Immune responses have evolved to defend hosts efficiently against the debilitating effects of parasites on host fitness. However, there are relatively few studies of the efficiency of the immune system in terms of providing hosts with an ability to defend themselves against parasitism. A meta-analysis of the literature on survival of birds in relation to non-specific immune response to challenge with an antigen or other measures of immune function demonstrated a mean effect adjusted for sample size of 0.43 across 12 studies. This observation shows that relatively simple estimates of non-specific immune responses often reliably predict a large and significant amount of variation in survivorship.