch of redstart with cuckoo egg. It is the left, slightly larger egg.

In interspecific brood parasitism, the parasitic behavior can always be determined. Usually only one egg is laid per nest. If the offspring grow up in alien nests, they are usually larger than the young of the host parents and can even become significantly larger than their host parents themselves. As a result of this size advantage, they receive more food than the offspring of the host parents and are often able to throw the other young birds and other eggs out of the nest shortly after hatching so that they remain alone in the nest.

Facultative Brood Parasitism

Interspecific brood parasitism can occur facultatively or obligatorily.

Parents who parasitize facultatively usually produce their own clutch, but increase their reproduction by laying eggs in host nests. The host parents usually raise the strange young next to their own, so that a strong synchronization between the parasite and the host is necessary with regard to egg size, incubation time and feeding habits.

Obligatory brood parasitism

Obligatory breeding parasites, of which around 100 species are known to date, do not look after their own clutch. They lay their eggs individually, rarely in pairs, in the nests of the host parents, whereby the choice of hosts can be very different both in number and type. Some species, such as cowbird , parasitize other blackbirds almost exclusively . All widow birds (Viduinae) parasitize individual species of finches (Estrildidae). Fine finch eggs and young are not pushed out of the nest by the hatching widow birds. Rather, they grow up together with their step-siblings. They not only show the same body plumage, pharynx markings, papillae or beak rim bulges as young splendid finches, but also resemble them in their begging movements and sounds. The specialization in certain species of hosts has advanced so far that successful males can master both their own species and the host's song.

Likewise, all representatives of the honey indicator (Indicatoridae) seem to be obligate breeding parasites.

Behavioral adaptations of the cuckoos that enable brood parasitism

Interspecific brood parasitism has established itself particularly successfully in the bird family of the cuckoo (Cuculidae), of which around 140 species 57 species are obligate parasites. A few others parasitize while maintaining their own brood care only when the opportunity is appropriate, a behavior that is evolutionarily viewed as a transition phase to obligatory brood parasitism. In cuckoos, which are obligated to be brood-parasitic, the parasitic quality of the behavior is very high, since the clutch or the young of the hosts are usually removed either by the laying female or later by the parasite that has just hatched. To this end, many types of cuckoo have developed a number of adaptations:

  • Synchronization of egg maturation : Egg maturation is likely stimulated by observing the nest building activities of a potential host.
  • Accelerated egg-laying : In the case of brood parasites, the eggs must be laid very quickly, as the host parents could react sensitively to disturbances. Some types of cuckoo can keep the finished egg in the fallopian tube , so that they can lay very quickly in the event of an opportunity. Edgar Chance was able to prove in a field study that the females of the cuckoo, which is also native to Central Europe, only need 10 seconds to lay their egg. Blue cuckoos only need three seconds to lay their eggs in the open nests of crows. They are dependent on this speed because crows are significantly larger in size, would attack females near the nest and can cause considerable injuries due to the difference in size.
  • Egg size and egg mimicry : Many cuckoos have their eggs adapted in color and size to those of their host parents, usually one host bird species is preferred. For the cuckoo, in the case of the bluish eggs, it was possible to clarify how the female cuckoo can match the laid eggs to the clutch: The females have W sex chromosomes on their W sex chromosomes (as with other birds, females have ZW chromosomes, males ZZ chromosomes ) both the preference for a certain host bird species (e.g. the common redstart with a bluish clutch) and the color (bluish) and pattern (uniform) of the egg. The males of the cuckoo, on the other hand, do not carry any genetic information regarding egg color, pattern and host species. This adaptation to individual host birds is lacking in the common jaguar cuckoo. However, their eggs are colored to match those of several species of crowbirds.
  • Removal of eggs and juveniles of the host bird : Cuckoos of some genera ( Cuculus , Caccomantis and Chrysococcis ) that have just hatched throw eggs and juveniles of the host parents out of the nest by certain shoveling movements of the back. Some species have developed a small hollow on their backs for this.
  • Higher assertiveness : especially the chicks of most parasitic cuckoo species, but also the chicks of some other breeding parasites hatch after a shorter incubation period than those of the host parents and grow very quickly, especially in the first few days. This gives them a decisive feeding and growth advantage over the other nestlings.

Josef Reichholf is of the opinion that the brood parasitism of the cuckoo occurring in Europe developed evolutionarily as a reaction to the competitive disadvantage compared to other songbirds that are capable of more efficient hunting techniques due to their size and construction. As a result, cuckoos often feed on hairy caterpillars and poisonous butterflies, which are spurned by other songbirds, but with which they could not feed their young. He also sees the eating of host bird eggs by female cuckoos (as many as they themselves put in the same nest) as a former preliminary stage of the current brood parasitism.

Characteristics of the host bird species

Most species of cuckoo specialize in a few host bird species. Suitable host bird species usually have the following characteristics:

The young bird of the hermit cuckoo is fed by a caprella .
  • Accessible nest : The nest must either be open at the top or, in the case of a ball nest, have an opening large enough for the female to slip into. Many brood parasitic species are also dependent on stand guard near the nest of the host bird species. This enables the brood parasites to observe the host bird's nest and to catch the moment when the host bird parents are not in the vicinity of the nest. For example, the nests of ground-breeding host bird species of the cuckoo are less parasitized the further they are from the hedges and forest areas that provide cover, from which the cuckoo can observe them.
  • Suitable body size for successful incubation of the brood parasite egg: Eggs of the brood parasite must neither be too small, because they would otherwise have too little contact with the breeding spot of the host bird, nor too large, because otherwise the host bird cannot incubate them successfully. From the parasitic parasites and their respective host bird species, which have so far been examined more closely, it has been deduced that the cuckoo's egg can be up to 46% smaller than that of the host bird species. Conversely, it cannot be more than 35 to 38 percent larger.
  • Food must be suitable for rearing the nestlings : Most brood parasitic bird species are insectivores. Accordingly, the host bird must also be an insectivore. There are exceptions to this, however: the oryx weaver parasitized by the gold cuckoo mainly eats grass seeds, but gold cuckoo nests still grow successfully in its nests.
  • Competitive disadvantage of nesting siblings : The nestlings of the brood parasitic species must be able to throw eggs and nestlings of the host bird out of the nest very early after hatching or, alternatively, they must show a stronger begging behavior in order to get enough food.
  • Sufficient number of parasitic nests : the host bird species not only have to be sufficiently frequent for brood parasitic species to offer an opportunity to lay eggs. They must also breed at a time when the brood parasite is also able to lay eggs.

Probably some brood parasites observe the behavior of the host parents after they have laid their eggs. Occasionally it has been found that the host parents' nests and clutches were destroyed by the brood parasites when they removed the egg that had been placed underneath.

Other kinds

Wasp bee

Brood parasitism is widespread among insects. For example, all wasp bees (see also cuckoo bees ) and some wasp species ( cuckoo wasps ) are obligate breeding parasites. Wasp bees, also called cuckoo bees because of their breeding biology, parasitize mainly sand bee species . Since most wasp bees are host-specific, a strong reproduction of these insects can lead to the collapse of the host species' populations, which, however, also leads to the local collapse of the parasitic species. There are also a number of obligatory breeding parasites among the hoverflies , for example ( Volucella pellucens ), the bumblebee hover fly .

The only fish that are known to have brood parasitism are the cuckoo whiskered catfish ( Synodontis multipunctatus and S. grandiops ), which mainly occur in Lake Tanganyika and which slide their eggs under the mouth-brooding cichlids .

History of science

Classification of individual species as brood parasites

An Indian koel (Eudynamys scolopaceus), historically the first bird species for which breeding parasitism is mentioned.
Red-axed cowbird, one of the brood parasites outside of the cuckoo

The oldest references to brood parasitism can be found in the Indian Vedas around 2000 BC. Chr .: is commenting on the behavior of the writings in these cuckoos belonging Eudynamys scolopaceus which is raised by another is called a bird. Further details are not given. It was not until around 375 AD that another document from this cultural area recorded that this species was used in crows. These are classified as the most common host birds even today. Greek natural philosophers already in the 4th century BC. Commented on the brood parasitism of the cuckoo, which is widespread in Eurasia. The next mention of a brood parasitic species was made by Jahangir , a ruler of the Mughal Empire (1605–1627), who recorded that Clamator jacobinus used Timalia as host birds.

In 1802, breeding parasitism was first described for a species of bird that did not belong to the cuckoo family: Félix de Azara , one of the most important Spanish South American researchers, stated in that year that the blackbird was a brood parasite in Paraguay and Argentina. In 1806 the French ornithologist François Levaillant noted that other members of the cuckoo family, namely Cuculus clamosus , Cuculus gularis and the golden cuckoo, are brood parasites. 1810 years later, Alexander Wilson reported that the brown-headed cowbird, another species outside of the cuckoo, does not raise its young itself. He observed a female of the brown-headed cowbird sitting on the nest of a roach vireo and discovered a little later that there was an egg in the nest that was clearly different from the other eggs in the clutch. In 1853, Alfred Brehm cited the first evidence for the jay cuckoo that it was a brood parasite. In 1861 it was certain that the red-eyed cowbird had its young birds raised by strange parents. In 1867 this behavior was discovered in a representative of the Honiganzeiger and thus found a third family in which representatives practiced brood parasitism. The Argentine-British ornithologist William Henry Hudson discovered in 1874 that brood parasitism also applied to the red-axed cowbird. He wrote about his discovery that he was no less proud than if he had discovered a new planet in the sky. In 1894 this behavior was also found in the giant cowbird, so that all five species of the genus show this behavior. Within the family of starlings , to which the cowbird belong, there are no other representatives that show brood parasitism. In 1879 another species from the cuckoo family was assigned to the brood parasites with the grimaceous cuckoo.

The classification of other species as brood parasites continued in the 20th century: With the widow birds , another family was found in 1907 that belongs to the brood parasites. The striped cuckoo was classified as a brood parasite in 1909 and the peacock cuckoo in 1914. In 1918 it was also discovered that the cuckoo duck is a brood parasitic species among duck birds . This was established for the genus Pachycoccyx in 1936; the British ornithologist Reginald Ernest Moreau pointed out in 1939 that there was sufficient evidence to classify the genus Cercococcyx as a parasite. For most species of this genus, however, it is still unclear what the typical host birds are. In contrast, the host birds of the pheasant cuckoo are known , but have not yet observed whether it removes other nestlings and eggs from the nest after hatching.

Causes of Brood Parasitism

European naturalists asked about the reasons for the behavior of the cuckoo very early on.

The English cleric Edward Topsell explained in his The Fowles of Heaven in 1614 the brood parasitism of the cuckoo with the miraculous work of God. The creator has compensated for the lack of parental instincts of this bird species in his benevolent way by the fact that other birds would take over the task of raising his young for him. The French anatomist Herissant (1752) and the British pastor and ornithologist Gilbert White (1789) could only explain the lack of parental care by an anatomical defect that made it impossible for them to hatch their eggs themselves. However, the English country doctor Edward Jenner refuted this thesis in 1788 through a practical experiment. He pushed two hatched wagtail eggs under a cuckoo nestling that was growing in the nest of a dunnock. These hatched successfully, which Jenner saw as evidence that the cuckoo is anatomically very well able to hatch eggs. Jenner argued against it that the cuckoo stayed too short a time in its summer quarters to be able to successfully raise its own young. In the marshland around Cambridge, the cuckoo stays only six weeks after laying eggs before it leaves for its winter quarters. But it would take eight weeks from the egg to the independent young bird. As early as 1824, the naturalist John Blackwall argued against it that the early migration of the cuckoo was more a consequence of brood parasitism than its cause and also pointed out that the removal of other eggs and young birds by the newly hatched cuckoo was a sensible behavior for a brood parasite . On the other hand, he was unable to explain the origin of this behavior. This was reserved for Charles Darwin , who in the eighth chapter of his major work On the Origin of Species , published in 1859 , explained the behavior of the cuckoo from an evolutionary development. He also pointed out the positive consequences of brood parasitism: the cuckoo, freed from parental care, can leave its summer areas earlier and the young bird grows up without food competitors. Darwin also argued that the host bird's acceptance of the cuckoo's egg was a misguided instinct.

Behavioral adjustments

European naturalists have dealt in great detail with the behavioral adaptations of the cuckoo that enable it to develop brood parasitism. It is probably the best studied species compared to other brood parasites.

The golden cuckoo shows numerous behaviors that can also be found in the cuckoo

The similarity between the cuckoo's egg and the host bird's eggs was already known in the 18th century. Towards the end of the 19th century, attempts were made to clarify whether the female cuckoo birds are able to adapt the color of their eggs or whether, like other female birds, they laid eggs whose shell color is always the same. To find out, ornithologists such as August Carl Eduard Baldamus collected specific series of cuckoo eggs, which one could be sure that they each came from a female because of the territorial behavior of the species. As it turned out that the eggs of a female were similar, one was already sure at that time that the cuckoo specialized in each host bird species.

A carefully carried out field study on several female cuckoos, which Edgar Chance carried out between 1918 and 1925, clarified essential points in the behavior of the cuckoo. He was able to prove that females actually prefer to seek out the nests of a host bird species, that they observe the nests of their host bird beforehand and that the eggs are laid directly in the host bird's nest within a few seconds. Since one observed again and again female cuckoos with an egg in their beak, it was previously thought possible that the female lays the egg sitting on the ground, then takes it in the beak and lays it in the nest of the host bird. Chance was able to prove that the eggs that female cuckoos carry in their beak are eggs from the clutch of the host bird. In 1921 he was even able to film the entire process of oviposition. The results of Chance's research are corroborated by a number of similar field studies below. It has also been shown in other brood parasitic species such as the golden cuckoo that the behavioral adaptations found in the cuckoo also occur in other brood parasitic species.

literature

Single receipts

  1. ↑ a b Davies: Cuckoos, Cowbirds and Other Cheats. P. 11.
  2. ↑ Davies: Cuckoos, Cowbirds and Other Cheats. P. 12.
  3. ↑ a b Davies: Cuckoos, Cowbirds and Other Cheats. P. 31 and p. 32.
  4. ↑ Davies: Cuckoos, Cowbirds and Other Cheats. P. 102.
  5. ↑ Frode Fossøy, Michael D. Sorenson, Wei Liang, Torbjørn Ekrem, Arne Moksnes, Anders P. Møller, Jarkko Rutila, Eivin Røskaft, Fugo Takasu, Canchao Yang, Bård G. Stokke: Ancient origin and maternal inheritance of blue cuckoo eggs . In: Nature Communications , Volume 7, Article Number 10272, January 12, 2016, doi: 10.1038 / ncomms10272 .
  6. ↑ Davies: Cuckoos, Cowbirds and Other Cheats. P. 100.
  7. ↑ a b Davies, p. 130
  8. ^ Davies, p. 131.
  9. ↑ a b Davies: Cuckoos, Cowbirds and Other Cheats. P. 14.
  10. ↑ Davies: Cuckoos, Cowbirds and Other Cheats. P. 3.
  11. ↑ a b Davies: Cuckoos, Cowbirds and Other Cheats. P. 19.
  12. ↑ a b Davies: Cuckoos, Cowbirds and Other Cheats. P. 15.
  13. ↑ a b Davies: Cuckoos, Cowbirds and Other Cheats. P. 18.
  14. ↑ Davies: Cuckoos, Cowbirds and Other Cheats. P. 20.
  15. ↑ Davies: Cuckoos, Cowbirds and Other Cheats. P. 22.
  16. ↑ Davies: Cuckoos, Cowbirds and Other Cheats. P. 24.
  17. ↑ Davies: Cuckoos, Cowbirds and Other Cheats. P. 17.
  18. ↑ Davies: Cuckoos, Cowbirds and Other Cheats. P. 4.
  19. ↑ a b c d Davies: Cuckoos, Cowbirds and Other Cheats. P. 5.
  20. ↑ Davies: Cuckoos, Cowbirds and Other Cheats. P. 8.
  21. ↑ Davies: Cuckoos, Cowbirds and Other Cheats. P. 9.
  22. ↑ Davies: Cuckoos, Cowbirds and Other Cheats. P. 29.
  23. ↑ Secrets of Nature: The Cuckoo's Secret (1922). Wild Film History, accessed July 16, 2016 .
  24. ↑ Davies: Cuckoos, Cowbirds and Other Cheats. P. 82 to p. 87.