This has been researched by Helen O. McCarthy and her team of University of Ulster scientists both in the laboratory and at Muck Island in Scotland.

This is due to two reasons: First, the birds forage in the snails' rocky shore habitat mainly during the breeding season (which lasts about 4 months each summer) but not at other times.

Thus, even when the gulls forage in periwinkle habitat, the snails tend to stay out of easy reach of the birds.

It is unknown whether M. piriformes spends the winter as resting eggs in the habitat, or as sporocysts in already-infected periwinkles.

In any case, snails harboring metacercariae generally die of desiccation before winter if they are not eaten.