Ibis (2008), 150, 799–806 © 2008 The Authors Journal compilation © 2008 British Ornithologists’ Union Blackwell Publishing Ltd Sex differences in circulating antibodies in nestling Pied Flycatchers Ficedula hypoleuca ELISA LOBATO,* SANTIAGO MERINO, JUDITH MORALES,† GUSTAVO TOMÁS,‡ JOSUÉ MARTÍNEZ-DE LA PUENTE, ESTRELLA SÁNCHEZ, SONIA GARCÍA-FRAILE & JUAN MORENO Departamento de Ecología Evolutiva. Museo Nacional de Ciencias Naturales. CSIC, C/José Gutiérrez Abascal 2, 28006, Madrid, Spain Sex differences in immune function are relatively well studied in vertebrate animals, although the patterns are not always clear in birds. The study of immune responses in nestlings of wild bird populations may constitute an appropriate way to investigate inherent intersexual differences while controlling for environmental conditions such as parasitism that affect male and female individuals growing in the same nest. We studied whether the cell-mediated immune response, as measured by phytohaemagglutinin (PHA) injection, and the levels of circulating antibodies differ between sexes of Pied Flycatcher nestlings Ficedula hypoleuca. No sex differences in nestling cell-mediated immune response were found, but females showed significantly higher levels of plasma immunoglobulins than males did. Although nestling birds may not have a fully functional humoral immune defence, our study indicates that sex differences in the humoral component exist at this early stage of life. Given the importance of antibodies in the fight against parasite, bacterial and viral infections, the intrinsic sex disparity in circulating antibodies may have important implications for the life history of each sex. Keywords: cell-mediated immune response, dimorphism, humoral response, immune system, passerines. A pattern of differential vulnerability to disease according to sex is often present in nature, as males are frequently reported to show higher rates of parasitic diseases than females (Alexander & Stimson 1989, Zuk 1990). These sex differences may be caused by differential exposure to pathogens due to reproductive roles or by intrinsic physiological differences between male and female phenotypes (reviewed in Zuk & McKean 1996). Specifically, sex differences in immune function may drive differences in susceptibility to disease between the sexes, as males may exhibit lower immune responses than females and subsequently may show higher susceptibility to infections (Møller et al. 1998, Nelson et al. 2002). In birds, the patterns of sexual differences in immune function are not always clear (reviewed in Has- selquist 2007). Intersexual differences in parasite susceptibility and immune function may be already present in early stages of life, and may be important to the future prospects of each sex, because immu- nological parameters have been shown to explain recruitment probability of nestling birds (Christe et al. 2001, Møller & Saino 2004, Cichón & Dubiec 2005, Moreno et al. 2005a). In fact, the pressure exerted by mite parasitism has been reported to induce sexual size differences in Pied Flycatcher Ficedula hypoleuca nestlings, with males experiencing a more detrimental effect from mites (Potti & Merino 1996). Similarly, ectoparasite effects on growth of Great Tit Parus major nestlings have been reported to affect males more than females (Tschirren et al. 2003). Several studies have investigated possible sexual differences in the cell-mediated immune response of nestling birds, with no significant differences being observed (Tella et al. 2000, Saino et al. 2002, Jovani et al. 2004, Müller et al. 2005, Rowland et al. 2007, Wilk et al . 2007). However, evidence of sexual differences *Corresponding author. Email: elobato@mncn.csic.es †Present address: Lab de Conducta Animal, Instituto de Ecología, UNAM, México DF. ‡Present address: Netherlands Institute of Ecology (NIOO- KNAW). Centre for Terrestrial Ecology. Postbus 40, 6666 ZG Heteren, The Netherlands.