261 BROOD SIZE MANIPULATION MODIFIES THE INTENSITY OF INFECTION BY HAEMATOZOA IN FEMALE BLUE TITS PARUS CAERULEUS JUAN A. FARGALLO l AND SANTIAGO MERINO Fargallo I.A. & Merino S. 1999 . Brood size manipulation modifies the intensity of infection by Haematozoa in female Blue Tits Parus caeruleus. Ardea 87(2): 261-268. We have studied the effects of a brood size manipulation on the infection by blood parasites in a Blue Tit Parus caeruleus population in central Spain. Overall, 98% of the birds were infected by at least one parasite species. Haemoproteus majoris was the most prevalent parasite (92%), followed by Trypanosoma spp. (48%), Leucocytozoon majoris (44%), and Hepatozoon parus (15%). An experimental brood size manipulation was carried out by means of adding two chicks. Nestlings from enlarged group were lighter than reduced and control groups. Also females from enlarged group had less weight than those from reduced and control groups. Results shown a higher intensity of infection by Trypanosoma and Leucocytozoon in females, but not in males, from enlarged group. Experimental brood size reduction had not detectable effects on infection status in either sex. There were no associ- ations between parasite infection and parental body measurements. The experiment modified the intensity of infection only in some haemoparasite species, suggesting that different haemoparasite species seem to have a dif- ferent response to the same manipulation of bird reproduction. Key words: Parus caeruleus - parasites - cost of reproduction - host fitness - immune system Departamento de Ecologia Evolutiva, Museo Nacional de Ciencias Natu- rales-CSIC, J. Gutierrez Abascal 2, E-28006 Madrid, Spain. Present address: Section of Ecology, Department of Biology, University of Turku, FIN-20014 Turku, Finland. E-mail: juafar@utu.fi. INTRODUCTION The influence of parasites on host fitness is receiving increasing attention, especially in stud- ies of coevolution and population regulation (Atkinson & van Riper 1991; Mszlller 1997). Anti- parasite adaptations of birds, as those mediated by the host immune system are thought to be energet- ically costly activities (M0ller 1997). Similarly, reproduction represents a considerable investment for animals in the wild with attendant costs (Linden & M0ller 1989; Lessells 1991). Conse- quently, there is a potential conflict between a host's level of reproductive effort and its ability to mitigate parasite infection (see Sheldon & Ver- hulst 1996). Initially, blood parasites were considered to be organisms of low pathogenicity in natural popula- tions (Weatherhead & Bennett 1992; Bennett et al. 1993; Davidar & Morton 1993), although they can produce morbidity and mortality in captive birds (Bennett et at. 1993). Recent studies show subtle but important effects of hematozoa on the life his- tories of their avian hosts (Ratti et al. 1993; Kor- pimaki et al. 1993; Allander & Bennett 1994; Kor- pimaki et al. 1995; Dufva 1996). In addition, some experimental studies in Great Tits Parus major have found a relationship between parental repro- ductive effort and the prevalence of blood para- sites (Norris et al. 1994; Richner et al. 1995; Oppliger et al. 1996; Allander 1997) showing an association between reproductive effort and para-