Elevated nest temperature has opposing effects on host species infested with parasitic nest flies

Environmental factors, such as elevated temperature, can have varying effects on hosts and their parasites, which can have consequences for disease outcomes. The individual direct effects of temperature must be disentangled to determine the net-effect in host-parasite relationships, yet few studies have determined the net-effects in a multi-host system. To address this gap, we experimentally manipulated temperature and parasite presence in the nests of two host species infested by parasitic blowflies (Protocalliphora sialia). We conducted a factorial experiment by increasing temperature (or not) and removing all parasites (or not) in the nests of eastern bluebirds (Sialia sialis) and tree swallows (Tachycineta bicolor). We then measured nestling morphometrics, blood loss, and survival and quantified parasite abundance. We predicted that if temperature had a direct effect on parasite fitness, then elevated temperature would cause similar directional effects on parasite abundance across host species. If temperature had a direct effect on hosts, and therefore an indirect effect on the parasite, parasite abundance would differ across host species. Heated swallow nests had fewer parasites compared to non-heated nests. In contrast, heated bluebird nests had more parasites compared to non-heated nests. The results of our study demonstrate that elevated temperature can have differential effects on host species, which can impact infestation susceptibility. Furthermore, changing climates could have complex net-effects on parasite fitness and host health across multi-host-parasite interactions.