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Brood parasites are organisms that use the strategy of brood parasitism, a kind of kleptoparasitism found among birds, fish or insects, involving the manipulation and use of host individuals either of the same (intraspecific brood-parasitism) or different species (interspecific brood-parasitism) to raise the young of the brood-parasite. This relieves the parasitic parent from the investment of rearing young or building nests, enabling them to spend more time foraging, producing offspring etc. Additionally, the risk of egg loss to raiders such as raccoons is mitigated, by having distributed the eggs amongst a number of different nests. As this behaviour is damaging to the host, it will often result in an evolutionary arms race between parasite and host.
Avian brood parasites
In many monogamous bird species, there are extra-pair matings resulting in males outside the pair bond siring offspring and used by males to escape from the parental investment in raising their offspring. This form of cuckoldry is taken a step further when females lay their eggs in the nests of other individuals. Intraspecific brood parasitism is seen in a number of duck species with females laying their eggs in the nests of others for example in the Goldeneye, Bucephala clangula.
Interspecific brood-parasites include the Old World cuckoos in Eurasia and Australia, American Coots, Cowbirds and Black-headed Ducks in the Americas, and indigobirds, whydahs, and the honeyguides in Africa. Seven independent origins of obligate interspecific brood parasitism in birds have been proposed. While there is still some controversy over when and how many origins of interspecific brood parasitism have occurred, recent phylogenetic analyses suggest two origins in Passeriformes (once in New World cowbirds: Icteridae, and once in African Finches: Viduidae); three origins in Old World and New World cuckoos (once in Cuculinae, Phaenicophaeinae, and in Neomorphinae-Crotophaginae); a single origin in Old World honeyguides (Indicatoridae); and in a single species of waterfowl, the black-headed duck (Heteronetta atricapilla).
Most avian brood parasites are specialists which will only parasitize a single host species or a small group of closely related host species, but four out of the five parasitic cowbirds are generalists, which parasitize a wide variety of hosts; the Brown-headed Cowbird has 221 known hosts. They usually only lay one egg per nest, although in some cases, particularly the cowbirds, several females may use the same host nest.
The Common Cuckoo presents an interesting case in which the species as a whole parasitizes a wide variety of hosts, including the Reed Warbler and Dunnock, but individual females specialize in a single species. Genes regulating egg coloration appear to be passed down exclusively along the maternal line, allowing females to lay mimetic eggs in the nest of the species they specialize in. Females generally parasitize nests of the species which raised them. Male Common Cuckoos will fertilize females of all lines, maintaining sufficient gene flow among the different maternal lines.
The mechanisms of host selection by female cuckoos are somewhat unclear, though several hypotheses have been suggested in attempt to explain the choice. These include genetic inheritance of host preference, host imprinting on young birds, returning to place of birth and subsequently choosing a host randomly ("natal philopatry"), choice based on preferred nest site (nest-site hypothesis), and choice based on preferred habitat (habitat-selection hypothesis). Of these hypotheses the nest-site selection and habitat selection have been most supported by experimental analysis.
Common adaptations of avian brood parasites
Among specialist avian brood parasites, mimetic eggs are a nearly universal adaptation. There is even some evidence that the generalist Brown-headed Cowbird may have evolved an egg coloration mimicking a number of their hosts.
Most avian brood parasites will remove a host egg when they lay one of their own in a nest. Depending upon the species, this can happen either in the same visit to the host nest or in a separate visit before or after the parasitism. This both prevents the host species from realizing their nest has been parasitized and reduces competition for the parasitic nestling once it hatches.
Most avian brood parasites have very short egg incubation periods and rapid nestling growth. This gives the parasitic nestling a head start on growth over its nestmates, allowing it to outcompete them. In many brood parasites, such as cuckoos and honeyguides, this short egg incubation period is due to internal incubation periods up to 24 hours longer in cuckoos than hosts. Some non-parasitic cuckoos also have longer internal incubation periods, suggesting that this longer internal incubation period was not an adaptation following brood parasitism, but predisposed birds to become brood parasites. In cases where the host nestlings are significantly smaller than the parasite nestling, the host nestlings will often starve to death. Some brood parasites will eliminate all their nestmates shortly after hatching, either by ejecting them from the nest or killing them with sharp mandible hooks which fall off after a few days.
It has often been a question why the majority of the hosts of brood parasites care for the nestlings of their parasites. Not only do these brood parasites usually differ significantly in size and appearance, but it is highly probable that they reduce the reproductive success of their hosts. So what possible benefits are gained from providing this parental care? Through studies in an attempt to answer this question evolved the "Mafia hypothesis". This hypothesis revolves around host manipulations induced by behaviors of the brood parasite. Upon the detection and rejection of a brood parasite's egg, the host's nest is depredated upon, its nest destroyed and nestlings injured or killed. This threatening response is indirectly enhancing selective pressures favoring aggressive parasite behavior that may result in positive feedback between Mafia-like parasite and compliant host behaviors.
There are two avian species that have been speculated to portray this mafia-like behavior, the brown-headed cowbird of North America, Molothrus ater, and the Great Spotted Cuckoo of Europe, Clamator glandarius. The Great Spotted Cuckoo lays the majority of its eggs in the nests of the European Magpie, Pica pica. It has been observed that the Great Spotted Cuckoo repeatedly visits the nests that it has parasitised, a precondition for the Mafia hypothesis. An experiment was run by Soler et al. from April to July 1990–1992 in the high-altitude plateau Hoya de Guadix, Spain. They observed the effects of the removal of cuckoo eggs on the reproductive success of the magpie, and measured the magpie's reaction; the egg was considered accepted if it remained in the nest, ejected if gone in between visits, or abandoned if the eggs were present but cold. If any nest contents were gone between consecutive visits, the nests were considered to have been depredated. The magpie's reproductive success was measured by number of nestlings that survived to their last visit, which was just before the nestling had been predicted to fledge from the nest. The results from these experiments show that after the removal of the parasitic eggs from the Great Spotted Cuckoo, these nests are predated at much higher rates than those where the eggs were not removed. Through the use of plasticine eggs that model those of the magpie, it was confirmed that the nest destruction was caused by the Great Spotted Cuckoo. This destruction benefits the cuckoo, for the possibility of re-nesting by the Magpie allows another chance for the cuckoo egg to be accepted.
Another similar experiment was done in 1996–2002 by Hoover et al. on the relationship between the parasitic Brown-headed Cowbird and a host, the Prothonotary Warbler, Protonotaria citrea. In their experiment, they manipulated the cowbird egg removal and cowbird access to the predator proof nests of the warbler. They found that 56% of egg ejected nests were depredated upon in comparison to 6% of non-ejected nests when cowbirds were not prevented from getting to the hosts' nest. Of the nests that were rebuilt by hosts that had previously been predated upon, 85% of those were destroyed. The number of young produced by the hosts that ejected eggs dropped 60% compared to those that accepted the cowbird eggs.
In this hypothesis, female cuckoos select a group of host species with similar nest sites and egg characteristics to her own. This population of potential hosts is monitored and a nest is chosen from within this group.
Research of nest collections has illustrated a significant level of similarity between cuckoo eggs and typical eggs of the host species. A low percentage of parasitized nests were shown to contain cuckoo eggs not corresponding to the specific host egg morph. In these mismatched nests a high percent of the cuckoo eggs were shown to correlate to the egg morph of another host species with similar nesting sites. This has been pointed to as evidence for nest- site selection.
A criticism of the hypothesis is that it provides no mechanism by which nests are chosen, or which cues might be used to recognize such a site.
Parental-care parasitism emphasizes the relationship between the host and the parasite in brood parasitism. Parental-care parasitism occurs when individuals raise offspring of other unrelated individuals. The host are the parents of offspring and the parasites are individuals who take advantage of either the nest or eggs within the family construct. Such dynamics occur when the parasites attempt to reduce their parental investment so they can invest the extra energy into other endeavors. 
Cost of the hosts
Given the detrimental effects avian brood parasites can have on their hosts' reproductive success, host species have come up with various defenses against this unique threat. Given that the cost of egg removal concurrent with parasitism is unrecoverable, the best defense for hosts is avoiding parasitism in the first place. This can take several forms, including selecting nest sites which are difficult to parasitize, starting incubation early so they are sitting on the nests when parasites visit them early in the morning, and aggressive territorial defense. Birds nesting in aggregations can also benefit from group defense.
The hosts reject offspring
The host may be the one that ultimately ends up raising offspring after they return from foraging. Once parasitism has occurred, the next most optimal defense is to eject the parasitic egg. According to parental investment theory, the host can possibly adopt some defense to protect their own eggs if they distinguish which eggs are not theirs. Recognition of parasitic eggs is based on identifying pattern differences or changes in the number of eggs.  This can be done by grasp ejection if the host has a large enough beak, or otherwise by puncture ejection. Ejection behavior has some costs however, especially when host species have to deal with mimetic eggs. In that case, hosts will inevitably mistake one of their own eggs for a parasite egg on occasion and eject it. In any case, hosts will sometimes damage their own eggs while trying to eject a parasite egg.
Among hosts not exhibiting parasitic egg ejection, some will abandon parasitized nests and start over again. However, at high enough parasitism frequencies, this becomes maladaptive as the new nest will most likely become reparasitized. Other behavior can include modifying the nest to exclude the parasitic egg, either by weaving over the egg or in some cases rebuilding a new nest over the existing one. For instance, American coots might kick the parasites’ eggs out, or build a new nest beside the brood nests where the parasites’ babies starve to death due to lack of food. 
Cost of the parasites
While parental-care parasitism significantly increased the breeding number of the parasite, only about half of the parasite eggs survived.  Parasitism for the individual (the brood parasite) also has significant drawbacks. As an example, the parasitic offspring of the bearded tits, Panurus biarmicus, compared to the offspring in non-parasitic nests, tend to develop much more slowly and often don’t reach full maturity.  Parasitic females however can adopt either floater traits or nesting traits. Floater females are entirely dependent on others to raise their eggs because they do not have their own nests. Hence, they reproduce significantly less because the hosts reject their ‘intruder’ eggs or they may just miss the egg-laying period of the bird they are trying to pass their eggs to. Nesting females who have their own nests may also be parasitic due to temporary situations like sudden loss of nests, or they lay surplus eggs, which overload their parental care ability.
The hosts raise offspring
Sometimes hosts to the parasitic birds are completely unaware that they are caring for a bird that is not their own. This most commonly occurs because the host cannot differentiate the parasitic eggs from their own. It may also occur when hosts temporarily leave the nest after laying the eggs. The parasites lay their own eggs into these nests so their nestlings share the food provided by the host. It may occur in other situations. For example, female eiders would prefer to lay eggs in the nests with one or two existing eggs of others because the first egg is the most vulnerable to predators. In doing so, when a female eider left the nest after laying the first egg, the probability that the predator attacked her egg decreased on account of presence of others’ eggs. There also are situations in which the parasitic offspring will kill the host nest mate during competition for resources. As an example, the parasite offspring of the cowbird chick would kill the host nest mates if food intake for each of them is low, but they would not do so if the food intake was adequate, as a result of their interactions with co-inhabitants of the nest. 
Insect brood parasites
There are many different types of cuckoo bees, all of which are brood-parasitic insects, laying their eggs in the nest cells of other bees, but they are normally referred to as kleptoparasites, rather than as brood parasites. A family of Cuckoo wasps also exist, many of which lay their eggs in the nests of Potter and Mud dauber wasps, and many other lineages of wasps in various families have evolved similar habits. Examples of cuckoo bees are Nomada and Epeoloides.
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