Fraga • GIANT COWBIRD PARASITISM IN ARGENTINA 
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mature and secondary native forest at Parque 
National Iguazu. 
Argentinean nests were elongated purses wider 
at the bottom, 47-65 cm long (n = 23) with 
entrance slits near the top. Nests were woven with 
strips of palm (Syagrus romanzoffianci) leaves and 
other fibers, often with blackish Marasmius 
hyphae inside, and lined at the bottom mostly 
with loose dry leaves. Only female caciques built 
nests, incubated eggs, and fed young. Males may 
behave as colony sentinels. The maximum host 
clutch size in non-parasitized nests was two eggs 
in 10 nests, and apparently one egg in one nest 
imean clutch - 1.91 eggs). Cacique eggs (n = 60) 
from Misiones were pinkish, variably spotted and 
scrawled in shades of brown and purplish brown. 
Mean ± SD length and width of 17 eggs from 17 
nests (parasitized or not) were 28.7 ± 1.3 X 19.4 
- 0.6 mm. Mean ± SD egg volume was 5.64 ± 
0.40 cm 3 . 
I observed 31 Giant Cowbird visits to five 
colonies, lasting 0.5 to 8 min, involving 1-6 
individuals including males and females. Giant 
Cowbird groups landed first at the highest branches 
of colony trees, but afterwards cowbird females 
visited nests above and below 5 m. Male cowbirds 
regularly produced shrill whistles, and females 
made short calls, but single females were silent on 
lour visits. Caciques reacted to visiting cowbirds 
with loud and rasping alarm calls, but the volume of 
colony noise was lower than in the presence of 
raptors or other nest predators. Male caciques 
attacked and chased cowbirds perched anywhere 
m the colony tree, but female attacks were usually 
localized around a nest or a group of nests. 
Extreme egg-laying dates for nine Giant Cow¬ 
ards eggs were 16 September and 20 December. 
Earlier dates are possible; the June-July cacique 
nesls °f 1996 were loo high to be examined, but 
c owhirds were seen near those colonies. 
Parasitism During the Egg Stage .—Giant 
Cowbird eggs occurred in 27 of 38 cacique nests 
'1%) found during the egg stage. Parasitized 
acsts contained one to six cowbird eggs (mean = 
18 e ggs). Most nests (17 of 27, 63%) were 
parasitized with one cowbird egg. 
Most Giant Cowbird eggs from Misiones were 
while and unmarked (39 of 46, 85%). The 
Gaining seven eggs were pale buff, marked 
w,l f' spots, blotches, and lines in brownish to 
^ackish brown, scattered over the surface. The 
barked eggs occurred at colonies near Iguazu 
Report, Parque Provincial Urugua-f, and Parque 
Provincial Esmeralda. White (n = 22) and spotted 
(n = 7) eggs did not differ in length (/ = -0.12, 
P = 0.90), width U = -0.75. P = 0,46), and 
volume (Mann-Whitney z = -0.72, P = 0.47). 
Mean ± SD length and width for all 29 cowbird 
eggs were 34.4 ± 0.2 X 24.6 ± 0.1 mm. Mean ± 
SD volume of 29 cowbird eggs was 10.92 ± 
1.29 cm 3 . Cowbird eggs were almost twice as 
large as host eggs (Mann-Whitney z. ~ 5.532, P < 
0.001). irrespective of color, and had thicker and 
rougher eggshells. 
Host-parasite Interactions During the Egg 
Stage. —I did not observe host eggs in three dump 
nests containing 5, 5. and 6 cowbird eggs, and the 
nests appeared abandoned during all visits (eggs 
cold, no females present). These dump nests had 
little lining material, which suggests that desertion 
may have occurred prior to host egg laying. The 
maximum number of host eggs observed in the 
remaining 24 parasitized nests was two eggs (6 
nests), one egg (16 nests), and zero eggs (2 nests) 
with a mean of 1,13 cacique eggs. Two-egg host 
clutches were significantly more frequent in non- 
parasitized nests (10/11) than in those containing 
cowbird eggs (6/24) (% 2 = 13.20, df = \. P < 
0.001). Cracked or crushed host eggs (n = 4) were 
observed in six parasitized clutches. Three 
cracked or crushed parasite eggs were also 
observed in nests with five and six cowbird eggs. 
1 did not observe caciques or cowbirds damaging 
or removing eggs. 
Host and Parasite Nestling*—' Only five of 14 
nests (35.7%) found in Ihe nestling stage had eggs 
or nestlings of Giant Cowbirds. Nest contents for 
this entire sample were 20 host nestlings, two 
cowbird nestlings, one host egg, and three 
cowbird eggs. None of the four eggs hatched. 
The incubation period for one Giant Cowbird egg 
was estimated at 12-13 days. 
Host and parasite nestlings differed in appear¬ 
ance and size. Feathered host nestlings had visible 
red rumps, bills with a small frontal casque 
colored pale yellowish gray, pale yellow flanges, 
and brownish eyes with bluish reflections. The 
three feathered cowbird nestlings had uniform 
blackish plumage, pinkish-ivory colored bills, 
yellow flanges, and conspicuous pinkish-ivory 
frontal casques edged with pale yellow. The eyes 
were dark brown and surrounded by a triangular 
facial patch of bare flesh-colored skin. The total 
length of the frontal shield plus bill was 23 mm 
for a partially feathered cowbird nestling at 
12 days of age; the equivalent length was 
