Coevolution of avian grooming and ectoparasite avoidance

Ectoparasites severely reduce avian fitness when they occur in large numbers or most cases, when they serve as intermediate hosts for avian pathogens. In however, ectoparasites occur in small populations with little or no effect on the host. These small populations may be the result of host-parasite coevolution. If a parasite exerts selection on its host and the host exerts reciprocal selection on the parasite, repeated bouts of coevolutionary change can lead to coadaptation of host defence and parasite resistance. Although host defence can prevent the build up of large parasite loads, energetic constraints will probably prohibit the evolution of perfect defence against parasites with coadaptations for resistance. Persistence of small loads on birds may therefore reflect a coevolutionary balance between defence and resistance. Coevolution is not the only explanation for the persistence of small loads, however. If small loads have no effect on the host, they may persist simply because a bird has nothing to gain by removing them. Coevolution cannot be assumed to occur in a given system, but must be empirically inferred, which entails a three-step process. First, the adaptive function of presumably coevolved traits must be documented. Second, reciprocal selection on those traits must be measured. Third, the opportunity for coevolutionary response to reciprocal selection must be demonstrated. In this chapter I report the results of three experiments designed to address the first step -documentation of the adaptive functions of avian grooming and ectoparasite avoidance.