Although immune function usually declines with age in humans and captive animals, little is known about whether immune function deteriorates with age in natural populations. Here we present evidence for such an age-related deterioration in humoral immune function from a wild population of the Collared Flycatcher (Ficedula albicollis). In this study, young (1-year old), mid-age (3-year old) and old (5-6-year old) females were challenged with a nonpathogenic antigen, sheep red blood cells (SRBC), while provisioning their nestlings. The level of antibodies against SRBC was measured thereafter. Old females showed markedly lower humoral immune response and produced fledglings of lower body mass in comparison with the other two age classes. Moreover, the age classes differed in the relationship between immune response and fledgling body mass with mid-age females showing a significant positive relationship while the relationship was negative but nonsignificant among young and old females. The results are discussed in light of existing theories of optimal resource allocation, ageing and the theory of terminal investment.