A comparative analysis of the relationship between the spleen-a major
organ of immunity-and helminths was undertaken with bird species, usin
g the phylogenetic regression technique. Species in which many individ
uals are infected with nematodes relative to the number of individuals
examined for the presence of helminths (termed 'relative nematode pre
sence') have significantly larger spleens, for a given body weight, in
females (though not in males). There was little indication that this
relationship depends on incidental ecological indices, the weights of
other organs, or the 'relative presences' of trematodes, cestodes or h
aematozoa. Combined with previous, experimental, work it suggests that
the avian spleen is important in resisting nematodes. Body weight is
correlated with the relative presence of helminths; but even after rem
oving body size effects, bird species which tend to be infected with t
rematodes are also more likely to be infected with cestodes. This pape
r indicates that the spleens of wild bird species are associated with
macroparasites in the natural environment.