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THE WILSON JOURNAL OF ORNITHOLOGY • Vol 123, No. 2, June 2011 
19.7 mm for a cacique nestling of the same age. 
Cowbird nestlings (n = 3) attained 90-130 g in 
body mass at days 12-15, while host nestlings 
fledged with body masses of 67 to 85 g (n — 14). 
Female Red-rumped Caciques in Misiones fre¬ 
quently reared broods of two host nestlings. 
However, the three cowbird nestlings were reared 
alone, as the host eggs remaining in the three nests 
did not hatch (2 cases), or the host hatchling died 
and was probably removed by the host female. 
Behavior of host and parasite nestlings during 
provisioning episodes (in 2 nests observed with 
the micro cameras) had parallel development. 
Nestlings during the first week were rather 
passive, often subject to brooding, and gaped at 
feeding females from the bottom of the nest. Nest¬ 
shaking caused by the incoming host female 
stimulated older nestlings of both species to climb 
the nest walls towards the entrance, gaping with 
stretched necks. Host and parasite nestlings in the 
last days of the nestling stage remained perched 
near the entrance most of the day with their bills 
outside the nest and clearly visible with binocu¬ 
lars. Bills of cowbird nestlings were larger and 
paler than those of host nestlings. The nestling 
period of one fledged cowbird was estimated to be 
18 days. 
Botflies (Philomis spp.) infested seven host 
nestlings and one cowbird nestling. Eight botfly 
larvae were observed on a cowbird nestling at 
4 days of age with a body mass of 41.5 g; 6 days 
later the number of visible larvae had increased to 
19 and nestling body mass had increased to 51 g. 
One non-infested cowbird nestling had a mass of 
71 g at day 9. Another cowbird nestling was 
infested with botflies prior to this study in one of 
my colonies (Jorge Anfuso and Silvia Elsegood, 
pers. comm.). Infested host nestlings (n = 7) had 
only 7-9 Philomis larvae, and no mortality or 
abnormal growth was detected. 
Reproductive Success of Host and Parasite.— 
Data are presented only for the 35 nests found in 
the egg stage. Precise estimates of success were 
difficult as fledglings moved into dense forest 
cover within 12 hrs of leaving the nest. The 35 
nests contained 67 cacique eggs, which produced 
a minimum of 11 fledglings (16.4%). The 
estimate is slightly increased to II fledglings 
from 61 eggs (18%) if the three dump nests did 
not contain host eggs. Only one of the 46 cowbird 
eggs (2.2%) was known to produce a fledgling. 
Monkeys were not seen in the man-made 
clearings used for nesting, and the main nest 
predators of cacique colonies were three toucan 
species: Toco Toucan (Ramphastos toco), Green¬ 
billed Toucan (/?. dicolorus). and Chestnut-eared 
Aracari (Pterofilossus castanotis). I saw two Toco 
Toucan raids destroying a colony of 16 nesis. park 
rangers witnessed similar cases, and several 
predation sequences were filmed by the \HK 
documentalists. A. A. Bodrati (pers. comm.ialso 
observed Saffon Toucanet (P. bailloni ) predation 
of cacique colonies in Misiones. Strong rain¬ 
storms knocked down several nests, affecting host 
and parasite success. Specific causes of hatching 
failure for Giant Cowbird eggs included egg- 
dumping (3 nests with 16 eggs), and poor 
synchronization with the host’s egg laying At 
least three cowbird eggs were laid 6-8 days before 
the host eggs hatched, i.e.. 10-12 days after the 
start of incubation. Those eggs remained in the 
nest but did not hatch. 
DISCUSSION 
Many aspects of the breeding biology of Red- 
rumped Caciques (including breeding season, 
nests, clutch size, egg coloration) agree with 
information obtained in eastern Brazil (Ducaand 
Marini 2004, 2008; Pizo 2009). Important excep¬ 
tions were larger colony sizes in Misiones and 
brood parasitism by the Giant Cowbird, which is 
rare or absent in large areas of eastern Brazil 
(Jaramillo and Burke 1999). 
The incidence of parasitism of Red-rumped 
Caciques is among the highest reported for Giant 
Cow birds; Fleischer and Smith (1992) reported a 
54.2% incidence for Yellow-rumped Caciques in 
Panama. Cowbirds were far less numerous than 
caciques in Misiones. and the 71% incidence of 
parasitism suggests high egg productivity of female 
parasites, as for other cowbirds (Scott and Ankney 
1983). Clutches parasitized with two or more eggs 
were 37% in my sample, almost identical to the 
37.5% reported for Yellow-rumped Caciques m 
Panama (Fleischer and Smith 1992). 
Mean measurements of Giant Cowbird eggs 
from northeastern Argentina were similar to those 
reported from Panama (35.8 X 24.9 mm) 
(Fleischer and Smith 1992) and Surinam • 34.0 
X 25.5 mm) (Haverschmidt 1966. 1967). indicat¬ 
ing considerable uniformity in size throughout an 
extensive geographic range. Variation in egg color 
is important. White unmarked eggs, the most 
common morph in northeastern Argentina. <'ccur 
less commonly in Panama (Fleischer and Smith 
1992), Costa Rica (Crandall 1914), and Venezuela 
