11 (2): 168-174 (2004) E C o SCIENCE Changes in reproductive strategies may affect fitness components differentially. Conflicts or trade-offs between different components of fitness are the foundation of life- history theory. Two of the most studied trade-offs have been those between (1) current and future reproduction or survival and (2) quantity and quality of offspring (Stearns, 1992). Both are closely implicated in the individual deter- mination of clutch size. Traditionally, two hypotheses have been proposed to explain individual variation of clutch size in birds (Nur, 1987). The individual optimiza- tion hypothesis, derived from Lack (1947), proposes that parents lay the number of eggs from which they can max- imize the number of recruits produced from a single sea- son (Perrins & Moss, 1975); thus, the number of eggs laid would match the parental ability to rear offspring for which no trade-offs are predicted (Pettifor, 1993). The trade-off hypothesis assumes a cost of reproduction of laying larger clutches and predicts trade-offs between clutch size and individual survival or fecundity and/or between quantity and quality of the progeny (Williams, 1966; Monaghan, Bolton & Houston, 1995). Evidence for the trade-off hypothesis arises mainly from experimental studies in which the clutch/brood size has been enlarged or reduced (Stearns, 1992; Monaghan & Nager, 1997). However, in descriptive studies there is less evidence of potential trade-offs in parents and offspring in relation to clutch/brood size (Merilä & Wiggins, 1995). Recent reviews focusing on host-parasite ecology underline the interaction between parasites and immune function and the potential impact of parasites on repro- ductive trade-offs (Sheldon & Verhulst, 1996; Møller, 1997; Zuk & Stoehr, 2002). Reproduction and immune response are costly activities competing for limiting resources within the individual (Møller, 1997; Martin, Scheuerlein & Wikelski, 2002), for which a higher invest- ment in reproduction may produce a relapse and/or sus- Clutch size and haemoparasite species richness in adult and nestling blue tits 1 Juan A. FARGALLO 2 & Santiago MERINO, Departamento de Ecología Evolutiva, Museo Nacional de Ciencias Naturales (C.S.I.C.), José Gutiérrez Abascal 2, E-28006 Madrid, Spain, e-mail: fargallo@mncn.csic.es Abstract: Diseases have been recently suggested as a potential cost that parents and offspring pay for an elevated reproductive effort in wild organisms. We investigated the relationship between clutch size and the number of blood parasite species (species richness) infecting adult and nestling blue tits (Parus caeruleus). Four protozoan (Haemoproteus majoris, Leucocytozoon majoris, Hepatozoon parus, and Trypanosoma avium) and one helminth (Microfilariae) species were found infecting adults and two protozoan species (T. avium and L. majoris) were found in nestlings. Females laying larger clutches showed a higher haemoparasite species richness, but infection in male parents was not associated with clutch size. The species richness of haemoparasites per nestling was also positively correlated with clutch size. Both relationships in female and nestling blue tits may be explained in terms of the trade-off between reproduction and number and quality of the progeny. This is the first report of a connection between clutch size and blood parasitemias of their offspring, and it suggests new tools for exploring potential trade-offs among fitness components in wild organisms. Keywords: blood parasites, fitness, life history, nestling viability, number of offspring, Parus caeruleus, reproductive effort, trade-offs. Résumé : On a récemment suggéré que les maladies représentent un coût potentiel que les parents d’animaux sauvages et leur progéniture doivent payer pour un effort de reproduction élevé. Nous avons vérifié s’il existe une relation entre la taille de la couvée et le nombre d’espèces de parasites sanguins (richesse) qui infectent les mésanges bleues (Parus caeruleus) adultes et juvéniles. Quatre espèces de protozoaires (Haemoproteus majoris, Leucocytozoon majoris, Hepatozoon parus et Trypanosoma avium) et une d’helminthe (Microfilariae) infectent les adultes, alors que deux espèces de protozoaires (T. avium et L. majoris) contaminent les jeunes au nid. Les femelles qui pondent le plus d’oeufs ont une richesse élevée en espèces d’hémoparasites. Par contre, les infections des mâles ne sont pas associées à la taille de la couvée. La richesse en espèces des parasites sanguins chez les jeunes au nid est également corrélée de façon positive à la taille de la couvée. Les relations observées chez les femelles et les oisillons pourraient s’expliquer par un compromis entre l’effort de reproduction et le nombre de jeunes et la qualité de la progéniture. Il s’agit de la première mention d’une relation entre la taille de la couvée et le parasitisme du sang des jeunes. Elle apporte de nouveaux outils permettant d’explorer les compromis potentiels entre les composantes du succès reproducteur chez les animaux sauvages. Mots-clés : cycle vital, échanges, effort de reproduction, nombre de jeunes, parasites du sang, Parus caeruleus, succès reproducteur, viabilité des jeunes au nid. Nomenclature: Baker, 1956; Peirce, 1981; Bennett & Peirce, 1989; 1992. Introduction 1 Rec. 2003-07-08; acc. 2003-11-12. 1 Associate Editor: André Desrochers. 2 Author for correspondence.