Résumé
Blow fly ( Protocalliphora spp.) larvae are nest ectoparasites that compete with nestlings for resources by feeding on blood. Their main direct consequences on nestlings are a reduction in growth, blood parameters, and physiological performance. The impact of Protocalliphora larvae on nestling development can be extremely variable between nests within years and between years due to varying food availability. However, there is also considerable within‐nest variation in nestling growth and development which has been attributed to an aggregation of ectoparasites on less resistant chicks; the tasty chick hypothesis. We tested for aggregated attacks of Protocalliphora within broods of nestling blue tits using radio‐labelled inulin to trace plasma, and hence blood, turnover. To estimate blood removal by Protocalliphora, we compared inulin turnover in naturally parasitized and experimentally deparasitized nestlings. Inulin turnover, and hence the blood loss attributed to Protocalliphora attacks, was significantly correlated with chick mass and size rank within the brood, with the smallest and lowest‐ranking chicks showing greatest inulin turnover. This is the first study demonstrating non‐random blood‐feeding by Protocalliphora larvae, a phenomenon which can render the statistical detection of their impact problematical by increasing within‐brood variance in developmental and physiological measures.