The Wilson Journal of Ornithology 123 ( 2 ): 277 - 282 , 2011 
GIANT COWBIRD {MOLOTHRUS ORYZIVORUS ) PARASITISM OF 
RED-RUMPED CACIQUES (CACICUS HAEMORRHOUS) IN 
THE ATLANTIC FOREST, NORTHEASTERN ARGENTINA 
ROSENDO M. FRAG A 1 
.ABSTRACT.—Host-parasite interactions between Red-rumped Caciques (Cacicus haemorrhous) and Giant Cowbirds 
{Molothrus oryzivorus) were studied at 52 nests in the Atlantic Forest. Misiones. Argentina. Cowbird eggs (1-6) occurred in 
27 of 38 cacique nests (71%) found during the egg stage. Giant Cowbird eggs were white and unmarked (85%), or marked 
and 'potted over pale buff (15%); the marked eggs somewhat resembled host eggs, but were twice as large. Four host and 
three parasite eggs were found punctured and broken. Only three cowbird nestlings were observed, and resembled 
oropendola \Psarocolius spp.) nestlings more than those of Red-rumped Caciques. Botflies (Philornis spp.) infested 
iowbiid and cacique nestlings, but there was no evidence of a cowbird-cacique preening mutualism. Received 23 June 
2010. Accepted 17 November 2010. 
The Giant Cowbird (Molothrus oryzivorus) is 
an obligate brood parasite in the New World 
tropics from Mexico to Argentina. Its better- 
known hosts are colonial nesting species of 
caciques ( Cacicus ) and oropendolas ( Psorocolius) 
(Tricdmann 1963, Robinson 1988, Jaramillo and 
Burke 1999). Host-parasite interactions of Giant 
Cowbirds have been mostly studied in Costa Rica 
(Crandall 1914, Webster 1994, Cunningham and 
Lewis 2006). Panama (Smith 1968, 1980), 
Venezuela (Schaffer 1957), Surinam (Haver- 
schmidt 1966, 1967), and Peru (Robinson 1988). 
However, field research on breeding of this 
cowbird has not been extensive. Commonly the 
nesLs of its cacique and oropendola hosts are 
inaccessible (Webster 1994), as colonies are built 
ra upper branches of tall isolated or emergent 
trees (Fraga 1989, Jaramillo and Burke 1999) and. 
Jl l unes. near nests of aggressive wasps or bees 
• Smith 1968). Important information on interac¬ 
tions of Giant Cowbirds and their hosts was 
obtained by Smith (1968, 1980) and Fleischer and 
Smith (1992) in Panama. 
Sniith (1968) reported an unusual and complex 
nteraction between Giant Cowbirds and two hosts 
,n Panama, the Chestnut-headed Oropendola 
1 P'arocolius wagleri) and the Yellow-rumped 
Cacique (Cacicus cela). The interaction included 
Possible cases of mutualism and egg mimicry 
^mith 1968 1980). Preening mutualism was 
re Ported when nestlings of Chestnut-headed 
(Oropendolas were infested by ectoparasitic botfly 
,an 'ae (Philornis spp.) that could cause nestling 
CICYTTP. Consejo National de Invcstigaciones (CON- 
dc la Argentina Espana y Matter). (3105) Diamante, 
Entre Rios, Argentina; e-mail: chfraga@yahoo.com 
mortality. Giant Cowbird nestlings apparently 
removed the larvae from themselves and their 
nest mates in botfly-infested broods, and were 
beneficial to the host. Botfly infestations were not 
found in host colonies built near nests of 
aggressive wasps. The cowbird nestlings compet¬ 
ed with host nestlings in these instances, and host 
adults were aggressive to the brood parasites. 
Hosts removed cowbird eggs not resembling their 
own in wasp-protected colonies, but incubated 
those with mimetic resemblance. 
Subsequent studies by Robinson (1988) did not 
find evidence of preening mutualism in Peruvian 
hosts including Yellow-rumped Caciques and 
Russet-backed Oropendolas (Psarocolius ungusti- 
frons). which were invariably aggressive to Giant 
Cowbirds. Webster (1994) also reported constant 
host aggressiveness to Giant Cowbirds in colonies 
of Montezuma Oropendolas ( P. montezuma) in 
Costa Rica. 
Giant Cowbirds in Argentina, according to 
Jaramillo and Burke (1999), are found only in 
northern Misiones Province, where the only 
potential host is the colonial nesting Red-rumped 
Cacique (Cacicus haemorrhous). A Giant Cow- 
bird egg was once reported from a nest of Red- 
rumped Caciques in Amazonian Brazil (Pinto 
1953), but recent studies on the cacique in eastern 
Brazil (Duca and Marini 2004, 2008; Pizo 2009) 
did not report parasitism. De la Pena (1987) 
reported one Giant Cowbird egg in one nest of 
Red-rumped Caciques from Misiones, Argentina, 
but cowbird interactions with this host remain 
unstudied. 
My objectives are to: (lj provide the first 
account of the host-parasite interaction between 
Red-Rumped Caciques and Giant Cowbirds , (2) 
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