An Early-Life Disruption of Gut Microbiota Has Opposing Effects on Parasite Resistance in Two Host Species.

Gut microbiota regulate multiple aspects of host health, including metabolism and the development of the immune system. However, we still know relatively little about how the gut microbiota influences host responses to parasitism in wild organisms, particularly whether host-microbiota interactions contribute to variation in parasitism across host species. The goal of this study was to determine the role of gut microbiota in shaping how birds respond to nest parasites and investigate whether this relationship varies between host species. Both eastern bluebirds (Sialia sialis) and tree swallows (Tachycineta bicolor) are parasitized by blow flies (Protocalliphora sialia), for which larval flies feed on nestlings' blood. We experimentally manipulated the gut microbiota of nestling bluebirds and tree swallows by dosing nestlings with an oral antibiotic or sterile water as a control. We then quantified nestling physiology (haemoglobin, glucose, parasite-specific IgY antibody levels), body morphometrics, and survival until fledging, as well as blow fly abundance and size. An experimental disruption of nestling gut microbiota increased parasite abundance in tree swallows, but decreased parasite abundance in bluebirds, which suggests that the disruption has opposing effects on resistance across host species. Furthermore, experimental treatment delayed parasite development and had variable effects on nestling body morphometrics and physiology across the two host species. Together, these results suggest that gut microbiota contribute to host differences in resistance to blow flies and can influence host-parasite interactions.