Brood parasite
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 (inter-specific 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.

# 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.
Inter-specific brood-parasites include the Old World cuckoos in Eurasia and Australia, cowbirds and Black-headed Ducks in the Americas, and indigobirds, whydahs, and the honeyguides in Africa. 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, 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 are thought to imprint upon the host species which raised them, and subsequently only parasitize nests of that species. Male Common Cuckoos will fertilize females of all lines, maintaining sufficient gene flow between 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 (Brian Peer, Scott Robinson, and James Herkert in The Auk 117(4):892-901).
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 could function either to prevent the host species from realizing their nest has been parasitized or to reduce 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 cases where the host nestlings are significantly smaller than the parasite nestling, the hosts 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.

## The "Mafia hypothesis"
It has often been a question as to 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 parasites egg, the hosts 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 2 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 parasitized, 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 magpies 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 magpies 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 depredated upon 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 its 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. Although there has not been a lot of experimentation performed to test this so-called Mafia hypothesis, these two experiments show rather convincing results.

## Common counteradaptations of 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.
Once parasitism has occurred, the next most optimal defense is to eject the parasitic egg. 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.

# 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.