Comparison of parasitism level in two sympatric passerines: The pied flycatcher and the great tit

Parasites: have negative effects on their hosts and the latter evolve adaptations to reduce the burden of the parasites. This co-evolution profess may result in similar host species in similar habitats, parasitised by similar parasite species, having widely different levels of parasitism. Here I test this idea by reviewing levels of parasitism in two passerines, the pied flycatcher Ficedula hypoleuca and the great tit Parus major, which are ecologically similar and share the same habitat and parasite species. When the abundance of ectoparasites in nests was compared or the same study sites and pear, I found that nests of pied flycatchers contain only half as many ectoparasites as those of the great tit. A similar trend was found in other studies of the ectoparasites of the two species. A further search through the literature showed that pied flycatchers also hare fev;er blood parasites than great tits. Several hypotheses to explain these differences in the level of parasitism are put forward and discussed. There is little or no support for hypotheses based on differential exposure, nest site selection, or variation in resistance through sexual selection. On the other hand the data are consistent with hypotheses based on two behavioural strategies (small clutch size, use of insecticidal material in the nest) or on differential parasite virulence. Thus similar host species may vary widely in their levels of parasitism. in the present ease. experimental studies are needed to conclusively distinguish between the possible mechanisms by which this difference has arisen.