EFFECTS OF A DIPTERAN ECTOPARASITE ON IMMUNE-RESPONSE AND GROWTH TRADE-OFFS IN BARN SWALLOW, HIRUNDO-RUSTICA, NESTLINGS

Parasites can have a profound effect on biology and evolution of the h osts, which are expected to have evolved physiological and development al mechanisms that allow them to minimise the costs imposed by parasit es. In this study we analyse the effects of a dipteran ectoparasite on barn swallow (Hirundo rustica) nestling biology including rate of som atic growth, plasma protein concentration, blood cell sedimentation ra te, hematocrit, concentration of leukocytes in peripheral blood, and T -lymphocyte cell-mediated immunocompetence. In a natural population, i ntensity of parasite infestation was positively correlated with growth of feathers. Nestlings in heavily infested nests may decide to alloca te more resources to feather growth thus fledging early. To test this hypothesis, the detrimental effects of parasites on nestlings, and the existence of trade-offs between competing growth processes, we inocul ated some nests with additional flies. Nestlings exposed to increased infestation had larger rate of feather growth but were in poorer condi tion than unmanipulated controls. Parasite inoculation resulted in lar ger concentrations of eosinophils and lymphocytes. Among siblings of b roods inoculated with parasites, those that had the largest rate of fe ather growth had the lowest rate of increase in tarsus length and body mass. We conclude that louse flies depress barn swallow nestling cond ition and influence their immune profile. However, they also enhance g rowth of a morphological character that may allow nestlings to reduce the impact of parasites. Nestlings apparently experience a trade-off b etween the competing demands for growing feathers and other somatic ch aracters.