Segura and Reboreda • BOTFLY PARASITISM OF RED-CRESTED CARDINALS 
113 
impact of botfly parasitism on host nestling 
survival between studies may be due to differ¬ 
ences in parasite intensity, which was three-fold 
higher in mockingbirds than in cardinals. 
Latency of parasitism, as in other studies 
(Arendt 1985a, 2000; Rabuffetti and Reboreda 
2007), was an important factor that influenced 
nestling survival. We found a positive relationship 
between nestling survival and age at time of 
parasitism. We also observed a negative associ¬ 
ation between latency and parasite intensity. The 
latter association possibly indicates that changes 
in the skin of nestlings as they grow (likely the 
presence of feathers after day 5-6) may prevent 
larvae from penetrating the skin. We did not find 
an association between nestling survival and 
parasite intensity, probably due to low intensity 
in this study. 
Nests in large continuous forest along river 
edges were more parasitized than those in small 
isolated patches, even when the distance between 
both types of patches was a few hundred meters. 
Bennett and Whitworth (1991b), in an experi¬ 
mental study with adult flies of the genus 
Protocaliphora , found that adult flies do not 
move large distances and that new infestations 
occur <50 m from where adults emerged. 
Dudaniec et al. (2010) also found lower levels 
of genetic relatedness in P. downsi when nests 
where located in more arid environments and host 
nesting density was lower. Open areas of 
grasslands that separate patches of forest may 
net as barriers for botfly dispersal. Alternatively, 
isolated patches of forest could have microclimate 
conditions (i.e., lower humidity and higher 
temperatures) that may reduce their suitability 
for botflies. O’Connor et al. (2010a) found habitat 
differences in Philornis parasite prevalence and 
intensity with higher levels of parasitism in moist 
forest highlands than arid lowlands on Floreana 
Island, in the Galapagos. 
Some authors have proposed birds should avoid 
selecting nesting sites where they are more 
exposed to parasites, and that selection for good 
sites would be the first line of defense against 
parasitism (Loye and Carroll 1991, 1998; O Con¬ 
nor et al. 2010b). We did not observe behavior by 
Red-crested Cardinals that suggested increased 
nse of isolated patches of forest and earlier onset 
°f breeding to avoid the higher parasite preva¬ 
lence typical of riverine forest patches. Use of 
nest sites with high botfly parasite intensity could 
Be the consequence of high host density; we have 
observations of high nesting density of Red- 
crested Cardinals in riverine forest study sites 
(LNS, unpubl. data). Kleindorfer and Dudaniec 
(2009) and Kleindorfer et al. (2009) found an 
effect of high host nesting density on high P. 
downsi parasite intensity in the Galapagos Islands. 
Little is known about the role of parasite 
infestation for host nest site selection behavior: 
more studies on these interactions are needed. 
ACKNOWLEDGMENTS 
We are grateful to Luis del Sotto and Emiliano F. Torres 
for allowing us to conduct this study at Estancia ‘La 
Matilde’. We thank Rachel E. McNutt, Danielle Castle, 
Sharon A. Fee, Kathleen Masterson, Anahi E. Formoso, 
Diego A. Masson, Diego I. Isaldo, Roberto F. Jensen, 
Tobias Mika, Ross H. Crandall, Marie M. Kalamaras, 
Rachel Buxton, Amy Nixon, Leigh Marshall, Yamila S. 
Obed, and Miguel A. Diferdinando for help in data 
collection and nest monitoring. We also thank Sarah A. 
Knutie and one anonymous reviewer for helpful comments 
to a previous version of this manuscript. We are grateful to 
•Base Aeronaval de Punta Indio’ for rainfall and temper¬ 
atures data in the study area. LNS was supported by a 
fellowship from Consejo Nacional de Investigaciones 
Cientificas y Tecnicas (CONICET). JCR is a Research 
Fellow of CONICET. 
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