Philornis infection in blue‐black grassquits: impact on nestlings and risk factors involved

Abstract

Parasitic botfly larvae (Philornis ssp., Diptera: Muscidae) are found in nests of several bird taxa, although prevalence and impact on nestling survival 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, and changes in nestling body shape. Additionally, we tested whether climatic conditions, nest morphology and habitat characteristics are associated with larvae abundance. Blue‐black grassquits had low breeding success (16% of eggs/nestlings survived to fledged; 19% of the nests fledged at least one), but most failures resulted from predation by vertebrate predators. We estimated that 1% of nestlings died due to botfly infestation, and the number of subcutaneous larvae (range 1–18) in a nestling's body did not predict fledging success. Infected chicks exhibited higher tarsus asymmetry. 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. Nesting areas 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. Alternatively, vegetation composition could influence local invertebrate diversity, which could provide a natural trophic buffer against adult Philornis. The present study brings to light new perspectives concerning bird–botfly interaction.

Grassquits adjust parental care in broods parasitized by native nest flies

“…Incubation and nestling periods lasted up to 10 days each (Carvalho et al, 2007). The age of nestlings in nests found after eggs had hatched was estimated by comparison with chicks of known age (appearance, weight, and size), or by date of fledging (following Biagolini Jr & Macedo, 2021) (hatching day: day = 0).…”

“…In the 2017–2018 and 2018–2019 breeding seasons, we video‐recorded nests during both incubation and nestling periods, and nestlings were handled in early and late development stages to record the number of Philornis larvae (Biagolini Jr & Macedo, 2021). In the 2010–2011 and 2011–2012 breeding seasons, data were primarily collected to answer questions on another subject (Diniz et al, 2015).…”

“…Video records allowed us to detect larval parasitism on the head and neck body regions of nestlings (Figure 1), so we were able to increase our sample size by using these videos as evidence of nest parasitism status in nests that lacked manual inspection of nestlings. This protocol is unlikely to bias our sample to head or neck parasitism because in 93% of the infected nests at least one grassquit nestling bear larvae in these body regions (Biagolini Jr & Macedo, 2021). We discarded from analyses: nests that were not inspected in the field in late stages and for which video records showed no evidence of larvae on the head or neck of nestlings, since larvae could be present elsewhere on their bodies; and nests with videos with poor image quality (e.g., when the camera was distant from the nest), which may have prevented us from confirming the presence of larvae.…”

“…Grassquits are parasitized by the subcutaneous and native nest flies P. glaucinis and P. trinitensis (Teixeira, 1999). Results from a recent study showed that Philornis parasitized 54 of 164 nestlings (32%) from 29 of 75 (38.7%) nests, and these nestlings exhibited greater tarsus asymmetry (Biagolini Jr & Macedo, 2021). However, parasitism was directly responsible for only 1% of nestling mortality.…”

“…However, parasitism was directly responsible for only 1% of nestling mortality. In addition, blue‐black grassquit nests built in territories with more abundant food resources had lower parasite loads, suggesting that food‐rich habitats may allow parents to invest more time in brooding nestlings, thus protecting offspring from the effects of fly parasitism (Biagolini Jr & Macedo, 2021).…”