Botfly Parasitism Effects on Nestling Growth and Mortality of Red-crested Cardinals

—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 of Philornis 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 of 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 characteristics 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 of 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) represent 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 Philornis and their hosts have been limited to a few species (i.e., P. downsi and Darwin’s finches in the Galápagos 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; O’Connor et al. 2010c), or to species distributed in tropical and subtropical regions (Dudaniec and Kleindorfer 2006). The genus Philornis includes ,50 species of flies, 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 unknown (Couri 1999, Teixeira 1999, Dudaniec and Kleindorfer 2006). Flies of this genus are distributed from central Argentina to the southern 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, Rabuffetti and Reboreda 2007, Quiroga 2009). Most studies indicate botfly parasitism produces 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 reported 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; Rabuffetti and Reboreda 2007) although this association has been less studied. Parasite prevalence (the percentage of nests with larvae) increases as the The Wilson Journal of Ornithology wils-123-01-13.3d 17/12/10 15:12:35 107 Cust # 10-053 1 Laboratorio de Investigaciones en Sistemas Ecológicos y Ambientales, Universidad Nacional de La Plata, Diagonal 113 # 469, B1904CCA, La Plata, Argentina. 2 Departamento de Ecologı́a, Genética y Evolución, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, C1428EGA Buenos Aires, Argentina. 3 Corresponding author; e-mail: [email protected] The Wilson Journal of Ornithology 123(1):107–115, 2011