During nest-box checks the larvae can sometimes be seen attached to the fleshy parts of the chick's head, legs, or underwings. After a number of blood meals, the larvae pupate. Adult flies emerge, and if the timing is right, they breed and lay eggs in the next available nest box. Flies that emerge at the end of the birds' breeding season may find no viable nests or chicks to lay their eggs on. These blowflies overwinter as adults outside the nest box and lay their eggs in the spring, starting the cycle all over again.
Blowflies are interesting creatures in their own right.
Their offspring's survival depends entirely on their ability to lay their eggs in nests at the appropriate time. This strong selective pressure has produced an animal that times its egg laying quite accurately. Nasonia are the most commonly encountered. Jewel wasps overwinter as larvae inside the blowfly pupal cases. In certain areas, nest boxes that are not emptied out at the end of the birds' breeding season will almost certainly contain Nasonia wasp larvae.