Philornis parasitism: impact on nestlings and risk factors involved

Parasitic botfly larvae (Philornis ssp., Diptera: Muscidae) are found in nests of several bird taxa, although prevalence and nestling tolerance vary considerably among species. Here we describe patterns of botfly infestation in blue-black grassquit (Volatinia jacarina) nestlings. We identified the most typically affected nestling body parts and assessed parasite prevalence, impact on nestling survival, changes in nestling body shape and mass index. Additionally, we tested whether climatic conditions, nest morphology and habitat characteristics are associated with larvae abundance. Blue-black grassquits had low breeding success (15%), but most failures resulted from predation by vertebrate predators. We estimated that only 1% of nestlings died due to botfly infestation, and the number of larvae in nestling body did not affect nest success. Infected chicks exhibited a higher body mass to tarsus length ratio, and higher tarsus asymmetry. Previous studies indicate that adult grassquits with a higher body mass index have lower dominance status and mating success. Thus, we argue that although botflies had a small impact on offspring survival, they may reduce fitness in adulthood. There was no evidence that environmental conditions and nest morphology are linked to the number of larvae on nestlings. Territories with higher food supply had lower infestation rates. Possibly, food-rich habitats allow parents to invest more time in offspring care (brooding nestlings), thus protecting them from fly attacks. The present study brings to light new perspectives concerning bird-botfly interaction