Avian epigenetic clocks: state of the art and call to action

DNA methylation-based epigenetic clocks are powerful biomarkers of chronological and biological age, estimating age from CpG-specific methylation patterns that integrate developmental history, environmental exposure, physiological stress, and stochastic epigenetic change. Despite rapid advances in mammals, avian epigenetic clocks remain scarce, limiting comparative inference and our understanding of how ageing varies across ecological and evolutionary contexts. Here, we review existing avian epigenetic clocks and evaluate their performance for age estimation and biological inference. We argue that birds provide a uniquely powerful system for resolving what epigenetic age represents, due to unique qualities such as external embryonic development, rapid postnatal growth, long-term field studies, and nucleated erythrocytes that enable minimally invasive longitudinal sampling. Together with variation in lifespan and life-history strategies, and expanding genomic and physiological resources, these features position birds as an ideal system for linking developmental environments to epigenetic ageing trajectories. We conclude that integrating epigenetic clocks into avian research will improve age estimation, demographic inference, and conservation applications, while enabling mechanistic tests of how development, environment, and physiology shape ageing. We call for expansion of publicly available DNA methylation datasets from known-age individuals across the avian tree of life, alongside standardized and reproducible analytical workflows to fully realise this potential.