We examined the effect of nestling diet quality on a suite of physiological, morphological and life–history traits in adult male zebra finches, Taeniopygia guttata. Compared with birds reared on a supplemented diet, nestlings reared on a seed–only diet showed a reduced rate of growth and reduced cell–mediated immune function as measured by an in vivo response to a T lymphocyte–dependent mitogen. There were no differences between birds reared on the two diets in any of the following adult traits: body size, primary sexual traits (testes mass, numbers of stored sperm, sperm function, velocity and morphology), secondary sexual traits (beak colour and song rate), serological traits or immunological traits. The only differences we detected were a lower body mass and a greater proportion of individuals with plumage abnormalities among those reared on a seed–only diet (this latter effect was transient). The fact that male zebra finches reared on a seed–only diet were, as adults, virtually indistinguishable from those reared on a supplemented diet, despite having reduced growth and immune function as nestlings, demonstrates that they subsequently compensated through the differential allocation of resources. Our results indicate that differential allocation is costly in terms of fitness since birds reared on a seed–only diet experienced a significantly greater mortality rate than those reared on a supplemented diet. This in turn suggests the existence of a trade–off between the development of traits important for reproduction, such as primary and secondary sexual traits and longevity.