The effects of nestling diet quality (limiting nutrient kl⁻¹) and quantity on the growth and adult phenotype of passerines was investigated in this study. An increase in nestling diet quality resulted in faster growth and larger adult size of the zebra finch Taeniopygia guttata. Tha ultrasound technique revealed differences in pectoralis thickness between groups. Mala zebra finch plumage may be affected by diet quality. Diet quality effects are likely to occur in wild zebra finch populations. A decrease in diet quality resulted in slower growth, and smaller nesting size at fledging, of house martins Delichon urbicum. Mechanisms for this effect in wild populations are discussed. Estimates of Field Metabolic Rate using the doubly-labelled water techniques were the highest yet published. Possible explanations are discussed. Brood manipulation affected great tit Parus major nestling growth, reduced broods grew faster than enlarged broods. Supplementary feeding did not alter diet quality but did increase food availability and result in faster growth of Supplemented broods. Breast Stripe size was affected by food availability, but this effect may not have been independent of body size. Those effects are likely to occur in wild populations, and their implications are discussed. The pattern of female mass loss supported the hypothesis feeding frequency is adjusted to maximise the difference between reproductive costs and benefits. Variation in diet quality may affect individual life-histories through effects on growth rate, differential growth, growth curve shape, fledging size, adult size, fledging plumage and adult plumage. The results of the study support a key assumption of the brood reduction hypothesis. Implications of the results for evolutionary and ecological studies are discussed. It is suggested that parents may maximise the quality of nestlings through maximising the quality of their nutrition, and that the quality of nestlings is a more important component of fitness than has been considered previously.