Condition-dependent immune defence in the Magpie: how important is ectoparasitism?

The immune system has high costs of maintenance and production and expressi on of immune function should be dependent on the condition of the individua l. The :fitness cost that parasites impose on hosts may then be reflected i n the investment that hosts make in immunity. Little is known about sources and consequences of intra- and interspecific variation in the presence and size of organs of the immune system in relation to ectoparasitism. Variati on in the presence and size of the bursa of Fabricius and spleen size was m easured to assess potential relationships between immune defence, nutrition al condition and ectoparasites in the Magpie Pica pica. We found three spec ies of lice (Mallophaga), two species of louse flies (Hippoboscidae), and t he larvae of a tick (Ixodidae) infecting magpies. Age explained a significa nt proportion of variation in prevalence and number of parasite species, fi rst-year birds being more parasitized than adults. Spleen size increased wi th nutritional condition, and the level of ectoparasitism negatively affect ed nutritional condition. Only one species of lice (Philopterus picae) was involved; the negative effect of this species on nutritional condition was more pronounced in males than females. These results are consistent with th e hypotheses of condition-dependent investment in the immune system and sex ual differences in susceptibility to parasite effects. Ectoparasites may th us play a role in modulating the relationship between condition and immune defence. We also found evidence for an interaction between the bursa and th e spleen. Magpies with bursa had larger spleens than those where it had atr ophied. This interaction did not affect ectoparasitism or nutritional condi tion. (C) 2001 The Linnean Society of London.