Older female little penguins Eudyptula minor adjust nutrient allocations to both eggs

• Body mass increases before laying suggest reliance on reserves for clutch production.
• Stable isotopes suggest little endogenous reserve input to eggs.
• Female age modulates individual variations in resource allocation strategies.
• Differential investment in eggs may favour siblings with greater chances to fledge.

How individuals allocate resources to reproduction is a fundamental concept in animal life history theory. While it is crucial to quantify the relative allocation of somatic reserves to reproduction by avian females; variation at the individual and intra-clutch level has generally been neglected. We investigated nutrient allocation to eggs of little penguins (Eudyptula minor, Forster, 1781) using automated body-mass recording and natural δ13C and δ15N values for plasma and blood cells of incubating females, and down feathers of their chicks. Further, we evaluated the role of female age (a proxy for experience or senescence) and condition as drivers of individual strategies within this population. Little penguins increased their body masses during a short period prior the onset of reproduction, suggesting that they were accumulating resources to cope with reproductive requirements. Estimated endogenous contribution to eggs was relatively low (~ 20%), and female age was revealed as a major factor modulating individual variations in resource allocation strategies. Older females accumulated larger amounts of reserves before laying (up to 40% body mass increases) and relied more on endogenous resources (up to 45%) for clutch production than younger females. Our findings suggest that females were able to adjust their endogenous investment between their two chicks, with somatic reserve inputs to heavier siblings showing a bimodal distribution (mean ± SD; 16.7 ± 5.9% and 36.5 ± 4.4%) that contrasted with the unimodal distribution observed for lighter siblings (17.8 ± 8.7%). Here, we provide evidence pointing to age and age-related changes as major factors modulating individual decisions on the amount of somatic reserves allocated to eggs. We additionally identified two distinct allocation strategies at the intra-clutch level that may favour siblings with greater chances to fledge, revealing a novel clutch size reduction strategy in seabirds that produce two eggs of similar size but involving a differential nutrient investment in eggs.