Understanding spatiotemporal effects of food supplementation on host–parasite interactions using community-based science

Supplemental feeding can increase the overall health of animals but also can have variable effects on how animals defend themselves against parasites. However, the spatiotemporal effects of food supplementation on host–parasite interactions remain poorly understood, likely because large-scale, coordinated efforts to investigate them are difficult. Here, we introduce the Nest Parasite Community Science Project, which is a community-based science project that coordinates studies with bird nest box ‘stewards’ from the public and scientific community. This project was established to understand broad ecological patterns between hosts and their parasites. The goal of this study was to determine the effect of food supplementation on eastern bluebirds (Sialia sialis) and their nest parasite community across the geographic range of the bluebirds from 2018 to 2021. We received 674 nests from 69 stewards in 26 states in the eastern United States. Nest box stewards reported whether or not they provided mealworms or suet near nesting bluebirds, then they followed the nesting success of the birds (number of eggs laid and hatched, proportion that hatched, number and proportion of nestlings that successfully fledged). We then identified and quantified parasites in the nests. Overall, we found that food supplementation increased fledging success. The most common nest parasite taxon was the parasitic blow fly (Protocalliphora sialia), but a few nests contained fleas (Ceratophyllus idius, C. gallinae and Orchopeas leucopus) and mites (Dermanyssus spp. and Ornithonyssus spp.). Blow flies were primarily found at northern latitudes, where food supplementation affected blow fly prevalence. However, the direction of this effect varied substantially in direction and magnitude across years. More stewards fed bluebirds at southern latitudes than at northern latitudes, which contradicts the findings of other community-based science projects. Overall, food supplementation of birds was associated with increased host fitness but did not appear to play a consistent role in defence against these parasites across all years. Our study demonstrates the importance of coordinated studies across years and locations to understand the effects of environmental heterogeneity, including human-based food supplementation, on host–parasite dynamics.