112 
THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123, No. 1, March 2011 
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FIG. 4. Growth rates of (A) body mass (g X day -1 ) (B) 
tarsus length (mm X day -1 ), (C) beak length (mm X day -1 ) 
and (D) wing length (mm X day -1 ) for Red-crested Cardinal 
nestlings that fledged from unparasitized nests (NP. n = 66) 
and from nests parasitized by P. seguyi botflies (P, n = 16). 
Growth rates were estimated as the slope of the linear 
regression between the values of each variable and the a«e of 
nestlings between 2 to 8 days of age (hatching day = age 0). 
X , = 0.86, P = 0.35; n = 67), or at the level of 
vegetation surrounding the nest tree (logistic 
regression: X 2 x = 2.67, P = 0.1, n = 67). There 
was a significant effect of landscape features and 
botfly parasite prevalence: nests in connected 
riverine woodlands had higher parasite prevalence 
than nests in small woodland patches more distant 
from the river (large riverine woodlands patch: 
32/46 nests; small woodlands patch: 5/21 nests- 
X 2 i = 4.0, P = 0.04). 
DISCUSSION 
Botfly parasitism of Red-crested Cardinals 
increased during the breeding season, but was 
not significantly different across years. Parasite 
prevalence and intensity had lethal (lower nestling 
survival) and sub-lethal (lower growth rate of 
nestlings that survive) effects. Nestling survival in 
parasitized nests was positively associated with 
age at which nestlings were parasitized. In 
addition, botfly parasitism was more prevalent in 
nests in large continuous patches of riverine 
woodland than in small isolated patches further 
from the river. 
Red-crested Cardinals start breeding in early 
October, but first records of botfly parasitism 
occurred in late December-early January. Simi¬ 
larly, Rabuffetti and Reboreda (2007), in a study 
conducted only 35 km north of our study site, 
observed low prevalence of botfly parasitism at 
the beginning of the Chalk-browed Mockingbird 
breeding season (mid Oct-early Dec), but high 
prevalence towards the end of the season (late 
Dec-Jan). Quiroga (2009) studied House Wrens 
500 km north our study area and reported botfly 
parasitism in late October-November and did not 
detect any seasonal trend in parasite prevalence. It 
is likely our study site is close to the southern 
limit of Philomis distribution, as studies of House 
Wrens (a regular host of botflies; Young 1993. 
Quiroga 2009) 100 km south of our study site did 
not detect parasitism (Llambias and Fernandez 
2009; Paulo Llambias, pers. comm.). The increas¬ 
ing absence of botfly parasitism approaching the 
southern limit of the parasite’s distribution could 
be the result of lower ambient temperatures that 
delay emergence of new adults early in the 
season. Bennett and Whitworth (1991a, b) report¬ 
ed population size of flies of the genus Proto- 
caliphora increased over the breeding season 
because new adults emerge while older flies 
persist. Thus, as the season advances, the number 
of adult flies increases which would explain the 
finding of higher parasite prevalence. 
Many studies report parasitic botflies ( Philomis 
spp.) have detrimental effects on nestling survival 
(Arendt 1985b, Delannoy and Cruz 1991, Fessl 
and Tebbich 2002, O’Connor et al. 2010c), 
growth (Young 1993), and malformation that 
may persist in adults (Galligan and Kleindorfer 
2009). Our results conducted in a temperate area 
close to the southern limit of the distribution of 
botflies indicate that botfly parasitism can lower 
host s reproductive success. Many nests had total 
and partial fledging success (83%), but we found 
lethal (lower nestling survival) and sub-lethal 
(lower growth rate of nestlings that survive) 
effects of parasitism. Our study also showed that 
low parasite intensity (6—7 larvae/nestling) could 
produce lethal effects. Botfly parasitism in our 
study decreased nestling survival from 0.86 to 0.6. 
This effect was less than reported by Rabuffetti 
and Reboreda (2007) for Chalk-browed Mocking¬ 
birds, where botfly parasitism decreased nestling 
survival from 0.78 to 0.3. Differences in the 
