The Wilson Journal of Ornithology 123(1): 107-115, 2011 
BOTFLY PARASITISM EFFECTS ON NESTLING GROWTH AND 
MORTALITY OF RED-CRESTED CARDINALS 
LUCIANO N. SEGURA 1 ' 3 AND JUAN C. REBORED A 2 
ABSTRACT.—We collected observational data in three consecutive breeding seasons to study interactions between the 
botfly Philornis seguyi and Red-crested Cardinals (Paroaria coronata) in a temperate zone near the southern limit ol 
Philomis distribution. We analyzed: (1) seasonal trends in prevalence of parasitism, (2) influence of botfly parasitism on 
nestling growth rate and survival, and (3) the association between nest site vegetation at different scales (i.e., nest tree, 
vegetation surrounding the nest tree, and landscape) and probability of botfly parasitism. Prevalence ol parasitism was 28% 
and was higher later in the breeding season. Botfly parasitism produced sub-lethal (lower growth rate of nestlings that 
survive) and lethal (lower nestling survival) effects. The lethal effect was negatively associated with age at the time 
nestlings were parasitized. Botfly parasitism was not associated with vegetation characteristies at the level of nesting tree or 
vegetation surrounding the nesting tree, but was associated with landscape features. Parasite prevalence was higher in large 
continuous woodland patches than in small isolated patches. However, we did not observe increased use ol isolated patches 
of forest by Red-crested Cardinals, suggesting that use of nest sites with high botfly parasite intensity could be the 
consequence of high host density. Received 6 April 2010. Accepted 19 October 2010. 
Nestling birds are hosts to a wide range of 
ectoparasites that capitalize on the brief period of 
rapid host development and resource availability 
(Loye and Carroll 1995). Three dipteran families 
(Calliphoridae, Muscidae, and Piophilidae) repre¬ 
sent most of the hematophagous parasites of birds 
(Uhazy and Arendt 1986, Ferrar 1987). Many 
species of the genus Philornis (botflies) within the 
Muscidae parasitize nestlings and adults of cavity 
and open-nesting birds in the Neotropics (Arendt 
1985a). Studies of the interactions between the 
genus Philomis and their hosts have been limited 
to a few species (i.e., P. downsi and Darwin’s 
finches in the Galapagos Islands) (Fessl et al. 
2001; Fessl and Tebbich 2002; Fessl et al. 2006a, 
b’ Dudaniec et al. 2006; Dudaniec et al. 2007; 
Huber 2008; Kleindorfer and Dudaniec 2009; 
0 Connor et al. 2010c), or to species distributed 
in tropical and subtropical regions (Dudaniec and 
Kleindorfer 2006). 
The genus Philomis includes —50 species of 
Hies, all ectoparasites of birds (Couri and 
Carvalho 2003, Dudaniec and Kleindorfer 2006). 
The life cycle of most of these species as well as 
relationships with their hosts is frequently un¬ 
known (Couri 1999, Teixeira 1999, Dudaniec and 
Kleindorfer 2006). Flies of this genus are 
distributed from central Argentina to the southern 
Laboratorio de Investigaciones en Sistemas Ecologicos 
) Ambientales, Universidad Nacional de La Plata, Diagonal 
* *3 # 469, B1904CCA, La Plata, Argentina. 
Oepartamento de Ecologia, Genetica y Evolucion, 
acultad de Ciencias Exactas y Naturales, Universidad de 
uenos Aires, C1428EGA Buenos Aires, Argentina. 
Corresponding author; e-mail: lsegura79@yahoo.com.ar 
United States (Couri 1999, Fessl et al. 2001). 
Botflies have been reported to parasitize at least 
127 species of birds without marked host 
specificity (Couri 1991, Teixeira 1999). Most 
botfly species have subcutaneous larvae (Couri et 
al. 2005) and nestlings can be parasitized as soon 
as they hatch (Arendt 1985b; Delannoy and Cruz 
1988, 1991; Spalding et al. 2002; Rabuffetti and 
Reboreda 2007). Botfly larvae feed on red blood 
cells (Uhazy and Arendt 1986) and remain in 
nestlings for 5-8 days (Arendt 1985b, Young 
1993, Rabuffetti and Reboreda 2007, Quiroga 
2009) when they leave the nestling as third instars 
and pupate in nest material (Uhazy and Arendt 
1986). Adult flies emerge after a pupation period 
of 1-3 weeks (Oniki 1983, Young 1993, Rabuf¬ 
fetti and Reboreda 2007, Quiroga 2009). 
Most studies indicate botfly parasitism produc¬ 
es sublethal (i.e., lower growth rates) or lethal 
effects on their hosts (Arendt 1985a, b; Delannoy 
and Cruz 1991; Young 1993; Fessl and Tebbich 
2002; Rabuffetti and Reboreda 2007). One of the 
predictor variables for nestling survival is parasite 
intensity (number of larvae/nestling) (Dudaniec 
and Kleindorfer 2006). Some studies have report¬ 
ed only 5-6 larvae caused nestling death (Arendt 
1985b, Delannoy and Cruz 1991), but others 
report similar intensities were not lethal (Nores 
1995) and were only associated with lower growth 
rates (Young 1993). The other variable that 
influences nestling survival is age at the time 
they are parasitized (Arendt 1985a, 2000; Rabuf¬ 
fetti and Reboreda 2007) although this association 
has been less studied. Parasite prevalence (the 
percentage of nests with larvae) increases as the 
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