Birds can show patterns of sexual size dimorphism as early as during the nestling stage. This raises the question of how the faster growing sex might reconcile the energetic and nutritional needs of a faster growth rate with resource allocation to other important life functions, such as the development of innate immune function. Innate immunity represents the main line of defence against diseases, and while some innate immune defences are already present at hatching, substantial development occurs throughout the nestling stage. Hence, this development may compete for resource allocation with growth, potentially affecting nestlings in a sex-specific way in species showing sexual size dimorphism at early age. However, little is known about how sex might shape life-history strategies early into the life cycle. In this two-year study, we molecularly determined the sex of Great tit (Parus major) nestlings. We measured morphometrics (mass, wing and tarsus) and carried out innate immunity assays (Hemolysis-hemagglutination assay, Bacteria Killing Assay, and Haptoglobin assay). We then compared size, mass and immune function among sexes shortly before fledging, likely reflecting the outcome of relative resource allocation during ontogeny. We also carried out a brood size manipulation experiment to simulate resource limitation in the nest. We found that male nestlings grew to a larger size at day 14 than their female siblings. However, we also found some indication that males developed a better immune defense than females albeit their faster growth. Thus, males manage to invest more heavily in both growth rate and immune defence, probably depending on males being dominant to females in the competition for parental feeding, resulting in higher resource acquisition.