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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123. No. 2. June 2011 
compare this interaction with previous studies of 
Giant Cowbirds and other hosts with particular 
emphasis on the possible egg and nestling 
mimicry by the parasite, and (3) evaluate the 
possible occurrence of a preening mutualism 
between caciques and cowbirds. 
METHODS 
I had field experience with Giant Cowbirds, 
their eggs and nestlings, in Panama and Costa 
Rica in 1985-1986 (Fraga 1989). I also had access 
to the extensive collection of eggs of this cowbird 
at the Smithsonian Tropical Research Institute 
obtained by N. G, Smith. Information on the 
Argentinean distribution of Giant Cowbirds and 
data on colony sizes of Red-rumped Caciques 
were obtained during field trips to Corrienlcs and 
Misiones provinces (1995-2007), and from the 
literature and museum specimens. Red-rumped 
Cacique nests (n - 52) from eight colonies were 
studied between October 1995 and November 
2004 at five localities in the Atlantic Forest of 
Misiones Province, Argentina. The localities 
were; Parque Naeional Iguazu (25 41' S, 54' 
27' W). a colony near Iguazu International Airport 
(25 43 S, 54 29' W), the town of Puerto Iguazu 
(25° 36' S, 54 34' W), Parque Provincial Uru- 
gua-f (25° 52' S. 54 II' W), and Parque 
Provincial Esmeralda (25" 53' S. 53° 53' W). 
These colonies contained 16 to 104 nests. Only a 
few nests at each colony (10-20%) were <5 m 
above ground level and most could be inspected 
without unduly stretching or damaging the struc¬ 
tures. Inspected nests were close (4-70 cm) to other 
nests, and built in the main trees of each colony. 
I observed and recorded cacique and cowbird 
behavior around the eight colonies during 130 hrs 
with 8 x 42 binoculars and tape recorders. Both 
Red-rumped Caciques and Giant Cowbirds are 
sexually dimorphic in size, iris color, and plumage 
brightness (Jaramillo and Burke 1999, pers. obs.). 
Red-rumped Caciques in Argentina and nearby 
Brazil are considerably smaller (males 93 g, 
females 66 g) than Giant Cowbirds (males 209 g, 
females 122 g) (Contreras 1983, Belton 1985* 
Dameu and Camperi 1994; specimen at Burke 
Museum, University of Washington, Seattle, USA). 
Digital micro-cameras were placed in some cacique 
nests containing nestlings in November 1996, by a 
crew producing a documentary film for Japanese 
National Television (NHK). 
Nests were found during both egg-laying and 
incubation periods (“egg stage”), and with at 
least one hatched host or parasite nestling 
(“nestling stage”). Nests were examined on2-6 
different days, until failure, hatching or fledging 
People living near the colonies also informed me 
on nest contents, fledging dates, and predation 
events. Estimated laying dates for Red-rumped 
Cacique eggs that hatched are based on die 
incubation period of 18 days reported by Duca 
and Marini (2004). Eggs were numbered with 
waterproof ink, and their maximum length and 
width was measured with digital calipers to the 
nearest 0.1 mm. I estimated egg volume with the 
formula 0.524 X / x w* 2 , where / and w were the 
egg maximum length and width (Preston 1974). 
Nestlings were described and photographed, and I 
obtained their bill measurements with digital 
calipers. Body mass of nestlings was measured 
with Pesola scales accurate to 0.5 g. Small 
nestlings were marked with waterproof inK and 
afterwards color bands. Host and parasite fledg¬ 
ling success are compared by using ratios of 
fledglings/eggs. I used Stalistica 5 (StatSoft, 
Tulsa. Oklahoma, USA) for statistical analysis. 
RESULTS 
13reeding. — Red -rurnped Caciques breed in 
Misiones Province and northeastern Corrientes 
Province within Argentina. Misiones colonies (n 
- 68) contained from seven to 104 nests at times 
spread over up to three neighboring trees: 
Corrientes colonies (n — 14) contained nine to 
30 nests. The southernmost Argentinean colony 
was observed at Paso de Ios Libres (29 41' S, 57 
04' W), Corrientes. I did not find Giant Cowbirds 
at this site in 1995. but A. A. Bodrati (pers. 
comm.) saw three individuals 50 km north of this 
site in 2007. Other colonial nesting cacique or 
oropendola were not observed nesting in this area- 
The eight studied colonies were in man-made 
clearings, near houses, bams, and other buildings 
and. when tolerated by people, were reused during 
many years. I did not observe nests of aggressive 
wasps in the colony trees. Nesting activities wen 
quite asynchronous in the larger colonies. The 
start of the cacique breeding season varied 
between years during my study, even at the same 
locality. Active colonies with a few ineuhaun? 
females were seen in the Iguazu area by mid-June 
1996 but, in the colder winter of 1997, nests were 
still unfinished at the end of August. Egg-laying 
continued at least until 21 December. Postbreed¬ 
ing cacique flocks with dependent fledglings 
occurred in December—March along trails in 
