Experimental studies in humans and laboratory species have shown that the decline of the immune system with age (immunosenescence) and the accumulation of oxidative damage to macromolecules are two key contributors to the onset and progression of the ageing process. Although laboratory models have provided important insights, the physiological basis of ageing in natural populations remains comparatively understudied, constraining our mechanistic understanding of the ageing process. The complexity of age-related physiological changes increases further in long-lived species, which appear to possess unique adaptations reducing immunosenescence and oxidative damage. However, studies investigating the underlying physiological mechanisms in long-lived birds have yielded contrasting results. In this study, we compared four markers of oxidative status and eight immune markers between younger and older breeders of a long-lived seabird, the Scopoli’s shearwater (Calonectris diomedea), to identify potential physiological signatures of ageing. Regardless of sex, older individuals exhibited higher levels of blood antioxidant enzymes, natural antibodies, and lymphocytes compared to younger birds, while levels of DNA damage and cellular effectors of innate immunity did not differ between age classes. These findings suggest that older shearwaters may upregulate antioxidant enzyme activity, possibly to cope with increased basal production of reactive oxygen species, in line with the oxidative stress theory of ageing. Alternatively, the higher antioxidant levels of older birds might reflect selective mortality of birds with reduced protection. In contrast to the oxidative status, the observed immune patterns do not support the immunosenescence hypothesis