Brood parasitism is a reproductive strategy in which certain bird species lay their eggs in the nests of others, leaving incubation and chick-rearing to unsuspecting hosts. This behavior allows parasites to maximize reproductive success while minimizing parental investment, often at great cost to the host species.
More than 100 bird species across multiple families practice some form of brood parasitism. Most are obligate parasites, meaning they never build their own nests, while others engage in facultative parasitism, laying eggs in both their own nests and those of other birds.
This article examines twelve major groups of brood parasitic birds, the types of parasitism they practice, their tactics of deception, and the responses of host species that have evolved to counteract them.
Cuckoos (Cuculidae)
- Type of parasitism: Obligate, interspecific.
- Host selection: Varies by species, some highly specialized.
- Main tactics: Egg mimicry, nestling eviction, stealth egg-laying.
- Notable adaptations: Chicks eject host eggs/chicks from the nest.
Cuckoos are among the most studied brood parasites. The common cuckoo (Cuculus canorus) lays eggs that closely mimic those of its host, tricking the host into incubating them. Cuckoo chicks hatch earlier than host chicks and instinctively push out all other eggs or nestlings, monopolizing parental care. Some cuckoo species exhibit specialized host preferences, while others have broader host ranges, parasitizing multiple species.

Host defenses vary widely, for instance, great reed warblers (Acrocephalus arundinaceus) have been observed rejecting cuckoo eggs, while others remain highly susceptible. Some cuckoo species, such as Asian koel (Eudynamys scolopaceus), do not eject host chicks but outcompete them for food by being more demanding. Certain hosts, like the superb fairywren (Malurus cyaneus), have even evolved the ability to recognize and reject cuckoo chicks, a rare adaptation.
Note: Not all cuckoos engage in brood parasitism. Species such as the yellow-billed Cuckoo (Coccyzus americanus) build and care for their own nests, highlighting an important evolutionary divergence within the family.
Honeyguides (Indicatoridae)
- Type of parasitism: Obligate, interspecific.
- Host selection: Hole-nesting species (barbets, kingfishers).
- Main tactics: Nestling aggression (hooked beaks used to kill host chicks).
- Notable adaptations: Extreme nestling violence ensures their survival.
Honeyguides employ a highly aggressive parasitic strategy. Their chicks hatch with sharp, hooked beaks, which they use to kill host nestlings shortly after hatching. This ensures that all parental investment is directed toward the honeyguide chick, allowing it to monopolize food and care. Unlike cuckoos, which eliminate competition by pushing eggs or nestlings out of the nest, honeyguide chicks physically attack and kill their foster siblings.

Studies in Africa have demonstrated that hosts like barbets and kingfishers have evolved defensive behaviors, including sealing their nest entrances after detecting a honeyguide egg. Some species also abandon parasitized nests, though this is not always successful in preventing parasitism.
The greater honeyguide (Indicator indicator) is the most well-documented species, but others, such as the lesser honeyguide (Indicator minor) and scaly-throated honeyguide (Indicator variegatus), also exhibit similar brood parasitic behaviors.
Indigobirds and whydahs (Vidua, Viduidae)
- Type of parasitism: Obligate, interspecific.
- Host selection: Specialists (each species mimics a specific finch host).
- Main tactics: Egg mimicry, chick vocal mimicry.
- Notable adaptations: Chicks develop host-like begging calls to avoid rejection.
Unlike cuckoos, indigobirds and whydahs don’t harm host chicks. Instead, their survival relies on precise vocal mimicry, where parasite chicks replicate the begging calls and feeding behaviors of the host’s own young, ensuring they receive food.
Each Vidua species is host-specific, meaning that individual species only parasitize one or a few closely related hosts. For example, the village indigobird (Vidua chalybeata) exclusively parasitizes the red-billed firefinch (Lagonosticta senegala), while the pin-tailed whydah (Vidua macroura) relies on common waxbills (Estrilda astrild) as hosts. This extreme host fidelity has driven rapid speciation in indigobirds and whydahs, as each parasite species evolves in tandem with its host.

Researchers have found that Vidua chicks imprint on their foster parents, learning their songs and social behaviors. As adults, these parasites seek out mates that sing the same species-specific song as their foster father, ensuring that their offspring will continue to parasitize the correct host species.
This form of cultural inheritance reinforces host specificity and plays a crucial role in the evolution of new Vidua species.
Cowbirds (Molothrus, Icteridae)
- Type of parasitism: Obligate, interspecific.
- Host selection: Generalist (can parasitize over 200 species).
- Main tactics: Stealth egg-laying, mafia behavior (check section below).
- Notable adaptations: Can lay up to 40 eggs per season.
Cowbirds are among the most aggressive brood parasites, using strategies such as egg destruction and “mafia behavior” to force hosts into accepting their eggs. Studies on prothonotary warblers (Protonotaria citrea) have shown that hosts which reject cowbird eggs often experience retaliatory nest destruction, discouraging future rejection.
The brown-headed cowbird (Molothrus ater) is one of the most widely studied parasitic birds and targets a broad range of host species, including yellow warblers (Setophaga petechia), which have evolved a unique defense: burying the parasite’s egg under a new layer of nest material.

Other frequently parasitized hosts include the song sparrow (Melospiza melodia), which sometimes ejects cowbird eggs, and the eastern phoebe (Sayornis phoebe), which usually accepts parasitism, often leading to reduced reproductive success.
Other cowbird species show regional variations in host selection. The shiny cowbird (Molothrus bonariensis), found in South America and the Caribbean, parasitizes a wide range of hosts, including tropical kingbirds (Tyrannus melancholicus) and rufous-bellied thrushes (Turdus rufiventris).
The screaming cowbird (Molothrus rufoaxillaris), on the other hand, has an extremely specialized host relationship, parasitizing almost exclusively grayish baywing (Agelaioides badius), to the point where it is nearly dependent on their nests for reproduction.
Because cowbirds do not mimic host eggs, they rely on rapid egg-laying, often depositing their eggs in the host’s nest in under a minute, reducing the chance of detection. Their chicks hatch earlier and grow quickly, often outcompeting host nestlings for food.
Black-headed duck (Heteronetta atricapilla)
- Type of parasitism: Obligate, interspecific.
- Host selection: Generalist (coots, ducks).
- Main tactics: Passive parasitism (no host harm).
- Notable adaptations: Chicks hatch fully developed and leave immediately.
Unlike cuckoos or cowbirds, black-headed ducks practice so-called “soft” or non-destructive brood parasitism, as they don’t harm host eggs or chicks. Each chick hatches fully feathered and highly precocial, then leaves the nest within a day or two. This allows them to bypass parental care without directly competing with the host’s own chicks for food or attention.

Because of this minimal impact, black-headed duck parasitism is often considered less harmful than that of other obligate brood parasites. However, studies suggest that host species like coots and ducks may still suffer reproductive costs, as parasitized nests sometimes experience increased egg loss or reduced incubation efficiency.
North American ducks
- Type of parasitism: Facultative, intraspecific/interspecific.
- Host selection: Primarily conspecifics, but some species target closely related ducks.
- Main tactics: Egg-dumping, nest takeover in rare cases.
- Notable adaptations: High nesting density favors parasitism, and weak egg recognition allows incubation of foreign eggs.
Several North American duck species engage in facultative brood parasitism, laying eggs in the nests of their own species or closely related waterfowl. This behavior is most common in dense breeding colonies, where nest proximity increases the chances of unnoticed parasitism. In some cases, host females incubate and raise the extra chicks, but excessive parasitism can lead to overcrowding, reduced hatching success, and nest abandonment.
The redhead (Aythya americana) is one of the most frequent brood parasites among North American waterfowl. Females often lay eggs in other redhead nests, but they are also known to parasitize canvasbacks (Aythya valisineria), which often incubate and hatch redhead ducklings alongside their own. While canvasbacks are frequent hosts, some individuals have been observed rejecting foreign eggs, suggesting potential evolutionary pressure for host defenses.

Other species engage in intraspecific parasitism more opportunistically. Lesser scaup (Aythya affinis), mallards (Anas platyrhynchos), and northern pintails (Anas acuta) have been observed laying eggs in conspecific nests, particularly when nesting sites are limited.
In cavity-nesting species, such as the hooded merganser (Lophodytes cucullatus) and wood duck (Aix sponsa), parasitic females sometimes deposit eggs in the nests of conspecifics or other cavity-nesting ducks, leading to mixed broods.
Despite the risks, facultative brood parasitism remains an effective strategy in highly competitive environments. By distributing eggs across multiple nests, parasitic females increase their reproductive success while reducing parental investment. However, excessive parasitism can lead to nest abandonment, limiting the success of both parasite and host.
South American ducks
- Type of parasitism: Facultative, intraspecific/interspecific.
- Host selection: Primarily conspecifics, but some species target closely related ducks.
- Main tactics: Egg-dumping, occasional interspecific parasitism.
- Notable adaptations: High nesting densities facilitate parasitism, and some species lack strong egg recognition.
Brood parasitism among South American ducks is less studied than in North America, but several species have been observed laying eggs in the nests of conspecifics or related species. This strategy is particularly common in dense breeding areas, where females take advantage of available nests to increase their reproductive output while minimizing parental investment.
The rosy-billed pochard (Netta peposaca) is one of the most well-documented facultative brood parasites in South America. Females have been observed laying eggs in the nests of other pochards, including conspecifics and potentially other diving ducks. This behavior increases clutch sizes in host nests, sometimes leading to overcrowding and reduced hatching success.

Another species known for intraspecific parasitism is the Brazilian teal (Amazonetta brasiliensis), which sometimes deposits eggs in the nests of other teals. This behavior has been recorded in wetland areas with high breeding densities, where nest sites may be limited.
Although less frequent, some cases of interspecific parasitism have been observed in other South American dabbling ducks, though more research is needed to determine how common this strategy is.
While facultative brood parasitism provides reproductive advantages, excessive parasitism can disrupt host incubation and increase nest abandonment rates, ultimately affecting overall breeding success.
European and Asian
- Type of parasitism: Facultative, intraspecific.
- Host selection: Primarily conspecifics, occasionally closely related diving ducks.
- Main tactics: Egg-dumping, exploiting high nesting densities.
- Notable adaptations: Weak egg recognition in some species allows foreign eggs to be incubated.
Facultative brood parasitism among European and Asian ducks occurs primarily in diving ducks and teals, particularly in high-density nesting areas where females take advantage of nearby nests to increase their reproductive success. While most cases involve intraspecific parasitism, some species have been recorded laying eggs in the nests of closely related ducks.
The common pochard (Aythya ferina) is one of the most well-documented facultative brood parasites in Europe. Females often lay eggs in conspecific nests, leading to clutch sizes exceeding what a single female could normally produce. This behavior has also been observed in tufted ducks (Aythya fuligula), particularly in dense breeding colonies. In both species, parasitized nests may suffer from overcrowding, leading to lower hatching success if too many eggs are present.

In dabbling ducks, Eurasian teals (Anas crecca) have occasionally been observed engaging in intraspecific parasitism, though this appears to be less frequent than in diving ducks. The extent of this behavior in other European and Asian dabbling ducks is not well documented, but it may occur in wetland habitats with high breeding densities.
While facultative brood parasitism allows females to increase their reproductive success, excessive parasitism can disrupt host incubation, reduce hatching success, and in extreme cases, cause nest abandonment.
Australian ducks
- Type of parasitism: Facultative, intraspecific.
- Host selection: Primarily conspecifics, occasionally closely related ducks.
- Main tactics: Egg-dumping in communal nesting areas.
- Notable adaptations: High nesting densities increase parasitism rates, and weak egg recognition allows foreign eggs to be incubated.
Brood parasitism in Australian ducks occurs mostly in high-density nesting areas, where females take advantage of nearby nests to offload eggs. This behavior has been observed primarily in dabbling ducks, where females deposit eggs in conspecific nests to increase reproductive success without fully investing in incubation and chick-rearing.
The Pacific black duck (Anas superciliosa) is one of the most well-documented facultative brood parasites in Australia. Nesting in dense wetland habitats, some females have been observed laying eggs in other Pacific black duck nests, leading to inflated clutch sizes that may impact hatching success.

Another known facultative brood parasite is the Australian wood duck (Chenonetta jubata), a cavity-nesting species that occasionally lays eggs in conspecific nests. This behavior is particularly common in areas with limited nesting sites, where females may exploit existing nests instead of building their own. While most cases involve intraspecific parasitism, there are occasional reports of wood ducks laying eggs in the nests of other cavity-nesting waterfowl.
Though facultative brood parasitism can enhance reproductive success, excessive parasitism may reduce hatching rates and lead to nest abandonment, especially in highly parasitized nests.
Weavers (Ploceidae)
- Type of parasitism: Facultative, intraspecific/interspecific.
- Host selection: Related weavers.
- Main tactics: Egg-dumping, possible nest usurpation.
- Notable adaptations: Unattended nest usurpation, laying eggs in abandoned or temporarily empty nests.
Some red-billed buffalo weavers (Bubalornis niger) engage in intraspecific brood parasitism, laying eggs in the nests of their own species. This strategy allows them to bypass the energy demands of rearing young while still contributing to the gene pool.
Unlike obligate brood parasites, these weavers typically nest communally, meaning that multiple females may contribute eggs to the same nest, sometimes making it difficult to distinguish parental roles.

The village weaver (Ploceus cucullatus) has also been observed practicing intraspecific parasitism, particularly in high-density colonies, where it becomes harder for host females to detect foreign eggs.
Another species, the southern masked weaver (Ploceus velatus), has been recorded laying eggs in conspecific nests, though at lower frequencies. In these cases, the parasitic females may increase their reproductive success without fully investing in chick-rearing.
Certain studies suggest that the complex, multi-chambered nests of some weavers might help facilitate parasitism by making it more difficult for hosts to recognize and reject foreign eggs. Despite these examples, most weaver species do not exhibit brood parasitism, and many are known for their intricate parental care and cooperative breeding behaviors.
Gulls (Laridae)
- Type of parasitism: Facultative, intraspecific.
- Host selection: Same species, dense colonies.
- Main tactics: Egg-dumping.
- Notable adaptations: Weak egg recognition in dense colonies allows successful egg-dumping.
In large nesting colonies, some gulls, such as herring gulls (Larus argentatus) and lesser black-backed gulls (Larus fuscus), lay eggs in the nests of their own species, particularly if their own nest is lost or if they fail to establish a suitable nesting site. This strategy allows them to retain some reproductive success despite nest failures.

In some cases, host gulls incubate and raise the extra chicks, but overcrowding can lead to lower survival rates due to increased competition for food. Studies suggest that high colony density and frequent nest takeovers may contribute to the frequency of intraspecific brood parasitism in gulls.
Geese and swans
- Type of parasitism: Facultative, intraspecific.
- Host selection: Primarily conspecifics, occasionally closely related species.
- Main tactics: Egg-dumping in nearby nests.
- Notable adaptations: High nesting densities increase parasitism rates, and weak territorial defense by some females allows nest intrusion.
Facultative brood parasitism in geese and swans occurs mainly in high-density breeding areas, where nest sites are often close together. Unlike parasitic ducks, geese and swans exhibit strong parental care, but some females take advantage of nearby nests to offload extra eggs, increasing their reproductive success without the added cost of incubation.
The Canada goose (Branta canadensis) is one of the best-documented facultative brood parasites among waterfowl. In dense breeding colonies, females sometimes lay eggs in the nests of neighboring geese, particularly when their own nest is disturbed or lost. This behavior leads to mixed broods, where host females unknowingly incubate and raise additional goslings.

In swans, mute swans (Cygnus olor) have been recorded engaging in intraspecific parasitism, with some females laying eggs in the nests of other swans, particularly when territory disputes prevent them from establishing a nest of their own. While less common than in geese, this behavior suggests that territorial pressures and nest site availability play a role in driving parasitic egg-laying.
Although facultative brood parasitism reduces the incubation burden for parasitic females, it can also lead to overcrowding, competition for parental care, and in some cases, higher chick mortality due to limited resources.
Types of brood parasitism
Brood parasitism in birds can be divided into two major forms: obligate and facultative, each with distinct strategies for exploiting host birds. Some species specialize in intraspecific parasitism, targeting their own kind, while others engage in interspecific parasitism, selecting hosts from different species.
Obligate brood parasitism
Obligate brood parasitism is a reproductive strategy in which birds never build their own nests or care for their young, instead relying entirely on host species for incubation and chick-rearing. This form of parasitism has led to specialized adaptations, including egg mimicry, rapid egg-laying, and aggressive nestling behavior.
The common cuckoo is one of the best-known examples, laying eggs that closely resemble those of its host to avoid detection. Similarly, honeyguides take parasitism a step further, hatching with sharp beaks, their chicks eliminate competition by killing host nestlings.
Another example is the brown-headed cowbird, which forces hosts to raise its offspring through stealthy egg-laying and, in some cases, retaliatory nest destruction.
Facultative brood parasitism
Facultative brood parasitism occurs when birds continue to raise their own broods but also lay eggs in the nests of others, usually of their own species. This behavior is often opportunistic, emerging in high-density nesting areas where multiple females compete for limited resources.
In North America, redhead ducks frequently lay eggs in the nests of other redheads, as well as in those of canvasbacks, increasing their reproductive success without additional parental investment. Wood ducks, particularly in dense populations, also engage in intraspecific parasitism, leading to unusually large clutch sizes in some nests.
Among waterfowl, Canada geese frequently deposit eggs in the nests of neighboring geese, resulting in mixed broods.
Intraspecific vs interspecific parasitism
Intraspecific brood parasitism occurs when a bird lays eggs in the nest of another female of its own species, as seen in mallards, redheads, and Canada geese. In contrast, interspecific brood parasitism involves laying eggs in the nests of different species, as practiced by cuckoos, cowbirds, and honeyguides.
Some species, like shiny cowbirds, switch between both strategies depending on environmental conditions and host availability.
Brood parasitism tactics and host responses
Brood parasitism has led to an ongoing evolutionary “arms race” between parasites and their hosts. Parasites have developed specialized strategies to exploit host parental care, while hosts have evolved defensive adaptations to minimize the impact of parasitism. Below are the primary tactics and manipulations used by brood parasites and the corresponding host responses.
Tactics used by brood parasites:
- Egg mimicry. Some parasite birds, such as common cuckoos and indigobirds, lay eggs that closely resemble those of their hosts. This reduces the likelihood of rejection and increases the chances of successful parasitism.
- Nestling aggression. Certain species, such as honeyguides and cuckoos, eliminate competition by attacking host offspring. Honeyguide chicks hatch with hooked beaks, which they use to kill host chicks, while cuckoo nestlings push host eggs or chicks out of the nest.
- Mafia behavior. Some cowbirds retaliate against hosts that reject their eggs. If a host removes a cowbird egg, the cowbird may return and destroy the host’s entire clutch, forcing future acceptance.
- Begging call mimicry. Indigobirds and whydahs mimic the begging calls of host nestlings, ensuring they receive food even when raised alongside host young.
- Rapid egg-laying. Some species, such as cowbirds, lay eggs in under a minute, reducing the risk of detection and host aggression.
Defensive strategies of host birds:
- Egg rejection. Some hosts, such as great reed warblers and American robins, can identify and eject foreign eggs from their nests before they hatch.
- Nest abandonment or egg burial. Species like yellow warblers bury foreign eggs under a new nest layer or abandon parasitized nests to start fresh.
- Aggressive nest defense. Magpies and some warblers physically attack brood parasites near their nests to prevent egg-laying, though persistent parasites often succeed.
- Chick discrimination. Superb fairywrens can identify and reject parasitic nestlings, preventing them from monopolizing parental care. A rare but pretty powerful defense tactic.
Victims of brood parasitism
Brood parasitism exerts significant pressure on host species, often reducing their reproductive success and leading to the evolution of various defenses. Some birds suffer extreme consequences, such as complete nest failure, while others have developed strategies to recognize and reject foreign eggs or chicks. Hosts vary in their susceptibility, with some species frequently parasitized across their range and others only targeted occasionally.
Songbirds and warblers
Many small passerines, particularly warblers, sparrows, and flycatchers, are frequent victims of interspecific brood parasites like cuckoos and cowbirds. The yellow warbler (Setophaga petechia) is heavily targeted by brown-headed cowbirds (Molothrus ater), often suffering complete reproductive loss when parasitized. This species has evolved an unusual response – burying the cowbird egg under a new nest layer to prevent it from hatching.
Similarly, prothonotary warblers (Protonotaria citrea) face extreme pressure from cowbirds and are known to abandon their nests after parasitism.
In Europe, the great reed warbler (Acrocephalus arundinaceus) is a primary host for common cuckoos (Cuculus canorus). While some individuals reject foreign eggs, others incubate and raise cuckoo chicks at the expense of their own young. Some cuckoo hosts, such as the superb fairywren (Malurus cyaneus), have evolved the ability to recognize foreign chicks, making them one of the few species that can reject cuckoo nestlings after hatching.
Ducks and waterfowl
Many waterfowl species are parasitized by their own kind, resulting in overcrowded nests and reduced incubation efficiency. The canvasback (Aythya valisineria) is a common victim of redhead (Aythya americana) parasitism, with some nests containing dozens of extra eggs.
Similarly, wood ducks (Aix sponsa) and goldeneyes (Bucephala clangula) experience high rates of intraspecific egg-dumping, sometimes leading to clutch sizes exceeding what a single female can successfully incubate.
Larger birds
Some corvids and finches are unexpected hosts for brood parasites. Magpies (Pica pica) are commonly parasitized by the great spotted cuckoo (Clamator glandarius), which does not remove host eggs but outcompetes host chicks for food. The chalk-browed mockingbird (Mimus saturninus) in South America is a major host for shiny cowbirds (Molothrus bonariensis), showing partial egg rejection behavior.
Certain estrildid finches are targeted by Vidua whydahs and indigobirds, whose chicks mimic the host’s begging calls to blend in. Common waxbills (Estrilda astrild) frequently raise pin-tailed whydahs (Vidua macroura), while red-billed firefinches (Lagonosticta senegala) are victims of the village indigobird (Vidua chalybeata).
40 years of observing brood parasites
Brood parasitism is one of the most fascinating reproductive strategies in the avian world, shaping the complex relationship between parasites and their hosts. From egg mimicry to aggressive retaliation, brood parasites have evolved remarkable adaptations to exploit host parental care. Meanwhile, hosts have developed increasingly sophisticated defenses to detect and reject foreign eggs, creating an ongoing cycle of adaptation and counter-adaptation.
At Planet of Birds, this phenomenon has been a subject of continuous study and observation for over four decades. Since 2010, we have extensively documented brood parasitism, analyzing the behaviors of both parasites and their hosts across multiple species and habitats. This article represents the culmination of years of field research and academic study, offering a comprehensive look into the strategies, impacts, and evolutionary significance of avian brood parasitism.
While much has been uncovered about these extraordinary interactions, new discoveries continue to emerge. The study of brood parasitism remains crucial to understanding the evolutionary pressures that shape bird behavior, as well as the broader ecological consequences of this reproductive strategy.