The effect of the ectoparasite Carnus hemapterus on immune defence, condition, and health of nestling European Bee-eaters

One way that parasites influence host survival and fitness is by competing with their hosts for resources that would otherwise be used for growth and immune system development. Thus resource availability, influenced by environmental conditions, may be particularly important during offspring development. This study examined the influence of a haematophagous ectoparasite Carnus hemapterus (Diptera: Milichiidae) in relation to environmental conditions on the growth, immune response, and health of nestling European Bee-eaters ( Merops apiaster ). The abundance of this bloodsucking ectoparasite per nest was experimentally manipulated to determine its impact on nestling body condition, immune system function in terms of T-cell-mediated and humoral immune response, and two health parameters, sedimentation rate and haematocrit level. Results indicate a significant negative effect of C. hemapterus intensity on nestling body condition and immune response, and on one blood parameter (haematocrit levels). Nestling body condition, γ-globulin levels, wing web swelling, and haematocrit levels increased when parasites were removed. Although a broad spectrum of fitness indicators seems to be affected, no difference in nestling mortality during the experiment was observed between the two treatment groups. Surprisingly, variations in environmental conditions (weather in particular) that likely affected nestling food availability or thermoregulatory demands had no additional effect on the impact of the parasite infestation.