Fraga • GIANT COWBIRD PARASITISM IN ARGENTINA 
281 
(Schaffer 1957). Eggs of this morph are clearly 
different and non-mimetic in nests of Red-rumpcd 
Caciques, and probably in nests of other cacique 
and nropendola hosts as well (e.g., Smith 1968). 
The spotted egg morph in Misiones had some 
resemblance to cacique eggs in color (but not 
si/c), but the quite different white morph was the 
most common in my sample. Thus, egg mimicry 
between Giant Cowbirds and Red-rumpcd Ca¬ 
ciques seems not to occur. Perhaps Red-rumped 
Caciques are accepters of cowbird eggs (sensu 
Rothstein 1975) and do not select for egg 
mimicry, but this idea requires experimental 
evidence (Cunningham and Lewis 2006). 
Host clutch reduction in parasitized nests 
suggests Giant Cowbirds damage host eggs, as 
reported for other cowbird species (Peer 2006). 
Cacique eggs may have been cracked or crushed 
by the impact of the larger and thick shelled Giant 
Cowbird eggs, particularly if the parasite females 
deposited their eggs near the entrance of the 
elongated nest. Egg laying by Brown-headed 
Cowbirds (M. ater) may damage host eggs in a 
similar way (Blankespoor et al. 1982. Weather- 
head 1991), The low reproductive success of 
Giant Cowbirds reported here seems surprisingly 
low for a specialized brood parasite. However, my 
data on contents of nests found with nestlings 
suggest a ratio of 10 host young for each cowbird 
young. I observed one Giant Cowbird fledgling in 
seven postbreeding cacique flocks in Misiones. 
which included 30+ host fledglings. 
Crandall (1914) first described two Giant 
Cowbird young from nests of Montezuma Or- 
opendolas in Costa Rica, broadly agreeing with 
m y findings. However, he did not mention the 
existence of a conspicuous pale frontal casque, 
Jn d tound no resemblance between host and 
Parasite nestlings. The resemblance between 
Giant Cowbird and Red-rumped Cacique nestlings 
ls s *‘ght in my experience. The pale bill of Giant 
Cowbird nestlings gives some resemblance to host 
flings. A pale bill is found in nestlings of the 
''Creaming Cowbird ( M. rufoaxillaris ) (Fraga 
a species ancestral to Giant Cowbirds 
'Johnson and Lanyon 1999); this similarity may 
n °l have evolved through mimicry. 
conspicuous pale frontal casque of Giant 
°wbird nestlings is unique among cowbird 
nestlings (Fraga 1979; Lowther 1993, 1995; 
J"vvther and Post 1999), but resembles similar 
Str uctures in nestlings of most oropcndola species. 
In Particular, the pale-colored casque occurs in 
young Crested Oropendolas (Psarocolius decu- 
tmnus), a common host of the Giant Cowbird 
(Goeldi 1894, Guimaraes 1926, Schaffer 1957). 
This casque is retained in adult Giant Cowbirds 
(Webster 2003), but becomes relatively small, 
black-colored, and partially covered by plumage. 
Webster (2003) also noticed an elongated skull in 
adult Giant Cowbirds, and those of oropendolas 
and caciques. His suggestion that Giant Cowbirds 
could be related to caciques and oropendolas and 
not to other cowbirds is clearly contradicted by 
DNA data (Lanyon 1992. Johnson and Lanyon 
1999). Mimicry is a plausible explanation for the 
general resemblance between the head parts of 
nestling Giant Cowbirds and oropendolas. 
Mutualism between Giant Cowbirds and their 
hosts reported by Smith (1968. 1980) in Panama is 
not supported by Argentinean data. First, nestling 
Giant Cowbirds were infested by Philomis larvae. 
Second, Giant Cowbird nestlings in my sample 
could not preen host nestlings because they were 
reared alone. This situation may arise from the 
large disparity in body mass between host and 
parasite nestlings. Third, cacique nestlings sur¬ 
vived light Philornis infestations, as reported for 
other Misiones passerines (Cockle and Bodrati 
2009). Last, nesting Red-rumped Caciques were 
aggressive to visiting Giant Cowbirds. My results 
agree with studies from Peru and Costa Rica 
(Robinson 1988, Webster 1994) in indicating a 
negative effect of Giant Cowbird parasitism. 
ACKNOWLEDGMENTS 
The crew of Nippon H5so Kyokai (NHK) documentalists 
(particularly Kenji Mori) started my interest in this study. 
Parques Nacionales (Argentina) and the Ministerio de 
Ecologiu (Misiones) provided the required permits. Many 
park rangers in Misiones (national and provincial), and 
AndriSs Basso and Lucio Aquerreta, helped my work or 
provided important information. 1 am particularly thankful to 
Jorge Anfuso and Silvia Elsegond for allowing me to study 
caciques around their house, and for gathering inlonnation on 
many aspects of this study. A. A. Bodrati and K. L. Cockle 
helped me in many ways, particularly at Esmeralda. Sharon 
Birks provided information on a specimen at the Burke 
Museum. The manuscript was improved by comments from 
A. A. Bodrati. K. L. Cockle, and J. I. Areta. 
LITERATURE CITED 
Belton. W. 1985. Birds of Rio Grande do Sul, Brazil. Part 
2. Formicariidae through Corvidae. Bulletin of the 
American Museum of Natural History 180:1-242. 
Blankespoor, G. W., J. Oolman, and C. Uthe. 1982. 
Eggshell strength and cowbird parasitism of Red¬ 
winged Blackbirds. Auk 99:363-365. 
