Young nest-bound birds often face a diversity of ectoparasites that typically feed on blood and can negatively affect nestling morphological and physiological traits, including hemoglobin concentration. While hemoglobin concentration can be correlated with nestling performance, such as body mass, it is unknown whether variation in hemoglobin is the direct proximate factor responsible for variation in morphology, or if nestling traits simply co-vary with hemoglobin concentration due to a number of similar factors influencing both traits. If hemoglobin concentration does directly influence the phenotype of nestlings, then the reduction in nestling hemoglobin concentration by ectoparasites may cause other negative effects commonly associated with parasitism. By experimentally reducing the hemoglobin concentration of nestling Tree Swallows (Tachycineta bicolor) using phenylhydrazine hydrochloride (PHZ), we tested whether reduced hemoglobin concentration could be the mechanism that causes changes in nestling morphology and behavior in response to increased parasitism. Parasite loads were manipulated in conjunction with the PHZ treatment. Nestlings injected with PHZ displayed no change in hemoglobin concentration 2 days post-injection, whereas control nestlings showed the typical increase that occurs during early development. By 6 days post-injection, the parasite, but not PHZ, treatment significantly affected hemoglobin concentration. Both treatments interacted to affect begging intensity, while size and growth rate of nestlings were only affected by the parasite load manipulation. This study demonstrates the potential use of PHZ in nestlings to tease apart the effects of parasitism and reduced hemoglobin, but the results suggest incorporating a second injection of PHZ to better replicate the pattern of hemoglobin reduction created by hematophagous ectoparasites.