Blood-sucking fly larvae are widespread parasites of nestling birds, but in many systems we lack knowledge of their basic biology. This study reports the first observation of an endemic Tasmanian fly species, Passeromyia longicornis (Diptera : Muscidae), parasitising the forty-spotted pardalote (Pardalotus quadragintus), another Tasmanian endemic. Because the forty-spotted pardalote is an endangered and declining songbird, P. longicornis is a species of interest to conservation biologists. Its larval form is an obligate, subcutaneous parasite of nestling birds, but before this study, there were just two published records of the species infesting avian hosts, and little known about its ecology or life cycle. This study documented hosts, prevalence, and larval life history of P. longicornis by locating and monitoring nests and ectoparasites of the forest bird community in south-eastern Tasmania. I also reared P. longicornis larvae in captivity to determine the length of the pupal stage in relationship to ambient temperature. Hosts of P. longicornis included forty-spotted pardalotes (87% prevalence across nests), striated pardalotes (Pardalotus striatus) (88% prevalence), and New Holland honeyeaters (Phylidonyris novaehollandiae) (11% prevalence). Both pardalote species were new host records. P. longicornis larvae burrowed under the skin of nestlings where they developed for 4–7 days, feeding on nestling blood. When fully grown, larvae dropped into the surrounding nest material and formed pupae. Length of the pupal stage was 14–21 days, and declined with increasing ambient temperature. Median parasite abundance was 15 larvae in infested forty-spotted pardalote nests and 11 larvae in infested striated pardalote nests. Nestling mortality was frequently associated with ectoparasite presence. This study provides the first survey of P. longicornis hosts, prevalence and life cycle, and shows that this species is likely a major player in the ecology of pardalotes, and possibly other forest bird species in Tasmania.