Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Datasheet

Molothrus bonariensis
(shiny cowbird)

Toolbox

Datasheet

Molothrus bonariensis (shiny cowbird)

Summary

  • Last modified
  • 27 September 2018
  • 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 bird...

Don't need the entire report?

Generate a print friendly version containing only the sections you need.

Generate report

Pictures

Top of page
PictureTitleCaptionCopyright
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 - 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 - CC BY-SA 2.0
Shiny cowbird (Molothrus bonariensis); male perched.
TitleMale
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.
TitleFemale
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.

Identity

Top of page

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

Top of page

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

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

Notes on Taxonomy and Nomenclature

Top of page

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).

Description

Top of page

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).

Distribution

Top of page

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

Top of page

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.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

North America

MexicoPresentIntroduced1996ISSG, 2011Reported in Quinta Roo (rare) (Avibase, 2012)
USAPresentPresent based on regional distribution.
-AlabamaPresentIntroduced1990ISSG, 2011
-FloridaPresentIntroduced1987ISSG, 2011
-GeorgiaPresentIntroducedISSG, 2011
-LouisianaPresentIntroducedISSG, 2011
-MainePresentIntroducedISSG, 2011
-MississippiPresent, few occurrencesAvibase, 2012
-New MexicoPresent, few occurrencesAvibase, 2012
-North CarolinaPresentIntroduced1990ISSG, 2011
-OklahomaPresentIntroduced1990ISSG, 2011
-South CarolinaPresentIntroduced1989ISSG, 2011
-TennesseePresent, few occurrencesAvibase, 2012
-TexasPresentIntroducedISSG, 2011
-VirginiaPresent, few occurrencesAvibase, 2012

Central America and Caribbean

AnguillaPresentAvibase, 2012
ArubaPresent, few occurrencesAvibase, 2012
BahamasPresentIntroducedISSG, 2011
BarbadosPresentIntroducedISSG, 2011
British Virgin IslandsPresent, few occurrencesAvibase, 2012
Cayman IslandsPresent, few occurrences
Costa RicaPresentAvibase, 2012
CubaPresentIntroduced1982ISSG, 2011
CuraçaoPresentIntroducedc 1985ISSG, 2011
DominicaPresentAvibase, 2012
Dominican RepublicPresentAvibase, 2012
Greater AntillesPresentISSG, 2011
GrenadaPresentAvibase, 2012
GuadeloupePresent, few occurrencesAvibase, 2012
HaitiPresentAvibase, 2012
JamaicaPresentIntroducedc. 1989ISSG, 2011; Avibase, 2012
Lesser AntillesPresentNative Not invasive ISSG, 2011
MartiniquePresentIntroduced Invasive ISSG, 2011; Avibase, 2012
PanamaPresentAvibase, 2012
Puerto RicoPresentIntroduced1955 Invasive ISSG, 2011
Saint Kitts and NevisPresent, few occurrencesAvibase, 2012
Saint LuciaPresent Invasive Post et al., 1990; Lowther and Post, 1999; Toussaint et al., 2009; ISSG, 2011; Avibase, 2012Classified as native by some and alien by others; spreading through islands without direct anthropogenic assistance.
Saint Vincent and the GrenadinesPresentNative Not invasive ISSG, 2011
Trinidad and TobagoPresentNative Not invasive ISSG, 2011
United States Virgin IslandsPresent, few occurrencesAvibase, 2012

South America

ArgentinaPresentNative Not invasive ISSG, 2011
BoliviaPresentNative Not invasive ISSG, 2011
BrazilPresentNative Not invasive ISSG, 2011
-AmazonasPresentNative Not invasive ISSG, 2011
-CearaPresentNative Not invasive ISSG, 2011
-Espirito SantoPresentNative Not invasive ISSG, 2011
-MaranhaoPresentNative Not invasive ISSG, 2011
-Mato GrossoPresentNative Not invasive ISSG, 2011
-Mato Grosso do SulPresentNative Not invasive ISSG, 2011
-Minas GeraisPresentNative Not invasive ISSG, 2011
-ParaPresentNative Not invasive ISSG, 2011
-ParanaPresentNative Not invasive ISSG, 2011
-PernambucoPresentNative Not invasive ISSG, 2011
-PiauiPresentNative Not invasive ISSG, 2011
-Rio de JaneiroPresentNative Not invasive ISSG, 2011
-Rio Grande do SulPresentNative Not invasive ISSG, 2011
ChilePresentIntroducedc. 1865 Invasive ISSG, 2011
ColombiaPresentNative Not invasive ISSG, 2011
EcuadorPresentNative Invasive ISSG, 2011
Falkland IslandsPresentAvibase, 2012
French GuianaPresentNative Not invasive ISSG, 2011
GuyanaPresentNative Not invasive ISSG, 2011
ParaguayPresentNative Not invasive ISSG, 2011
PeruPresentNative Not invasive ISSG, 2011
SurinamePresentNative Not invasive ISSG, 2011
UruguayPresentNative Not invasive ISSG, 2011
VenezuelaPresentNative Not invasive ISSG, 2011

Habitat

Top of page

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

Top of page
CategoryHabitatPresenceStatus
Terrestrial-managed
Cultivated / agricultural land Present, no further details
Disturbed areas Present, no further details
Urban / peri-urban areas Present, no further details
Terrestrial-natural/semi-natural
Natural grasslands Present, no further details
Riverbanks Present, no further details

Hosts/Species Affected

Top of page

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

Top of page

Nutrition

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).

Reproduction

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.

Eggs

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

Top of page

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).

Other

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

Top of page
CauseNotesLong DistanceLocalReferences
Escape from confinement or garden escape Yes
Pet trade Yes Yes

Impact Summary

Top of page
CategoryImpact
Native fauna Negative

Impact

Top of page

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

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

Risk and Impact Factors

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

Uses List

Top of page

General

  • Pet/aquarium trade

Prevention and Control

Top of page

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).

Bibliography

Top of page

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. http://fm1.fieldmuseum.org/aa/Files/lowther/CBList.pdf.

 

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. http://www.birds.cornell.edu/bfl/speciesaccts/brzcow.html

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 http://www.nmnh.si.edu/botany/projects/cpd/sa/sa-viii.htm

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. http://www.conabio.gob.mx/invasoras/index.php/Especies_invasoras_-_Aves

Cornell Laboratory of Ornithology. 2003. All about Birds: Shiny cowbird Molothrus bonariensis Order PASSERIFORMES - Family ICTERIDAE Brown-headed Cowbird (Molothrus ater). http://www.birds.cornell.edu/AllAboutBirds/BirdGuide/Shiny_Cowbird.htmlhttp://www.birds.cornell.edu/AllAboutBirds/BirdGuide/Shiny_Cowbird.html

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

Cruz, A. and Andrews, R.W. 1997. The breeding biology of the pied water-tyrant and its interactions with the shiny cowbird in Venezuela, J. Field Ornithol. 68:91-97. (Abstract)

Cruz, A., and Andrews, R.W. 1989. Observations on the breeding biology of passerines in a s easonally flooded savanna in Venezuela. Wilson Bulletin. 101 (1): 62-76.

Cruz, A., Manolis, T.H., and Andrews, R.W. 1995. History of shiny cowbird Molothrus bonariensis brood parasitism in Trinidad and Tobago. Ibis. 137 (3): 317-321.

Debrot, A.O. and Prins, T.K. 1992. First record and establishment of the shiny cowbird in Curacao, Caribbean J. Sci. 28:104-105. (Abstract)

Fraga, R.M. 1985. Host-parasite interactions between chalk-browed mockingbirds and shiny cowbirds, Ornithol. Monog. 36:829-844. (Abstract)

Friedmann, H. 1929. Subgenus Molothrus: Molothrus bonariensis. In Springfield, C.C. Thomas (ed). The Cowbirds: a Study in the Biology of Social Parasitism.

Fulton, J.T. 1990. The shiny cowbird (Molothrus bonariensis) reaches Alabama, Alabama Birdlife 37:1-3. (Abstract)

Grzybowski, J.A. and Fazio III, V. W. 1991. Shiny cowbird reaches Oklahoma. American Birds 45:50-52. (Abstract)

Hutcheson, W.H. 1990. Shiny cowbird collected in South Carolina: first North American specimen, Wilson Bull. 102:561. (Abstract)

Isacch, J. P. 2002. Nectarivorous Feeding by Shiny Cowbirds: a Complex Feeding Innovation. Wilson's Bulletin. 114 (3): 412-414.
Kattan, G.H. 1996. Growth and provisioning of shiny cowbird and house wren host nestlings. Journal of Field Ornithology. 67 (3): 434-441.

Kluza, D.A. 1998. First record of shiny cowbird (Molothrus bonariensis) in Yucatan, Mexico. Wilson Bulletin. 110 (3): 429.

Lopez-Ortiz, R., Ventosa-Febles, E.A., Reitsma, L.R., Hengstenberg, D., and Deluca, W. 2002. Increasing nest success in the yellow-shouldered blackbird Agelaius xanthomus in southwest Puerto Rico. Biological Conservation. 108 (2): 259-263.

Lovette, I.J., Bermingham, E. and Ricklefs, R.E. 1999. Mitochondrial DNA Phylogeography and the Conservation of Endangered Lesser Antillean Icterus Orioles, Conservation Biology 13(5): 1088 - 1096.

Lowther, P.E. 2004. Lists of Victims and Hosts of the Parasitic Cowbirds (Molothrus), Host Lists. The Field Museum: Chicago. http://fm1.fieldmuseum.org/aa/staff_page.cgi?staff=lowther&id=417

Marin, M. 2000. The Shiny Cowbird (Molothrus bonariensis) in Chile: Introduction or Dispersion? Its Hosts and Parasitic Trends, Ornitologia Neotropical 11 285 - 296.

Mason, P. 1986. Brood parasitism in a host generalist the shiny cowbird Molothrus bonariensis I. The quality of different species as hosts. Auk. 103 (1): 52-60 "Mason, P. 1986. Brood parasitism in a host generalist the shiny cowbird Molothrus bonariensis I. The quality of different species as hosts. Auk. 103 (1): 52-60

Mason, P., and Rothstein, S.I. 1986. Coevolution and avian brood parasitism cowbird Molothrus bonariensis eggs show evolutionary response to host discrimination. Evolution. 40 (6): 1207-1214.

Mermoz, M.E. and Reboreda, J.C. 1994. Brood Parasitism of the Shiny Cowbird, Molothrus bonariensis, on the Brown-and-yellow Marshbird, Pseudoleistes virescens, The Condor 96: 716 - 721.

Mermoz, M.E. and Reboreda, J.C. 1999. Egg-laying Behaviour by Shiny Cowbirds Parasitizing Brown-and-yellow Marshbirds, Animal Behaviour 58: 873–882.

Mermoz, M.E., and Reboreda, J.C. 2003. Reproductive success of Shiny Cowbird (Molothrus bonariensis) parasitizing the larger Brown-and-Yellow Marshbird (Pseudoleistes virescens) in Argentina. The Auk. 120 (4): 1128-1139.

Nair, R. 2002. Behavioural Ecology Research Group. Brain Plasticity and Memory in Birds, ASAB (The Association for the Study of Animal Behaviour) Meeting. http://users.ox.ac.uk/~kgroup/memory.html

North American Bird Information Web Site. Undated. Observations of Shiny Cowbird in the United States of America.

Oppel, S., Schaefer, H.M., and Schmidt, V. 2003. Description of the nest, eggs, and breeding behavior of the endangered Pale-Headed Brush-Finch (Atlapetes pallidiceps) in Ecuador. Wilson Bulletin. 115 (4): 360-366.

Oppel, S., Schaefer, H.M., Schmidt, V. and Scroder, B. 2004. Cowbird Parasitism of Pale-headed Brush-finch Atlapetes pallidiceps: Implications for Conservation and Management, Bird Conservation International 14: 63 - 75.

Papandrea, B. 2000. Intellicast’s Climate Guide to South America. WSI/INTELLICAST.

Paredes, M., Weir, E., and Gil, K. 2001. Reproduction of the bird Mimus gilvus (Passeriformes: Mimidae) in Maracaibo, Venezuela. Revista de Biologia Tropical. 49 (3-4): 1143-1146.

Pereira, L.E., Suzuki, A., Moraes Coimbra, T.L., Pereira de Souza, R., and Bocato Chamelet, E.L. 2001. Ilheus arbovirus in wild birds (Sporophila caerulescens and Molothrus bonariensis). Revista de Saude Publica. 35 (2): 119-123.

Picman, J. and Honza, M. 2002. Are House Wren Troglodytes aedon eggs unusually strong? Test of the predicted effect of intraspecific egg destruction. Ibis. 144: e57-e66.

Post, W. 1993. First specimen of the shiny cowbird, Molothrus bonariensis (Aves: Emberizidae) in North Carolina. Brimleyana. 0 (19): 205-208.

Post, W., Cruz, A. and McNair, D.B. 1993. The North American invasion pattern of the shiny cowbird. Journal of Field Ornithology. 64 (1): 32-41.

Post, W., Nakamura, T.K., and Cruz, A. 1993. Patterns of shiny cowbird parasitism in St. Lucia, West Indies, Southwestern Puerto Rico. Condor. 92 (2): 461-469.

Prather, J.W., and Cruz, A. 2002. Distribution, abundance, and breeding biology of potential cowbird hosts on Sanibel Island, Florida. Florida Field Naturalist. 30 (2): 21-35.

Robert, M., and Sorci, G. 1999. Rapid increase of host defence against brood parasites in a recently parasitized area: The case of village weavers in Hispaniola. Proceedings of the Royal Society of London - Series B: Biological Sciences. 266 (1422): 941-946.

Sackmann, P., and Reboreda, J.C. 2003. A Comparative Study of Shiny Cowbird Parasitism of Two Large Hosts, the Chalk-Browed Mockingbird and the Rufous-Bellied Thrush. The Condor. 105: 728-736.

Salvador, S.A. 1984. Study of parasitism in raising shiny cowbirds Molothrus bonariensis and chalk-browed mockingbirds Mimus saturninus in Villa Maria, Cordoba, Argentine. Hornero. 12 (3): 141-149.

Schmidt, V. and Schaefer, H.M. 2003. Pale-headed Brushfinch Recovery Project in Southwestern Ecuador 2002-2003 (Final Report).

Smith, P. W. Undated. Shiny Cowbird Molothrus bonariensis. Florida's Breeding Bird Atlas.

Sorenson M.D. and Payne R.B. 2001. A Single Ancient Origin of Brood Parasitism in African Finches: Implications for Host-parasite Coevolution, Evolution Int J Org Evolution 55(12): 2550 - 2567.

Studer, A., and Vielliard, J. 1988. First etho-ecological data on the Brazilian Icterid Curaeus forbesi Sclater 1886 Aves Passeriformes. Revue Suisse de Zoologie. 95 (4): 1063-1078.

Sykes, P.W., and Post, W. 2002. First specimen and evidence of breeding by the shiny cowbird in Georgia. Oriole. 66 (3-4): 45-51.

USGS Patuxent Wildlife Research Center. 2000. Shiny cowbird Molothrus bonariensis. Patuxent Bird Identification InfoCenter. http://www.mbr-pwrc.usgs.gov/Infocenter/i4961id.html

Webster, S.J., and Lefebvre, L. 2001. Problem solving and neophobia in a columbiform-passeriform assemblage in Barbados. Animal Behaviour. 62 (1): 23-32.

Woodworth, B.L. 1997. Brood parasitism, nest predation, and season-long reproductive success of a tropical island endemic. Condor. 99 (3): 605-621.

Woodworth, B.L. 1999. Modeling Population Dynamics of a Songbird Exposed to Parasitism and Predation and Evaluating Management Options. Conservation Biology. 13 (1): 67-76.

References

Top of page

Astie AA Reboreda JC, 2006. Costs of egg punctures and shiny cowbird parasitism on creamy-bellied thrush reproductive success. The Auk, 123:23-32.

Avibase, 2012. Avibase - the world bird database. Molothrus bonariensis - Map. http://avibase.bsc-eoc.org/species.jsp?lang=EN&avibaseid=AA7798040CC37113&sec=map&ssver=1.

Bond J, 1957. 2nd Supplement to the checklist of birds of the West Indies. Birds of the West Indies. Philadelphia, USA: Academy of Natural Sciences of Philadelphia.

Carter MD, 1986. The parasitic behavior of the bronzed cowbird Molothrus aeneus in South Texas USA. The Condor, 88:11-25.

Cruz AJ, Wiley JW, Nakamura TK, Post W, 1989. The shiny cowbird Molothrus bonariensis in the West Indian region - biogeography and ecological implications. In: Biogeography of the West Indies [ed. by Woo, C. A.]. Gainsville, Florida, USA: Sandhill Crane Press, 519-540.

Fiorini VD, Tuero DT, Reboreda JC, 2009. Shinycowbirds synchronize parasitism with host laying and puncture host eggs according to host characteristics. Animal Behaviour, 77(3): 561-568.

Friedmann H, Kiff LF, 1985. The parasitic cowbirds and their host. Proceedings of the Western Foundation of Vertebrate Zoology, 2:226-304.

Hamilton WJIII, Orians GH, 1965. Evolution of brood parasitism in altricial birds. The Condor, 67:361-382.

ISSG, 2011. Global Invasive Species Database (GISD). Invasive Species Specialist Group of the IUCN Species Survival Commission. http://www.issg.org/database

Krabbe N, Juina M, Sornoza AF, 2010. Marked population increase of Pale-headed Brush-finch Atlapetes pallidiceps in response to cowbird control. Journal of Ornithology, 152(2):219-222.

Krauss U, 2012. 161 Invasive Alien Species present in Saint Lucia and their current status. Caribbean Alien Invasive Species Network (CIASNET), 12 pp. http://www.ciasnet.org/wp-content/uploads/2010/08/IAS-present-in-SLU-May-2012-revision.pdf

Lowther P, Post W, 1999. Shiny Cowbird (Molothrus bonariensis). In: The Birds of North America No. 399 [ed. by Poole, A. \Gill, F.]. Philadelphia, USA: The Birds of North America, Inc.

Mason P, 1986. Brood parasitism in a host generalist, the Shiny Cowbird: I. The quality of different species as hosts. The Auk, 103:52-60.

Massoni V, Reboreda JC, 2002. A neglected cost of brood parasitism: egg punctures by shiny cowbirds during inspection of potential host nests. The Condor, 104:407-412.

Mayr E, 1965. The nature of colonization in birds. Academic Press, New York. In: The Genetics of Colinizing Species [ed. by Baker, H. G. \Stebbins, G. L.]. New York, USA: Academic Press, 29-43.

Medina-Miranda R, Lopez-Ortiz R, Ramos-Álavarez K, Cruz-Burgos J, McKenzie P, Swinnerton K, Ortega C, Cruz A, Liu I, Lacy R, Miller P, Diaz-Soltero H, Traylor-Holzer K, 2013. Yellow-shouldered Blackbird/Shiny Cowbird Population and Habitat Viability Assessment Workshop. Final Report. Apple Valley, Minnesota, USA: IUCN SSC Conservation Breeding Specialist Group.

Nakamura TK, Cruz A, 2000. The ecology of egg-puncture behavior by the Shiny Cowbird in southwestern Puerto Rico. In: Ecology and Management of Cowbirds and their Hosts: Studies in the Conservation of North American Passerine Birds [ed. by Smith, J. N. M. \Cook, T. L. \Rothstein, S. I. \Robinson, S. K. \Sealy, S. G.]. Austin, USA: University of Texas Press, 178-186.

NatureServe, 2012. InfoNatura. Animals and Ecosystems of Latin America. Molothrus bonariensis - Shiny Cowbird. http://www.natureserve.org/infonatura/.

Peer BD, 2006. Egg destruction and egg removal by avian brood parasites: adaptiveness and consequences. The Auk, 123:16-22.

Peer BD, Sealy SG, 1999. Laying time of the bronzed cowbird. Wilson Bulletin, 111:137-139.

Porrata-Doria T, 2006. MSc Thesis. Mayaguez, Puerto Rico: University of Puerto Rico, Mayaguez Campus.

Post W, Nakamura TK, Cruz A, 1990. Patterns of Shiny Cowbird Parasitism in St. Lucia and Southwestern Puerto Rico. The Condor, 92:461-469.

Post W, Sykes PW, 2011. Reproductive Status of the Shiny Cowbird in North America. Journal of Ornithology, 123(1):151-154.

Reetz MJ, Musser JM, Kratter AW, 2010. Further Evidence of Breeding by Shiny Cowbirds in North America. Journal of Ornithology, 122(2):365-369.

Scott DM, Weatherhead PJ, Ankney CD, 1992. Egg-eating by female brownheaded cowbirds. The Condor, 94:579-584.

Sealy SG, 1992. Removal of yellow warbler eggs in association with cowbird parasitism. The Condor, 94:40-54.

Skutch A, 1976. Parent Birds and their Young. Austin, Texas, USA: University of Texas Press, 503 pp.

Smith JNM, Cook TL, Rothstein SI, Robinson SK, Sealy SG, 2000. Ecology and management of cowbirds and their hosts [ed. by Smith, J. N. M. \Cook, T. L. \Rothstein, S. I. \Robinson, S. K. \Sealy, S. G.]. Austin, USA: University of Texas Press, 400 pp.

Strausberger BM, 1998. Evident nest-searching behavior of female brown-headed cowbirds while attended by males. Wilson Bulletin, 110:133-136.

Toussaint A, John CL, Morton MN, 2009. The Status and Conservation of Saint Lucia's Forest Birds. Technical Report No. 12 to the National Forest Demarcation and Bio-Physical Resource Inventory Project. Helsinki, Finland: FCG International Ltd.

Tuero DT, Fiorini VD, Reboreda JC, 2007. Effects of Shiny Cowbird Molothrus bonariensis parasitism on different components of House Wren Troglodytes aedon reproductive success. Ibis, 149(3):521-529.

US Fish and Wildlife Service, 1996. In: Yellow-Shouldered Blackbird (Agelajus xanthomus) Revised Recovery Plan. US Fish and Wildlife Service, 109 pp.

Wiley JW, 1986. Growth of shiny cowbird and host chicks. Wilson Bulletin, 98(1):126-131.

Woodworth B, 1999. Modeling Population Dynamics of a Songbird Exposed to Parasitism and Predation and Evaluating Management Options. Conservation Biology, 13(1):67-76.

Contributors

Top of page

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

    Distribution Maps

    Top of page
    You can pan and zoom the map
    Save map