Thriving in an arid environment: High prevalence of avian lice in low humidity conditions

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Abiotic factors have been reported to cause variation in parasite pressure among host populations, but such relationships have not been studied in detail, and conflicting evidence exists about the nature of these relationships. Here we study within- and between-population variation in chewing lice parasitization in an arid bird species, the Trumpeter finch (Bucanetes githagineus), and the influence of ambient humidity on parasite load. We found a high prevalence of 2 chewing lice even in particularly dry years and a positive effect of humidity on prevalence at a monthly scale. Nonetheless, our results clearly reveal that lice can become abundant at low ambient humidity conditions and that birds in arid environments are not necessarily under lower ectoparasitic pressure than birds in humid regions. We suggest that lice may adapt their life cycle to overcome the most critical period by synchronizing the more resistant phase (eggs) to the period when relative humidity is lowest (i.e., summertime). We stress that studies on the effect of ecological factors on host-parasite relationships should consider detailed aspects of the life cycle of the latter and the main biological traits of the different stages of the parasites.