We compared haematocrits and blood haemoglobin concentrations of nestling House Wrens Troglodytes aedon in nests with relatively large numbers of haematophagous larvae of the blow fly Protocalliphora parorum (>6.4 large larvae/nestling) and nests in which exposure to larvae was severely reduced or eliminated (0–2.0 larvae/nestling). Heavily parasitized nestlings showed no reduction in haematocrit levels but a significant 28% reduction in haemoglobin levels. Previous research on avian response to chronic blood loss suggests that parasitized nestlings may show normal volumes of red blood cells as a result of a rapid production of new cells in response to blood loss and, additionally, as a result of swelling of individual cells left in the bloodstream after parasite feeding. Rapid erythropoiesis places high numbers of haemoglobin‐poor, immature cells into the bloodstream which would explain the reduced haemoglobin levels of parasitized nestlings. Our results refute the conclusion that larval feeding has “little direct effect” on nestling wrens in this parasite–host system, drawn earlier by Johnson and Albrecht (1993) based on examination of haematocrits alone. Our results also strongly suggest that researchers wanting to measure effects of blood‐feeding parasites on birds measure haemoglobin levels, not haematocrits.

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This study asked whether parent House Wrens (Troglodytes aedon) in a Wyoming population increased rates of food delivery to broods parasitized by hematophagous blow fly larvae and mites. We observed no significant difference in rates at which pairs fed nestlings at nests with naturally heavy infestations of fly larvae (6–19 larvae per nestling) and nests in which nestling exposure to larvae was experimentally eliminated or severely reduced (0–2 larvae per nestling). The apparent failure of parents to compensate nestlings for resources lost to parasites may, in part, explain the reduced rates of mass gain by parasitized nestlings that we observed and the presence of reduced hemoglobin levels in parasitized nestlings documented in a companion study. Parasitized nestlings may be too weak or anemic to intensify begging activity which would alert parents to their increased need for food. Alimentación de Polluelos de Troglodytes aedon Afectada por Ectoparásitos Hematófagos: Una Prueba de la Hipótesis de Compensación Parental Resumen. Nos preguntamos si los individuos parentales de Troglodytes aedon de una población de Wyoming aumentan las tasas de alimentación sobre polluelos parasitados por ácaros y larvas de moscas hematófagas. No observamos diferencias significativas en la tasa a la cual las parejas alimentaron a los polluelos entre nidos altamente infectados naturalmente por la larva de la mosca (6–19 larvas por polluelo) y nidos en que la exposición a las larvas fue eliminada o severamente reducida experimentalmente (0–2 larvas por polluelo). En parte, la aparente falta de compensación por parte de los padres sobre la pérdida de recursos de los polluelos infectados puede explicar sus bajas tasas de ganancia de peso y los niveles reducidos de hemoglobina documentados en un estudio asociado al presente. Los polluelos parasitados probablemente se encuentran muy débiles o anémicos como para intensificar la actividad de reclamo lo que alertaría a los padres sobre su mayor necesidad de alimento.