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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123, No. 1, March 2011 
host breeding season advances (Arendt 1985a, b; 
Young 1993; Rabuffetti and Reboreda 2007), 
although some studies did not find a trend (Nores 
1995, Fessl and Tebbich 2002, Quiroga 2009). 
The role of nest-site vegetation on prevalence 
and intensity ol botfly parasitism has received 
little research attention, except for the work of 
O’Connor et al. (2010a) who reported higher 
prevalence and intensity of botfly ( P . downsi) 
parasitism in moist forest highlands than in arid 
lowlands of the Galapagos Islands and suggested 
that size and continuity of forest patches could 
influence botfly dispersal ability. Understanding 
the role of nest-site vegetation on parasite 
infestation may help predict the likelihood of 
parasitism for a given host in a given environment 
(Loye and Carroll 1998). 
We used a large set of observational data 
collected during three consecutive breeding sea¬ 
sons to study the interactions between Philornis 
seguyi and Red-crested Cardinals ( Pciroaria 
coronata ), a species that has been previously 
reported as a host of botflies in central Argentina 
(De la Pena et al. 2003). We examined: (1) 
seasonal trends in parasite prevalence and inten¬ 
sity, (2) the influence of botfly parasitism on 
nestling growth and survival, and (3) the associ¬ 
ation between nest-site vegetation at different 
scales and probability of botfly parasitism. We 
hypothesized that survival of Red-crested Cardi¬ 
nal nestlings may be negatively associated with 
parasite intensity and positively associated with 
age at time ot parasitism. We expected nests in 
small isolated patches of forest would have lower 
parasite prevalence than those in large continuous 
patches, because grassland areas that separate 
isolated from continuous patches may act as 
barriers for dispersal. 
METHODS 
Study Site. —The study was conducted near the 
town of Punta Indio, Buenos Aires Province, 
Argentina (35 20' S, 57° 11' W). The vegetation at 
the study site consists of woodlands arranged in 
several strips (50-100 m in width and up to several 
km in length) parallel to the edge of the “de la 
Plata” River surrounded by small areas of 
grassland. In addition, there are also small patches 
of forests (between 10 and 70 m in diameter) more 
distant from the edge of the river surrounded by 
large areas of grassland. These small patches are 
separated from woodland strips by 300-1,200 m. 
The woodlands are dominated by Celtis tala 
(Tala) and Scutia buxifolia (Coronillo). The annual 
rainfall for the study site is 891 mm and rainfall 
during the study period varied between 772 (2005) 
and 845 mm (2007). We collected data during 
Red-crested Cardinal breeding seasons (early Oct- 
mid Feb) 2005-2006 to 2007-2008. Average 
annual rainfall during the breeding season is 
typically 441 mm and, during the breeding seasons 
of 2005-2006, 2006-2007, and 2007-2008, it 
was 431, 439, and 487 mm, respectively. Ranges 
of mean monthly ambient temperatures during 
the study period were 14.8 (Oct) to 21.9° C (Jan) 
in 2005-2006, 17.0 (Oct) to 23.2° C (Feb) in 
2005-2006, and 17.1 (Oct) to 24.6° C (Jan) in 
2007-2008. 
Study Species. —Red-crested Cardinals inhabit 
semi-open areas with scattered trees and shrubs 
from east central Argentina to southern Brazil, 
Paraguay, eastern Bolivia, and Uruguay (Ridgely 
and Tudor 1994). Their nests at our study site 
were at a height of 2-6 m, primarily in Talas and 
secondarily in Coronillos and Molles ( Schinus 
longifolius) (Segura and Arturi 2009). Nests are 
open-cups with external and internal diameters of 
13 and 6.5 cm, respectively, a depth of 4.5 cm and 
lateral translucent walls of 2 cm in width (LNS, 
unpubl. data). The wall of the nest is built with 
thin dry branches of Tala and small stems of grass 
while the chamber is lined with thin rootlets, 
vegetation fibers, and cattle hair. Clutch size 
varies between two and four eggs, nestlings hatch 
after 12 days of incubation and fledge ~14 days 
after hatching (LNS, unpubl. data). Average mass 
of nestlings at hatching is 3-3.5 g and 30-31 g at 
time of fledging (LNS, unpubl. data). 
The species of botfly recorded previously in our 
study area is P. seguyi (Couri et al. 2005, Rabuffetti 
and Reboreda 2007). We collected botfly larvae from 
Chalk-browed Mockingbird ( Mimus satuminus, 
n = 21), House Wren ( Troglodytes aedon, n = 9) 
and Bay wing ( Agelaioides badi us, n = 3) nestlings, 
all identified (Martin Quiroga, INALI-CONICET, 
Argentina) as P. seguyi (Garcia 1952, Couri et al. 
2009). Quiroga (2009) described the life cycle of 
this species. P. seguyi larvae feed and develop 
subcutaneously in the host for 5—6 days reaching a 
length of ~8-9 mm and a mass of 0.11-0.13 g. 
Larvae drop from the host to undergo pupation, 
emerging as adult flies after 9-10 days. Host larvae 
do not pupate at the bottom of the nest due to the 
scarce material that forms the cardinal nest, but 
drop to the ground where they undergo pupation 
(LNS, unpubl. data). 
