Hosts exert selection pressures on their parasites and it is often
assumed that host–parasite coevolution with each host is
less intense in a generalist parasite than for a parasite with a narrow
host range. Selection pressure on the parasite, however,
is rather determined by host specificity, i.e. the relative importance
each host, than simply by the range of hosts. The
determination of host specificity requires an assessment of the prevalence
and intensity of parasite infestation within each
host's nests, as well as the local abundance of each host species.
the hen flea, Ceratophyllus gallinae, is a rather
generalist parasite of birds it could be concluded that there has
been weak coevolution with each of its hosts. By reviewing
the literature on the prevalence and intensity of hen flea infestations
in bird nests we estimated the number of individuals
produced in the nest of each host species. The comparative analysis shows
(1) that the prevalence of infestation is highest
in hole-nesting avian families, (2) that prevalence and intensity of
infestation among bird families are highly correlated,
and (3) that hole-nesting Paridae have the highest intensities of infestation
and harbour the majority of the flea population.
These results underline the fleas' potential for coevolution with
Paridae despite their extensive host range.