Does behavioural flexibility facilitate host switching by parasites?

Baldwin (1896) suggested that behavioural flexibility could allow organisms to persist in novel environments, thus buying time for the evolution of genetic adaptations to the new environment. This has proven true for free-living organisms invading novel habitats. Behavioural flexibility could also allow parasites to exploit novel hosts, but this hypothesis has not been tested, despite the fact that parasitism is one of the most common lifestyles on earth.• In this study, I compare the behavioural flexibility of two relatively host specific groups of feather lice (Phthiraptera: Ischnocera) that parasitize pigeons and doves: `wing' lice (Columbicola columbae and C. passerinae) and `body' lice (Campanulotes compar and Physconelloides eurysema). • Wing and body lice are similar in many aspects of their natural history, but they differ in their relationship to host body size. Wing louse size is tightly correlated with host body size, whereas body louse size is not. Even so, experiments have shown that wing lice can establish on different sized novel hosts just as well as body lice. • Behavioural flexibility may facilitate the establishment of wing lice on different sized novel hosts. To test this hypothesis, I experimentally transferred wing and body lice to a series of different sized novel host species. Once the louse populations established (two generations) I compared the microhabitat preferences of wing and body lice on novel hosts vs. native host controls. • Wing lice shifted their microhabitat use on novel host species, and the magnitude of the shift was correlated with host size. In contrast, body lice rarely shifted microhabitat and when they did, the shift was not correlated with host size. • Behavioural flexibility may play a pivotal role in the ability of wing lice to establish on different sized novel host species, and could be an important factor for other parasites faced with novel hosts.