Nestling birds become infested when adult bird blow flies lay eggs directly on nestlings or on nest material near nestlings. These eggs are tiny (less than 2mm in length) and may be laid individually or in clusters. Egg laying usually occurs shortly after nestlings hatch, but multiple infestations may occur in nestlings that spend several weeks in the nest.
Fly eggs hatch within 24-48 hours after they are laid and the tiny larvae must feed immediately. In some species young larvae will crawl into nestlings' ears or nostrils and remain there until they get larger. In most species larvae spend 15-45 minutes feeding on nestlings and then return to the nest. Larvae attach firmly to nestlings with the aid of mouth hooks (called the prothoracic fringe), which are unique to this genus of blow fly. One species (Trypocalliphora braueri) burrows directly into the flesh of nestlings and spends its larval period in the nestling. After hatching, larvae go through two molts (3 instars) as they grow larger and suck more blood. Larvae need at least 3 blood meals to reach maturity. Mature larvae are 7-17mm (about 1/4-3/4 inches) long, depending on species, and they can consume a large amount of blood in a single feeding. Mature larvae pose the greatest hazard to nestlings, because they consume much more blood than when they are younger. If many larvae near maturity at the same time, the impact on nestlings can be severe (Whitworth and Bennett 1992). I have observed over 1000 larvae in a single raven nest.
Once larvae mature they spend several days in a non-feeding prepupal stage and then they pupate. The pupal period usually lasts 7-14 days, or more, depending on the ambient temperature, after which the adult fly emerges. In most species, pupation occurs shortly after nestlings fledge. After adult flies emerge from the pupa they mate and the female searches for a nest with young birds to infest. Adult flies that emerge toward the end of the nesting season will overwinter and infest nestlings the next spring. Adult flies overwinter in areas protected from the weather like under dead bark, in hollow trees or in old bird nests.