Cooperative Fish and Wildlife Research Units Program: Montana Cooperative Wildlife Research Unit
Education, Research and Technical Assistance for Managing Our Natural Resources

Summary 1. Variation in ontogeny and strength of immune defense mechanisms can be integrally related to variation in life history strategies and determined by trade-offs during development. However, little is known about the ontogeny of immune function in wild birds, especially in altricial birds and in a comparative context across altricial species with diverse life history strategies. 2. In this study we examined the ontogeny of constitutive immunity in a group of 22 Passerine species sampled in tropical Venezuela and north temperate Arizona. 3. Our results show activity of constitutive components of the immune defense at 1-3 days post-hatching, and an increase in immune activity with age. Inter-specific variation in immune activity at hatching was mainly explained by extrinsic factors mediated by the mother (egg size and egg temperature), suggesting an important role of maternal effects on offspring immunity at hatching. In contrast, the increase in agglutination activity with age suggests that immune function in older nestlings reflects intrinsic development. The increase in immune activity was greater in species that hatched with lower initial levels, and was somewhat negatively related to growth rate across species. 4. Our results suggest slower intrinsic development of immune function may be compensated by larger maternal contributions. Slower intrinsic development of immune function, in turn, may reflect a trade-off with faster somatic growth. Our study highlights the importance of both maternal (extrinsic) and endogenous (intrinsic) contributions to variation in immune function across altricial species that may reflect an important axis of developmental strategies.