In this new study, we followed blood mitochondrial DNA copy number (mtDNAcn) within individual zebra finches (Taeniopygia guttata) across two thirds of the species' normal lifespan. mtDNAcn is widely used as a cellular marker linked to mitochondrial function and physiological condition, yet how it changes within individuals over time and whether it relates individual performance remains poorly understood outside laboratory models.

We found that mtDNAcn declined early in life within individuals but was not related to an individual's performance (growth) during that stage. However, individuals with higher mtDNAcn later in life showed better flight performance (flight speed). These findings highlight the importance of longitudinal approaches to understand mitochondrial dynamics across the lifespan and suggest that mtDNAcn may be a useful marker of individual quality in ageing-related traits.