Data from: Dimorphism from an early age: sex differences in size and immune function in great tit nestlings

The dataset contains data on great tit nestlings morphometrics (mass,
wing, and tarsus) and innate immune function (agglutination, lysis,
haptoglobin concentration, and bacteria killing capability), and was
collected in a nestbox population in southern Sweden during two
consecutive breeding seasons (2022 and 2023). In both years, a brood size
manipulation experiment was carried out. 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 in the life cycle. In this
two-year study, we compared size, mass and immune function
(Hemolysis-hemagglutination assay, Bacteria Killing Assay and Haptoglobin
assay) between sexes of great tit (Parus major) nestlings, likely
reflecting the outcome of relative resource allocation during ontogeny. To
account for variation in environmental quality, we evaluated
sex-differences in relation to a brood size manipulation and in relation
to seasonal progression. 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.