Experimental effects of within-brood genetic variation on parasite resistance in a wild bird host

Hosts can differ in parasite susceptibility across individuals, populations, and species. Genetic variation can influence parasite susceptibility by affecting host resistance to parasitism. For example, genetic variation among related individuals, such as within broods of offspring, might be a key factor influencing within-brood resistance to ectoparasitism. The goal of this study was to determine if within-brood variation of eastern bluebirds (Sialia sialis) affects susceptibility to ectoparasitic blowflies (Protocalliphora sialia). To address the goal, we conducted a partial cross-fostering study for which half of the nestlings were cross-fostered (experimental) or not (control). Nestling physiology (i.e., glucose, hemoglobin, and parasite-specific IgY antibody levels), morphometrics (i.e., mass, tarsus length, bill length, and first primary feather), survival, nestling status (resident versus fostered), and sex were characterized. We also quantified parasite abundance, life stage, and pupal size. We found that experimental nests had fewer parasites and more larvae than pupae compared to control nests, which suggest that within-brood genetic variation affects parasite abundance. However, this effect was driven by the sex ratio with the experimental nests, with female-biased nests having fewer parasites than male-biased nests. Treatment did not affect nestling morphometrics, physiology, or survival at the nest level. Within experimental nests, resident females had significantly higher hemoglobin levels when compared to fostered females. Resident and fostered males and fostered females had significantly higher glucose levels than resident females. Together, these results suggest that resident females were fed upon less than fostered females and may have been less stressed than males and fostered females. Overall, our study demonstrates the importance of considering within-brood variation and nestling sex in understanding host-parasite interactions.