一般影響
紫輝牛鸝(Molothrus bonariensis)會摧毀或吞食受害者的蛋。他們讓宿主負擔孵蛋要育雛的責任。牛鸝的蛋較早孵化，使宿主自己的幼雛普遍面臨較大的競爭壓力。宿主受影響的程度，取決於許多元素，包括宿主和牛鸝繁殖季節的重疊情形，宿主照顧牛鸝雛鳥的體能狀況，以及宿主是否演化出對抗牛鸝寄生的行為機制。沒有發展出共同演化的物種往往更容易受到傷害(Cruz等人, 1995)。
紫輝牛鸝目前已威脅到一些西印度島嶼瀕危的鳥類(南美大陸和其他島嶼已經充滿了紫輝牛鸝)。例如，在波多黎各島上的黃肩黑鳥(請參閱 Agelaius xanthomus在IUCN瀕危物種紅皮書)，一般認為紫輝牛鸝寄生是使其瀕臨滅絕的主要原因(Lopez-Ortiz 等人, 2002)。另一本地區特有物種，波多黎各維綠鵑(Vireo latimeri)，也受到紫輝牛鸝寄生的威脅，紫輝牛鸝消滅了當地森林保護區的族群(波多黎各最大的乾燥森林保護區)(Woodworth 1999)。事實上，一般認為巢被掠捕和寄生是北溫帶雀形目鳴禽繁殖失敗的首要原因(Woodworth 1999)(很難想像，棲息地喪失明顯比築巢失敗較不重要)。在紫輝牛鸝的原生地南美大陸地區，育雛寄生的威脅已經影響到一些脆弱物種，例如瀕臨滅絕的白頭叢雀(請參閱 Atlapetes pallidiceps在IUCN瀕危物種紅皮書 )(Oppel 等人,2004)和瀕危的福布斯黑鳥(請參閱 Curaeus forbesi在IUCN瀕危物種紅皮書 )(Studer 和Vielliard, 1988)。
宿主和豐度和健康顯著地受到紫輝牛鸝威脅，宿主和豐度和健康卻不一定是維持牛鸝族群重要的因素。相反地，很多大陸性鳥類，受牛鸝影響很小，卻是維持牛鸝族群重要的因素(替牛鸝的維持和隨後的蔓延提供“儲存庫”)(Oppel 等人, 2004)。受害者和宿主完整清單請參考：Lowther, 2004, 牛鸝受害者和宿主清單。
地點特有的影響：Buenos Aires (Argentina) 
寄生: Humid grassland hosts include chestnut-capped blackbirds Agelaius ruficapillus, frequency of parasitism 48%; yellow-winged blackbirds A. thilius, frequency of parasitism 36.6%; scarletheaded blackbirds Amblyramphus holoseriseus, frequency of parasitism 14.3%; brown-and-yellow marshbirds Pseudoleistes virescens. Dry grassland and woodland hosts include chalk-browed mockingbirds Mimus saturninus, frequency of parasitism 78.1%; rufous-collared sparrows Zonotrichia capensis; firewood gathers Anumbius annumbi; rufous horneros Furnarius rufus; bay-winged cowbirds Molothrus badius; rufous-bellied thrushes Turdus rufiventris (Mermoz and Reboreda 1999). Cordoba (Argentina) (Argentina)
寄生: The chalk-browed mockingbird (Mimus saturninus) is probably the best host for Molothrus bonariensis in Villa Maria (Cordoba, Argentina). The cowbirds will puncture and remove the eggs of the hosts (Salvador, 1984). Argentina
降低本地生物多樣性: The brown-and-yellow marshbird (Pseudoleistes virescens) is an effective host species of Molothrus bonariensis. It inhabits marshes and moist grasslands in the east-northeast of Argentina, Uruguay and adjacent areas of Brazil and it is sympatric (occupying the same or overlapping geographic areas without interbreeding) with M. bonariensis in all its distribution (Ridgely and Tudor 1989, in Mermoz and Reboreda 1994). Hispaniola Is. (Atlantic Ocean)
寄生: It exploits the introduced village weaver (Ploceus cucullatus) as its host, reducing nesting success and productivity of the weaver (Cruz, 1989). Greater Antilles (Atlantic Ocean)
降低本地生物多樣性: As the shiny cowbird has moved through the Antilles host species have been shown to include vireos, the Dendroica warblers, Myiarchus flycatchers and Agelaius blackbirds. Icterids (orioles, blackbirds, cowbirds and allies) are favoured shiny cowbird hosts and endemic Antillian island birds are especially vulnerable to brood parasitism. Some island populations face the combined threat of habitat loss and intensive brood parasitism (Post & Wiley 1977; Cruz et al. 1985 1989; Post et al. 1990, in Lovette, Bermingham and Ricklefs 1999). Lesser Antilles (Atlantic Ocean)
降低本地生物多樣性: As the shiny cowbird has moved through the Antilles host species have been shown to include vireos, the Dendroica warblers, Myiarchus flycatchers and Agelaius blackbirds. Icterids (orioles, blackbirds, cowbirds and allies) are favored shiny cowbird hosts and endemic Antillian island birds are especially vulnerable to brood parasitism. Some island populations face the combined threat of habitat loss and intensive brood parasitism (Post & Wiley 1977; Cruz et al. 1985 1989; Post et al. 1990, in Lovette, Bermingham and Ricklefs 1999). In the Lesser Antillies, for example, the endangered Martinique Oriole (Icterus bonana) population declined primarily as a result of intensive brood parasitism by M. bonariensis (Babbs et al. 1987, in Lovette, Bermingham and Ricklefs, 1999). North Andros Is. (Bahamas)
降低本地生物多樣性: Black-cowled orioles (Icterus dominicensis) have been observed with shiny cowbird fledglings (Balts 1995). Distrito Federal (Brazil) (Brazil)
降低本地生物多樣性: The destruction of native cerrado vegetation is expected to increase the pressure on native cerrado birds already affected by the parasitism of M. bonariensis (Cavalcanti and Pimentel, 1988). Chile
降低本地生物多樣性: While Friedmann and Kiff (1985) gave 201 host species, for Chile only 15 species have been reported as hosts (of which only six have been observed raising young). Host bird species that have been noted feeding young shiny cowbirds in Chile (with earliest recorded sighting of feeding) are: Pyrope pyrope (Barros, 1946), Elaenia albiceps (M. Marín, unpubl. data), Mimus thenca (Rottman, 1970), Zonotrichia capensis (the rufous-collared sparrow), Agelaius thilius (the yellow winged blackbird) and Diuca diuca (the Diuca finch) (Barros, 1946) and Bullock (1940). Agelaius thilius appears to be the most affected by the shiny cowbird, receiving up to six eggs per nest. In some localities (such as in some areas in the marshes of the central provinces) surveys have revealed not one nest to be free of shiny cowbird eggs.
Other species that might be able to raise young cowbirds in northern Chile are: the bran-coloured flycatcher (Myiophobus fasciatus) and the vermilion flycatcher (Pyrocephalus rubinus) which are hosts elsewhere (fide Friedmann and Kiff, 1985, in Marín, 2000). Valle del Cauca (Colombia)
降低本地生物多樣性: Molothrus bonariensis prefers to parasitise nest of the house wren (Troglodytes aedon) in the Cauca Valley, Colombia. One study (Katten 1996) found that cowbirds tended to lay their eggs in coincidence with the wrens’ laying period. Of 185 eggs laid in 40 nests, 47% were laid during the 3-day laying period of wrens; many however, were laid before wrens started laying (35%) or after the wrens completed their clutch and initiated incubation (18%). This sample includes only nests in which wrens actually laid, and it underestimates the proportion of cowbird eggs laid prematurely (i.e. before the wrens’ laying period), because cowbirds laid 82 eggs in 20 nests that wrens abandoned before laying. Sometimes these eggs were laid too early during nest construction and were buried as wrens continued adding nesting material. Wrens abandoned 92 of 95 nests that were parasitized with more than two eggs, but accepted cowbird eggs when parasitized with one or two eggs regardless of when they were laid. These factors make the house wren a valuable host because they are able to accept almost any egg at any time (rather than only accepting similar-looking eggs that are laid at the same time as theirs). They are also smaller than cowbirds, giving cowbird chicks an competitive advantage over their small wren siblings. Cowbird chicks usually hatch before wrens, giving them more of a head start. Wrens are abundant in this area, and have an extended breeding season, providing a high availability of nests almost year-round (Alvarez-López et al. 1984, in Kattan 1996). Other potential hosts in this area include water-tyrants (Fluvicola pica), black-billed thrushes (Turdus ignobilis), blue-grey tanagers (Thraupis episcopus), scrub tanagers (Tangara vitriolina) and crimson-backed tanagers (Ramphocelus dimidiatus) (Katten, 1996). Cuba
降低本地生物多樣性: There have been observations of M. bonariensis chicks being fed by black-whiskered vireos (Vireo altiloquoos), tawny-shouldered blackbirds (Agelaius humeralis) and Greater Antillean [Cuban] orioles (Icterus dominicensis melanopsis) (Llanes Sosa and Hernandez de Armas 1995, O.H. Garrido pers. comm. 1998, in Lowther 2004). Predicted hosts in Cuba are Cuban vireos (Vireo gundlachi), zapata wrens ( see Ferminia cerverai in IUCN Red List of Threatened Species), olive-capped warblers (Dendroica pityophila), yellow-headed warbles (Teretistris fernandinae), oriente warblers (Teretistris fornsi), zapata sparrows (see Torreornis inexpectata in IUCN Red List of Threatened Species), eastern meadowlarks (Sturnella magna), red-shouldered blackbirds (Agelaius assimilis) and Cuban blackbirds (Dives atroviolaceus) (Cruz et al. 1998, in Lowther 1998). Yunguilla valley (Ecuador)
威脅瀕危物種: The pale-headed brushfinch (see Atlapetes pallidiceps in IUCN Red List of Threatened Species) is a critically endangered species threatened with habitat loss and limited to a small side valley (Yunguilla Valley) of the Rio Jubones drainage system in southern Ecuador. While widespread conversion of habitat to agricultural land probably caused the species' rareness and is of concern for long-term survival, researchers found that during the breeding season 2002, 61% of pairs breeding in the Yunguilla Reserve were parasitised by cowbirds and during the breeding season 2003, 66% of pairs were parasitised. This was realised to constitute a significant and immediate threat to the tiny remaining population of A. pallidiceps (Oppel et al., submitted, in Schmidt and Schaefer, 2004). Due to its restricted range and timy population the species is classified as critically endangered. The total population has been estimated to consist of less than 100 individuals (Birdlife International, 2000, Krabbe, N., in press, in Schmidt and Schaefer). Jamaica
降低本地生物多樣性: The following species are predicted to be hosts in some islands of the West Indies inlcuding Jamaica: thick-billed vireo (Vireo crassirostris), Jamaican white-eyed vireo (Vireo modestus) and Jamaican oriole (Icterus leucopteryx). The expected host status has since been proved correct for the Jamaican oriole (Cruz et al., 1998, Raffaele et al., 1998, in Lowther, 1998). Martinique
威脅瀕危物種: The decline of the endangered Martinique oriole (see Icterus bonana in IUCN Red List of Threatened Species) has resulted from the combination of intensive brood parasitism by the shiny cowbird and habitat alteration (Babbs et al. 1987, in Lovette, Bermingham and Ricklefs 1999).
降低本地生物多樣性: The main hosts of the shiny cowbird in Martinique are the black-whiskered vireo (Vireo altiloquus) and the yellow warbler (Dendroica petechia). Puerto Rico
威脅瀕危物種: The yellow-shouldered blackbird (see Agelaius xanthomus in IUCN Red List of Threatened Species) has been endangered in Puerto Rico since 1976, mostly due to parasitism by M. bonariensis (Lopez-Ortiz et al. 2002).
The Puerto Rican vireo (Vireo latimeri) is a small, insectivorous, tropical vireo endemic to the island of Puerto Rico. The vireo is subject to brood parasitism by the Molothrus bonariensis (shiny cowbird). M. bonariensis is capable of extirpating the population of the Puerto Rican vireo (Vireo latimeri) from the Guánica Forest reserve unless the vireo population is sustained by immigration from source populations or unless direct management intervention takes place. Saint Lucia
降低本地生物多樣性: The main hosts of the shiny cowbird in St Lucia are the black-whiskered vireo (Vireo altiloquus) and the yellow warbler (Dendroica petechia). Trinidad and Tobago
寄生: Molothrus bonariensis parasitises three main species in Trinidad and Tobago: the house wren (Troglodytes aedon), the red-breasted blackbird (Leistes militaris) and the yellow-hooded blackbird (Agelaius icterocephalus). There are also records of parasitism of the tropical kingbird (Tyrannus melancholicus). M. bonariensis does not appear to affect the reproductive success of any of these species (Cruz et al., 1995). Florida (USA) (United States (USA))
寄生: No definite record of parasitism by this species is known yet for Florida, but red-winged blackbird (Agelaius phoeniceus) has been claimed to have been a shiny cowbird host. Prairie warbler (Dendroica discolor), black-whiskered vireo (Vireo altiloquus) and northern cardinal (Cardinalis cardinalis) are known to have served as cowbird hosts within the Florida range of the brown-headed cowbirds (Paul 1989, Stevenson and Anderson 1994, Cruzet al. 1998, Cruz et al. 2000, in Lowther, 2004). Uruguay
降低本地生物多樣性: The rufous hornero (Furnarius rufus) is one of the more important hosts of the shiny cowbird in Uruguay (Mason and Rothstein, 1986).
The brown-and-yellow marshbird (Pseudoleistes virescens) is another effective host species of M. bonariensis. It inhabits marshes and moist grasslands in the east-northeast of Argentina, Uruguay and adjacent areas of Brazil and it is sympatric with M. bonariensis in all its distribution (Ridgely and Tudor 1989, in Mermoz and Reboreda 1994). Northern Venezuela (Venezuela)
寄生: In a site in Manacaibo Molothrus bonariensis was found to parasitise 80% of the nests of the tropical mockingbird (Mimus gilvus) between April and June, but no parasitism was recorded later in the breeding season of the tropical mockingbird (Paredes et al. 2001). Cruz and Andrews (1989) identified the main hosts of M. bonariensis in the Ilanos of western Venezuela as the pied water-tyrant (Fluvicola pica) and the white-headed marsh-tyrant (Arundinicola leucocephala). Subsequently Cruz and Andrews (1997) studied the parasitism rates of M. bonariensis in a seasonally inundated savanna in the llanos and found that only 3 of 47 parasitized nests were known to have successfully fledged cowbird young, suggesting that this bird is not an optimal host species.
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