Information on individual age is a fundamental aspect in many ecological and evolutionary studies. However, accurate and non-lethal methods that can be applied to estimate the age of wild animals are often absent. Furthermore, since the process of ageing is accompanied by a physical decline and the deterioration of biological functions, the biological age often deviates from the chronological age. Epigenetic marks are widely suggested to be associated with this age-related physical decline, and especially changes in DNA methylation are suggested to be reliable age-predictive biomarkers. Here, we developed separate epigenetic clocks for ageing and development in a small passerine bird, the great tit (Parus major). The ageing clock was constructed and evaluated using erythrocyte DNA methylation data of 122 post-fledging individuals, and the developmental clock using 67 pre-fledging individuals from a wild population. Using a leave-one-out cross-validation approach, we were able to accurately predict the ages of individuals with median absolute deviations of 0.40 years for the ageing and 1.06 days for the development clock. Moreover, using existing data from a brood-size manipulation, we show that nestlings from reduced broods are estimated to be biologically older compared to control nestlings, while they are expected to have higher fitness. These epigenetic clocks provide further evidence that, as observed in mammals, changes in DNA methylation of certain CpG sites are highly correlated with chronological age in birds and this opens up new avenues for broad applications in behavioural and evolutionary ecology.
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