Nestling diet, secondary sexual traits and fitness in the zebra finch

We examined the effect of nestling diet quality on a suite of physiological , morphological and life-history traits in adult male zebra finches, Taenio pygia guttata. Compared with birds reared on a supplemented diet, nestlings reared on a seed-only diet showed a reduced rate of growth and reduced cel l-mediated immune function as measured by an in vivo response to a T lympho cyte-dependent mitogen. There were no differences between birds reared on t he two diets in any of the following adult traits: body size, primary sexua l traits (testes mass, numbers of stored sperm, sperm function, velocity an d morphology), secondary sexual traits (beak colour and song rate), serolog ical traits or immunological traits. The only differences we detected were a lower body mass and a greater proportion of individuals with plumage abno rmalities among those reared on a seed-only diet (this latter effect was tr ansient). 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, demo nstrates that they subsequently compensated through the differential alloca tion of resources. Our results indicate that differential allocation is cos tly 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 develo pment of tl airs important for reproduction, such as primary and secondary sexual traits and longevity.