Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide


Molothrus bonariensis
(shiny cowbird)



Molothrus bonariensis (shiny cowbird)


  • Last modified
  • 20 November 2019
  • Datasheet Type(s)
  • Invasive Species
  • Natural Enemy
  • Preferred Scientific Name
  • Molothrus bonariensis
  • Preferred Common Name
  • shiny cowbird
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Metazoa
  •     Phylum: Chordata
  •       Subphylum: Vertebrata
  •         Class: Aves
  • Summary of Invasiveness
  • M. bonariensis, the shiny cowbird, is a brood parasite, relying on a host bird to incubate its eggs and rear its chicks. It is not host-specific, and will lay eggs in the nests of numerous other species of birds, only some of which will a...

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Shiny Cowbird (Molothrus bonariensis); chick being fed by adult Rufous-collared Sparrow (Zonotrichia capensis). Parque da Independência, Museu do Ipiranga, São Paulo. November 2007.
TitleChick being fed by adult Rufous-collared Sparrow
CaptionShiny Cowbird (Molothrus bonariensis); chick being fed by adult Rufous-collared Sparrow (Zonotrichia capensis). Parque da Independência, Museu do Ipiranga, São Paulo. November 2007.
Copyright©Dario Sanches/via Wikimedia Commons - CC BY-SA 2.0
Shiny Cowbird (Molothrus bonariensis); chick being fed by adult Rufous-collared Sparrow (Zonotrichia capensis). Parque da Independência, Museu do Ipiranga, São Paulo. November 2007.
Chick being fed by adult Rufous-collared Sparrow Shiny Cowbird (Molothrus bonariensis); chick being fed by adult Rufous-collared Sparrow (Zonotrichia capensis). Parque da Independência, Museu do Ipiranga, São Paulo. November 2007.©Dario Sanches/via Wikimedia Commons - CC BY-SA 2.0
Shiny cowbird (Molothrus bonariensis); male perched.
CaptionShiny cowbird (Molothrus bonariensis); male perched.
CopyrightReleased into the public domain by its author, Marina Torres.
Shiny cowbird (Molothrus bonariensis); male perched.
Male Shiny cowbird (Molothrus bonariensis); male perched.Released into the public domain by its author, Marina Torres.
Shiny cowbird (Molothrus bonariensis); female perched.
CaptionShiny cowbird (Molothrus bonariensis); female perched.
CopyrightReleased into the public domain by its author, Marina Torres.
Shiny cowbird (Molothrus bonariensis); female perched.
FemaleShiny cowbird (Molothrus bonariensis); female perched.Released into the public domain by its author, Marina Torres.


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Preferred Scientific Name

  • Molothrus bonariensis (Gmelin, 1789)

Preferred Common Name

  • shiny cowbird

Other Scientific Names

  • Molothrus bonariensis subspecies maxillaris Lafresnaye

International Common Names

  • Spanish: tordo lustroso; tordo renegrido; tordo vaquero
  • French: vacher luisant

Summary of Invasiveness

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M. bonariensis, the shiny cowbird, is a brood parasite, relying on a host bird to incubate its eggs and rear its chicks. It is not host-specific, and will lay eggs in the nests of numerous other species of birds, only some of which will accept and rear the chicks. M. bonariensis reduces the clutch size of the host they parasitize by removing or destroying some of their eggs (Nakamura and Cruz, 2000). Nestling competition between parasite and host chick may be detrimental to the success of the host offspring (Wiley, 1986b). Due to its parasitic lifestyle, it is negatively affecting some threatened bird species that are already at risk due to habitat loss.

M. bonariensis has expanded its range in its native South America, established exotic populations beyond its native range in the Caribbean, and has reached the North American continent.

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Metazoa
  •         Phylum: Chordata
  •             Subphylum: Vertebrata
  •                 Class: Aves
  •                     Order: Passeriformes
  •                         Family: Icteridae
  •                             Genus: Molothrus
  •                                 Species: Molothrus bonariensis

Notes on Taxonomy and Nomenclature

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M. bonariensis belongs to the family Icteridae, which includes five species of parasitic cowbirds that form the natural genus Molothrus (as determined by phylogenetic analyses of mitochondrial DNA sequences) (Lowther, 2004). Molothrus includes the giant cowbird M. (formerly Scaphidura) oryzivorus and excludes the non-brood parasitic bay-winged cowbird Agelaioides (formerly Molothrus) badius. Several subspecies of M. bonariensis exist (see Description).


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Field identification should be based on the presence of a slender conical bill, a uniform dull blue-black plumage and squared-off tail, and a solid dark eye-colour (Kluza, 1998). Males have a purplish shine on their head, neck, breast and upper back and a blue shine on their wings, whilst females are grey-brown with whitish eyebrows and throats (The Cornell Laboratory of Ornithology, 1999). Nestlings have flesh-coloured skin with scattered tufts of blackish down. The oral flanges range from white to yellow and the mouth-lining is reddish.

Seven subspecies are recognised and differ markedly in size. The smallest subspecies is M. bonariensis minimus (males average 39g; females average 32g), the largest is M. bonariensis cabanisii (males average 64g; females average 56g), with the nominal M. bonariensis bonariensis being intermediate (males average 56g; females average 45.6g) (Wiley 1986; Kattan 1996; Mermoz and Reboreda 2003; ME Mermoz., pers. comm., 2005).

The sexually dimorphic subspecies M. bonariensis minimus has a total length of about 18 cm (Lowther and Post, 1999). The adult male has glossy purple-black plumage on its head and torso and duller black plumage on the wing, rump and abdomen. Juvenile males are mostly dull brown, but sometimes a mixture of glossy purple-black and brown. The adult female has an ashy-brown crown cut by a lighter coloured supercilium. The breast and abdomen are a streaked ashy-brown over a lighter coloured brown. The torso is dull brown and most present a lighter-colour on the tips of the feathers. Immature females differ by having dull ashy-brown feathers. M. bonariensis of both sexes and all life stages have black conical bills and legs, and brown eyes (Porrata-Doria, 2006).


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Native Range

M. bonariensis was historically confined to South America and parts of the Caribbean such as Trinidad and Tobago (Cruz et al., 2005). There is not a single country in South America that is not inhabited in part or in whole by (at least) one subspecies of M. bonariensis. As might be expected of a species present across such an extended range and variety of environments, it comprises numerous (seven) subspecies: bonariensis (the most widely ranging subspecies occurs through many parts of South America, between sea level and up to 3500m), occidentalis (in western Peru and south-western Ecuador), aequatorialis (in coastal regions of Ecuador and south-western Colombia) cabanisii (in north-western Colombia), venezuelensis (in northern Venezuela), minimus (in Guyana, Suriname, French Guiana, north-eastern Brazil and some Caribbean islands) (Friedmann, 1929) and riparius (Eastern Peru and neighbouring regions of Brazil) (Ortega, 1998, Jaramillo and Burke, 1999, in M. E. Mermoz., pers. comm., 2005).

Known Non-Native Range

M. bonariensis has expanded its range in America to new areas in South America (including central Chile) and the Caribbean (including Puerto Rico, the Bahamas, Barbados, Hispaniola and Cuba). It has also expanded to parts of Central America as well as some states in southern USA (Marín, 2000; Baltz, 1995; Friedmann, 1929; Robert and Sorci, 1999 and Kluza, 1998). Some populations in the USA may be vagrant (NatureServe, 2012), however, M. bonariensis now has a permanent population in Florida, having first arrived, apparently unaided, in 1985 from the Caribbean (Avery and Tillman, 2005). There is evidence of egg-laying in Florida (Reetz et al., 2010) and breeding individuals have also been reported from South Carolina (Post and Sykes, 2011). 

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Last updated: 10 Feb 2022
Continent/Country/Region Distribution Last Reported Origin First Reported Invasive Reference Notes

North America

Antigua and BarbudaPresentIntroduced1959
ArubaPresent, Few occurrences
British Virgin IslandsPresent, Few occurrences
Cayman IslandsPresent, Few occurrences
Costa RicaPresent
CuraçaoPresentIntroducedFirst reported: c 1985
Dominican RepublicPresentIntroduced1972
GuadeloupePresent, Few occurrences
JamaicaPresentIntroducedFirst reported: c. 1989
MexicoPresentIntroduced1996Reported in Quinta Roo (rare) (Avibase, 2012)
Puerto RicoPresentIntroduced1955Invasive
Saint Kitts and NevisPresent, Few occurrences
Saint LuciaPresentInvasiveClassified as native by some and alien by others; spreading through islands without direct anthropogenic assistance.
Saint Vincent and the GrenadinesPresentNative
Trinidad and TobagoPresentNative
U.S. Virgin IslandsPresent, Few occurrences
United StatesPresentPresent based on regional distribution.
-MississippiPresent, Few occurrences
-New MexicoPresent, Few occurrences
-North CarolinaPresentIntroduced1990
-South CarolinaPresentIntroduced1989
-TennesseePresent, Few occurrences
-VirginiaPresent, Few occurrences

South America

-Espirito SantoPresentNative
-Mato GrossoPresentNative
-Mato Grosso do SulPresentNative
-Minas GeraisPresentNative
-Rio de JaneiroPresentNative
-Rio Grande do SulPresentNative
ChilePresentIntroducedInvasiveFirst reported: c. 1865
Falkland IslandsPresent
French GuianaPresentNative


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M. bonariensis is a native inhabitant of the South American Pampas. The Pampas cover c. 900,000 km² between latitudes 28°-39°S and longitudes 50°-65°W in the southernmost part of Brazil, the whole of Uruguay and the central-eastern part of Argentina. The climate is mild with precipitation of 600-1200mm more or less evenly distributed through the year. The soils are very rich and the dominant vegetation types are grassy prairie and grass-steppe in which numerous species of the grass tribe Stipeae (specifically genera Stipa and Piptochaetium) are particularly conspicuous. There is an almost absolute lack of native trees, except along main watercourses (CPD, Undated).

The climate of central Argentina and Uruguay (Buenos Aires, Cordoba, Rosario, Santa Fe, Mar Del Plata, Montevideo, Punta del Este, Colonia Sacremento) is naturally changeable (as this region is in the mid-latitudes). Winters are cool to mild and summers are very warm and humid. Rainfall is fairly uniform throughout the year but is a little heavier during the summer. Annual rainfall is heaviest near the coast and decreases gradually further inland. Rain in late spring and summer usually arrives in the form of brief heavy showers and thunderstorms. More general rainfall occurs during the remainder of the year as cold fronts and storm systems move through. Although cold spells during the winter often send night-time temperatures below freezing, snow is quite rare. In most winters, a few light snowfalls occur over inland areas. Snow is extremely rare near the coast (Papandrea, 2000).

M. bonariensis is able to adapt to a wide variety of habitat-types other than its native Pampas. It is common in cultivated land in its native region (much of which has been modified to graze cattle or plant soybeans). In Chile, M. bonariensis is common in the marshes of the central provinces (Marín, 2000). In this country dry years with little or no snow have been noted to correspond to higher abundances of cowbirds (Rahmer, in Friedmann 1929). Subspecies bonariensis avoids heavily forested areas while subspecies cabanisii may occur in the lower borders of cloud forests. Subspecies aequatorialis is found in a variety of ecosystems, from dry sandy habitats with stunted vegetation, to mangrove forests, to impenetrable jungle (Friedmann, 1929). In Ecuador in the area around the Rio Jubones drainage system in the Yunguilla valley M. bonariensis has been reported to favour warm dry habitats (Oppel et al. 2004). M. bonariensis avoids the following regions in South America: the Amazonian forests, the High Andes and southern Patagonia (Friedmann, 1929; Fraga, 1985, Wiley, 1985, in Mermoz and Reboreda 1994).

Habitat List

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Terrestrial ManagedCultivated / agricultural land Present, no further details
Terrestrial ManagedDisturbed areas Present, no further details
Terrestrial ManagedUrban / peri-urban areas Present, no further details
Terrestrial Natural / Semi-naturalNatural grasslands Present, no further details
Terrestrial Natural / Semi-naturalRiverbanks Present, no further details

Hosts/Species Affected

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The shiny cowbird is an extreme host generalist; its eggs have been found in the nest of over 201 species (Friedmann and Kliff, 1985; Mason, 1986). The ability to successfully parasitize different host species has facilitated its spread throughout the West Indies, allowing for the opportunistic exploitation of species encountered in these new regions (Cruz et al., 1989).

Biology and Ecology

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Omnivorous: Studies indicate that nestling cowbirds require a diet composed of protein, which most passerine species will provide for their young (Mason, 1986). For example, pale-headed brush-finch (Atlapetes pallidiceps) feed their fledglings (and any cowbird parasitic fledglings in their nest) small invertebrates such as crickets, caterpillars, adult Lepidoptera, beetles and earthworms (Lumbricidae) (Schmidt and Schaefer, 2003). Resident cowbirds have been noted to eat a greater proportion of insects in their diet, while migrant birds rely on seeds to a greater extent (Friedmann, 1929). They have been observed feeding on the nectar of flax (Phormium tenax) flowers (Isacch, 2002).


M. bonariensis is an obligate brood parasite, that is, it has completely abandoned the task of building nests, incubating eggs, and feeding and rearing nestlings. It is an extreme generalist, basing its reproductive success on its high fecundity. Although the number of eggs laid per breeding season is unknown, an estimate would be 20-30, as has been found for the closely-related brown-headed cowbird (M. ater) (P Lowther, personal communication). Whereas M. ater removes host eggs from the nest, M. bonariensis instead punctures host eggs, leaving them in the nest (Smith et al., 2000).

The breeding season of M. bonariensis is October to January in Argentina, but may be extended in the South American tropics. Shiny cowbirds have been known to synchronise breeding with that of their high quality hosts (Wiley, 1988, in Mermoz and Reboreda 2003). In Ecuador M. bonariensis visited the Yunguilla Reserve during the breeding season of the pale-headed brush-finch, a resident host (Schmidt and Schaefer, 2003). The reproductive success of cowbirds depends on the traits of the host; cowbird chicks in the nests of smaller hosts such as the house wren, Troglodytes aedon, have high survival rates due to cowbirds having a competitive edge over their “siblings” (Katten, 1996, in Katten 1997). Cowbird chicks in the nests of large hosts such as the chalk-browed mockingbird, Mimus saturninus, have lower survival rates (Fraga, 1985, in Katten 1997). Although it is large, the brown-and-yellow marshbird, Pseudoleistes virescens, has “helper” birds that aid in chick rearing, which increases chick survival rates (Mermoz and Reboreda, 1994). Other traits that increase the value of a cowbird host include: the construction of open nests, low nest attentiveness during egg-laying, a clutch size of 4 to 5 eggs, an extended breeding season and a long incubation period. Some hosts reject unusual looking eggs or eggs laid before or after their own by pushing them out of the nest, building the nest over them or abandoning their nest (Friedmann, 1929; Wiley, 1988; Schmidt and Schaefer, 2003; Kattan, 1996; Mermoz and Reboreda, 1994). Cowbirds have a short incubation period which can give them a competitive advantage. For example, cowbird chicks hatch 1 to 4 days before brown-and-yellow marshbird chicks, which may give them up to a 4-day head-start on their nest-mates (Mermoz and Reboreda, 2003).

Shiny cowbirds monitor host nests in their territory; they may adaptively adjust the timing between parasitism and host egg-laying according to the host species. Cowbird eggs also have shorter incubation periods than hosts of similar size (Briskie and Sealy, 1990; Kattan, 1995), so synchronization of parasitism generally results in cowbird chicks hatching before host chicks, giving the parasite a 1-2 days advantage in the competition for food with their nestmates (Briskie and Sealy, 1990; Peer and Bollinger, 1997; Mermoz and Reboreda, 2003).

Fiorini et al. (2009) found that cowbirds were more likely to synchronize parasitism with the egg-laying period of the host if the host was a chalk-browed mockingbird compared with if it was a house wren (and egg puncturing was also more intense for mockingbird parasitism). It may be that the synchronicity between parasitism and host laying in the case of mockingbird parasitism helps to ensure that the cowbird egg hatches before the mockingbird eggs so the cowbird chick can gain a competitive advantage. On the other hand, it may be that egg-laying can be sloppier in the case of wren parasitism, since the smaller wren hatchlings are less able to compete even if they hatch first. Katten (1996) found that 33% of cowbird eggs were laid in coincidence with the host’s laying period, while 55% were laid before and 12% after. There appears to be no specificity in terms of which host-species M. bonariensis prefers in any area. High cowbird density or low host nest availability may induce shiny cowbirds to be wasteful, laying eggs on the ground or in nests crowded with up to 36 other cowbird eggs (Kattan, 1996). Such nests are necessarily abandoned by the host.


The incubation period of M. bonariensis is about 11 - 13 days. Eggs have an extraordinary diversity in the colour and markings and can be pure white or flesh coloured with sparsely or densely scattered pink or red flecks. Some may have fine marks like pen scratches while others may have large chocolate brown spots. There is no such thing as characteristic markings in the eggs of this species although the eggs of the same individual show a "family resemblance". In general, eggs may be white-immaculate or spotted; spotted eggs may have a white, pale grey or pale blue background with a variable pattern of grey and reddish-brown spots. In size they may vary from 20mm x 26mm to 18mm x 22mm (Friedmann, 1929).

Means of Movement and Dispersal

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Introduction Pathways to New Locations

Natural Dispersal

Seasonal movements of M. bonariensis are well known in Argentina, where a bird banded in September in Cordoba was found in November in Rio Negro (600km south) (Lucero, 1982, in Marín, 2000). Natural migration has been the cause of its spread to the Antille, North America and Chile (Mermoz 2004).


M. bonariensis has benefited from the modification of the environment by humans during the last century, particularly by the creation of urban spaces and other open habitats with few trees (Cruz et al., 1985; Ridgely and Tudor, 1989; Canevari et al., 1991, in Isacch, 2002). The flexibility displayed by M. bonariensis in both its host generalist strategy and omnivorous diet may be responsible for their range expansion, as such generalist behaviour enables them to exploit novel, modified habitats and food sources (Isacch, 2002).

Pet/Aquarium Trade

In the early 1900s M. bonariensis was a popular cage bird, and large-scale importations took place from Argentina to markets in the central regions of Chile, including Rancagua and Santiago. Both Reed (1913; 1934) and Barros (1921; 1946; 1956) suggested that the Chilean population of M. bonariensis originated from caged birds that were liberated or had escaped in or near Santiago (Marin et al., 1989, in Marin 2000). Interestingly, these populations, and not the populations in northern Peru, are believed to be the origin of populations established more recently in southwest Peru and northern Chile (in Marín, 2000).
Debrot and Prins (1992) suggest that aviarists may have encouraged its spread on the island of Curacao (Netherlands Antilles) due to its presence in a local bird show in 1991, and observation of a probable captive escapee on the island.

A genetic estimates study (Porrata-Doria, 2006) indicated that the population in the expanded range is differentiated, and that there is no or very low gene flow between the expanded and original range populations.  This suggests that M. bonariensis minimus does not migrate, and that the expanded populations, once founded, have not been supplemented with other migrating individuals.

Local Dispersal Methods

Escape from confinement: see 'Pet/aquarium trade' (above).

M. bonariensis has benefited from the modification of the environment by humans during the last century (see above).

Pathway Causes

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CauseNotesLong DistanceLocalReferences
Escape from confinement or garden escape Yes
Pet trade Yes Yes

Impact Summary

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Native fauna Negative


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Original text compiled by IUCN SSC Invasive Species Specialist Group (ISSG)

M. bonariensis affects its hosts by destroying or devouring their eggs. They burden hosts that accept responsibility for their eggs with the additional and significant costs involved in incubating eggs and feeding and rearing the hatchlings. Competitive pressure on host young is also increased due to the generally relatively early hatching of (and sometimes relatively larger) cowbird hatchling (see Tuero et al., 2007 for further discussion of mechanisms of impact on the host). The extent to which a host species is affected depends on a number of elements, including the overlap between the host and cowbird breeding season in a region, the physical ability of the host to care for cowbird chicks, and the presence or absence of a host species’ evolved behavioural response to cowbird eggs. Species that have not co-evolved with brood parasitism are often more vulnerable. (Cruz et al. 1995). The expansion of M. bonariensis into areas of the Caribbean where it is non-native within the last century has brought them into contact with avian communities that have not experienced brood parasitism. Certain Caribbean birds are therefore at greater risk of harm from cowbird contact than mainland birds, since they have not co-evolved with cowbirds (Cruz et al., 2005).

Indeed, shiny cowbirds are currently a threat to several vulnerable bird species on some Caribbean islands where they have spread to (from continental South America and other Caribbean islands already populated with the shiny cowbird). For example, on the island of Puerto Rico the yellow-shouldered blackbird (see Agelaius xanthomus in IUCN Red List of Threatened Species) is thought to be endangered mainly due to parasitism by M. bonariensis (Lopez-Ortiz et al. 2002). Another species endemic to this region, the Puerto Rican vireo (Vireo latimeri), is also threatened by the brood parasitism of M. bonariensis, which threatens to wipe out the local population in the Guánica Forest reserve (Puerto Rico's largest dry forest reserve) (Woodworth 1999). It is cited as a brood parasite on the endemic oriole (see Icterus laudabilis in IUCN Red List of Threatened Species) in Saint Lucia. In fact, nest predation and parasitism are believed to be the primary causes of reproductive failure in northern temperate passerine songbirds (Woodworth 1999) (although it is hard to imagine that habitat loss is a significantly less important factor relating to nesting failure). In regions of continental South America where the shiny cowbird is native, brood parasitism threatens some vulnerable species already affected by habitat loss, for example the critically endangered pale-headed brush-finch (see Atlapetes pallidiceps in IUCN Red List of Threatened Species ) (Oppel et al 2004) and the endangered Forbes' blackbird (see Curaeus forbesi in IUCN Red List of Threatened Species) (Studer and Vielliard, 1988).

Host species whose health and abundance are threatened significantly by M. bonariensis are not necessarily important for the sustainment of cowbird populations (meaning their decline will not affect abundance of M. bonariensis). Conversely, a large wide-ranging continental species that is minimally affected as a whole by cowbirds may play an important role in sustaining populations of M. bonariensis (which provide “reservoirs” for the sustainment and subsequent spread of the species) (Oppel et al. 2004). For a full list of victims and hosts please see: Lowther (2012)Host list of the brood parasitic cowbirds.

Threatened Species

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Threatened SpeciesConservation StatusWhere ThreatenedMechanismReferencesNotes
Agelaius xanthomus (yellow-shouldered blackbird)EN (IUCN red list: Endangered); USA ESA listing as endangered speciesPuerto RicoParasitism (incl. parasitoid)US Fish and Wildlife Service (1996); ISSG (2011)
Atlapetes pallidicepsCR (IUCN red list: Critically endangered)ISSG (2011)
Curaeus forbesiEN (IUCN red list: Endangered)ISSG (2011)
Icterus laudabilis (St Lucia oriole)NT (IUCN red list: Near threatened)Saint LuciaCompetition - monopolizing resources; Parasitism (incl. parasitoid)
Vireo latimeriLC (IUCN red list: Least concern)Puerto RicoISSG (2011)

Risk and Impact Factors

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Impact outcomes
  • Threat to/ loss of native species
Impact mechanisms
  • Competition - monopolizing resources
  • Parasitism (incl. parasitoid)

Uses List

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  • Pet/aquarium trade

Prevention and Control

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Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Original text compiled by IUCN SSC Invasive Species Specialist Group (ISSG)

Smith (1999) suggested cowbird control to be justified if the parasitism level exceeds 60% over 2 years, however, small isolated bird populations facing multiple threats may be non self-sustaining at levels as low as 20%. The approaches used to manage shiny cowbirds are described and discussed below:

Physical: Most removal programmes in North America to control brood parasitism by the related brown-headed cowbird (Molothrus ater) rely on large cage-traps for cowbird control. Selective shooting has also been applied to remove cowbirds, but has yielded mixed results. Although site-specific shooting may be an effective complementary tool to support landscape-scale management, shooting alone may not always be sufficient to significantly reduce cowbird parasitism rates (Eckrich et al. 1999, Whitfield, 2000, in Oppel et al., 2004).

Another effective option is to monitor host nests during the breeding season, constantly removing cowbird eggs and chicks. Host eggs must be clearly distinguishable from cowbird eggs. When nest monitoring is required the host nests should not be approached while either parent is close to the nest, and damage to the surrounding vegetation should be kept to a minimum to avoid creating gaps around the nest and encouraging predation. While this method is intrusive and requires a considerable level of skill, nest manipulation is efficient and cost-effective, especially in areas where trapping is impractical (Schmidt and Schaefer, 2003; Oppel et al., 2004).

Habitat factors to consider: Factors influencing the intensity of cowbird parasitism include the type of microhabitat nests are built in (including the level of nest concealment and the structural diversity of vegetation); forest bird nests in cleared areas may be more vulnerable to cowbird parasitism. A study of the endangered pale-headed brush-finch in Ecuador, revealed land use to be a major factor determining the impact of cowbird parasitism. In cattle-grazed areas breeding rates of the brush-finch were two times greater than in ungrazed areas due to a decrease in cowbirds numbers (correlated with a decrease in bird diversity and abundance in the grazed areas) (Oppel et al. 2004).

On the other hand, M. bonariensis is associated with dry open habitats (rather than moist forest habitats) and its range expansion (in Chile and parts of the Caribbean) may have been facilitated by the conversion of forested areas to early successional habitats (as well as the lack of native brood parasites in the case of some Caribbean islands) (Marín 2000; Post and Wiley, 1977, Cruz et al. 1995).

Cowbird control has to be maintained for an infinitely long time, as cowbird populations at a regional level are not affected by most removal programmes. Despite often leading to reduced parasitism rates, cowbird removal has only occasionally triggered an evident increase in the target host population, and it has been suggested that habitat quality or quantity might be more limiting than cowbird parasitism rates alone (Oppel et al. 2004). Nevertheless, cowbird control has been implicated in the recovery of the endangered pale-headed brush-finch, since a shooting programme to control M. bonariensis began in 2003 (Krabbe et al., 2010). A control programme involving trapping and removal of cowbird eggs and nestlings from artificial nesting structures has been associated with a steep reduction in parasitism of yellow-shouldered blackbird nests and an increase in the blackbird population (Cruz et al., 2005).

Reversing the decline of the yellow-shouldered blackbird (Agelaius xanthomus) and adequately assuring the persistence of the species will require either the near-complete and immediate elimination of M. bonariensis from Puerto Rico, or the near-complete elimination of the impacts of M. bonariensis on A. xanthomus. The current strategy of removing cowbirds from A. xanthomus nests is probably more feasible than removing all M. bonariensis from southwestern Puerto Rico. However, even these aggressive management measures (either removing all M. bonariensis or removing all M. bonariensis eggs from A. xanthomus nests) may not be sufficient to protect the A. xanthomus from extinction and allow it to recover to safer numbers, unless other causes of mortality are also reduced (Medina-Miranda et al., 2013).


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Smith JNM, Cook TL, Rothstein SI, Robinson SK, Sealy SG, eds., 2000. Ecology and management of cowbirds and their hosts. Austin, USA: University of Texas Press, 400 pp.

Lowther, P. E. 2010. Lists of victims and hosts of the parasitic cowbirds (Molothrus). Version 22 September 2010. The Field Museum. Chicago, Illinois, USA.


References from GISD

Baltz, M. E. 1995. First records of the Shiny Cowbird (Molothrus bonariensis) in the Bahama Archipelago. The Auk. 112 (4): 1039.

Baltz, M. E. and D. E. Burhans. 1998. Rejection of artificial parasite eggs by gray kingbirds in the Bahamas (Abstract). Condor 100: 566-568.

BFL (Birds in Forested Landscapes). Undated. Brood Parasite: Bronzed Cowbird (Molothrus aeneus). Cornell Lab of Ornithology.

Catherine Levy, Jamaica

Cavalcanti, R.B. and Pimentel, T.M. 1988. Shiny cowbird parasitism in central Brazil, Condor 90:40-43. (Abstract)

Cavalcanti, R.B., and Pimentel, T.M. 1988. Shiny cowbird parasitism in central Brazil. Condor. 90 (1): 40-43.

Centres of Plant Diversity and Endemism (CPD). Undated. South America: Centres of Plant Diversity and Endemism: VIII. Southern Cone

CONABIO. 2008. Sistema de información sobre especies invasoras en México. Especies invasoras - Aves. Comisión Nacional para el Conocimiento y Uso de la Biodiversidad. Fecha de acceso.

Cornell Laboratory of Ornithology. 2003. All about Birds: Shiny cowbird Molothrus bonariensis Order PASSERIFORMES - Family ICTERIDAE Brown-headed Cowbird (Molothrus ater).

Cruz, A. 1989. The decline of an adaptation in the absence of a presumed selection pressure. Evolution. 43 (1): 55-62.

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Reviewed by: Myriam E. Mermoz, Departamento de Ecología, Genética y Evolución Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires, Argentina

      Compiled by: National Biological Information Infrastructure (NBII) & IUCN/SSC Invasive Species Specialist Group (ISSG)

    Last Modified: Sunday, July 03, 2005

    23/06/13 Updated by:

    E Ventosa, Department of Natural and Environmental Resources, Puerto Rico

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