Functional Ecology 2005 19, 1001–1007 © 2005 British Ecological Society 1001 Blackwell Publishing, Ltd. Increase in the constitutive innate humoral immune system in Leach’s Storm-Petrel (Oceanodroma leucorhoa) chicks is negatively correlated with growth rate R. A. MAUCK,*† K. D. MATSON,‡ J. PHILIPSBORN* and R. E. RICKLEFS‡ *Biology Department, Kenyon College, Gambier, OH 43022, USA, and ‡Department of Biology, University of Missouri-St. Louis, 8001 Natural Bridge Road, St. Louis, MO 63121–4499, USA Summary 1. Using a simple technique for assessing constitutive innate immune function recently adapted for use in wild populations, we characterize changes in avian immune system development by repeated measurements of individuals over the period of nestling growth in a wild population of Leach’s Storm-Petrels (Oceanodroma leucorhoa). 2. We measured levels of natural antibodies (NAb) during the early, middle and late phases of storm-petrel development and related these levels and NAb rate of change to mass and wing length growth. We used natural variation in nestling growth to assess the influence of nutritional status on the development of innate immunity. 3. NAb levels increased over the first 50 days of chick development; however, rate of increase was inversely proportional to wing growth. 4. Initial titre levels were inversely proportional to rate of change in NAb levels over the first 50 days of immune development. This suggests that individuals with low initial NAb levels accelerate immune development to reach adult levels, whereas individuals with high initial levels do not. 5. As in previous studies, our results demonstrate an inverse relationship between growth rate and development of components of the avian immune system. While such a relationship is consistent with the idea that immune function development involves trade-offs, the processes involved are more complex than simple energy allocation. Key-words: Development, innate humoral immune system, natural antibodies, seabirds, nestling Functional Ecology (2005) 19, 1001–1007 doi: 10.1111/j.1365-2435.2005.01060.x Introduction Immune function is increasingly seen in the context of life-history trade-offs (Sheldon & Verhulst 1996; Norris & Evans 2000; Martin et al . 2001; Tella, Scheuerlein & Ricklefs 2002; Schmid-Hempel 2003). The ability to respond effectively to parasites and pathogens has been shown to have fitness benefits in terms of body condition, survival and reproductive success (e.g. Horak et al . 1999; Johnsen et al . 2000; Alonso-Alvarez & Tella 2001; Christe et al . 2001; Svensson, Sinervo & Comendant 2001; Råberg & Stjernman 2003). With benefits come costs; resources allocated to the immune system are not available for other functions. For example, experi- mentally elicited immune responses have been shown to decrease avian nestling growth (Fair, Hansen & Ricklefs 1999; Alonso-Alvarez & Tella 2001; Whitaker & Fair 2002; Nilsson 2003; Soler et al . 2003; Brommer 2004a), degrade adult body condition (Alonso-Alvarez & Tella 2001; Sanz et al . 2004) and increase metabolic rate (Ots et al . 2001). Many studies (reviewed in Tella et al . 2002) have employed the phytohaemagglutinin (PHA) skin test to assess non-specific cellular immunity (commonly referred to as cell-mediated immunity, or CMI). Although the PHA response involves proliferation of T cells, PHA also attracts other immune cells, including basophils and heterophils, to the injection site and causes inflam- mation (Smits, Bortolotti & Tella 1999). Response to PHA is typically positively correlated with nestling body condition or food availability, sometimes mediated through an effect of brood size or season (e.g. Merino, Moller & de Lope 2000; Dubiec & Cichon 2001; Hoi-Leitner et al . 2001; Westneat et al . 2004). Much recent work on immune function in develop- ing birds has focused on measuring induced immuno- logical responses, a subset of immune function that involves adaptive humoral immunity (e.g. antibody responses to specific antigens) and cellular immunity †Author to whom correspondence should be addressed. E-mail: mauckr@kenyon.edu