Little is known about the genus Philornis (comprising ~50 species), a group of muscid flies that parasitise birds and may be highly detrimental to host nestlings. Philornis species affect at least 115 species of bird, particularly in the Neotropics. The main distribution of Philornis is in Central and South America, extending to the southern United States. Larvae of the genus Philornis reside in bird nests and may feed on either nestling faeces (coprophagous scavengers), the blood of nestlings (semi-haematophagous parasites), or on nestling tissue and fluid (subcutaneous parasites). Depending on the species of Philornis, larval development can occur in bird faeces, in nesting material or inside nestlings. Nestling mortality depends on the species of Philornis, the intensity of infestation and nestling susceptibility, which in turn depends on the nestling species, age, brood size, body condition and the anatomical site of infestation. Consequently, variable effects of Philornis parasitism are observed in relation to nestling growth, development and fledging success. The impetus for this review is the recent discovery of Philornis downsi on the Galapagos Archipelago, combined with high Philornis-induced mortality in Darwin’s finches. The potential for ectoparasites such as Philornis to compromise the viability of small, isolated bird populations is highlighted by this recently documented parasite invasion.