: Seasonal changes in the impact of parasites on hosts
should result in seasonal changes in immune function. Since both
ectoparasites and endoparasites time their reproduction to that of
their hosts, we can predict that hosts have been selected to show an
annual peak in their ability to raise an immune response during the
reproductive season. We found large seasonal changes in immune
function between the breeding and the nonbreeding season for a
sample of temperate bird species. These changes amounted to a
decrease in spleen mass from the breeding to the nonbreeding season
by on average 18% across 71 species and a seasonal decrease in Tcell-mediated
immunity by on average 33% across 13 species. These
seasonal changes in immune function differed significantly among
species. The condition dependence of immune function also differed
between the breeding and the nonbreeding season, with individuals
in prime condition particularly having greater immune responses
during breeding. Analyses of ecological factors associated with interspecific
differences in seasonal change of immune function revealed
that hole-nesting species had a larger increase in immune
function during the breeding season than did open nesters. Since
hole nesters suffer greater reduction in breeding success because of
virulent parasites than do open nesters, this seasonal change in immune
function is suggested to have arisen as a response to the increased
virulence of parasites attacking hole-nesting birds