Nestlings of many bird species are hosts to hematophagous ectoparasites. Parasitism of nestlings is usually sub-lethal, but its effects can extend into the fledgling and adult stages. Nestling hosts lose enough blood to become anemic, but the effects of reduced oxygen-carrying capacity on metabolic rate are poorly understood. This study examined the consequences of parasitism by larval blow flies Protocalliphora sialia for nestling tree swallows Tachycineta bicolor. We found that nestlings with more parasites had higher whole-animal RMR. We evaluated the role of increased erythropoiesis as a potential mechanism explaining increased RMR by drawing blood from some birds. However, we found no evidence that blood loss, as a proxy for hematophagous parasitism, increased metabolic rate. Future work should examine the metabolic costs of upregulation of immunity and how elevated RMR due to ectoparasites affects nestlings as they develop into independent adults.