Acknowledgements
We are indebted to Peter H. Becker for setting up the long-term individual-based common tern study at the Banter See and to Götz Wagenknecht and the many scientists, students and field assistants who collected the data across the years. We express our gratitude to Johanna Klughammer, Amelie Nemc and Christoph Bock for their advice on sequencing strategy and for performing the RRBS sequencing. We also thank Michaela Schwarz for help with DNA extraction, Kristian Ullrich for his support in solving bioinformatics hurdles, and Melanie J. Heckwolf and our epigenetic journal club for inspiring discussions on DNA methylation. The study was performed under licenses of the city of Wilhelmshaven and the Lower Saxony State Office for Consumer Protection and Food Safety, Germany. This study was supported by the Max Planck Society through a MPRG grant (MFFALIMN0001 to ML), a sequencing grant (PSLIMN6000 to ML and SB), and the ”Norddeutscher Wissenschaftspreis 2018” (BSM, SB and ML).

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Data accessibility

The common tern reference genome we developed (bSteHir1) is accessible in NCBI (https://www.ncbi.nlm.nih.gov/bioproject/560234), as well in Genome Ark (https://vgp.github.io/genomeark/Sterna_hirundo/). All RRBS data will be deposited into the public database at ENA https://www.ebi.ac.uk/ena. The final dataset and our R code will be uploaded to Dryad upon acceptance of the manuscript.

Author Contributions

SB collected the blood samples and life history data on the terns, ML and SB designed the study and ML organized sequencing of the samples. BSM analyzed the methylation data and performed the statistical analyses with the help of MM. The reference genome was generated as part of the Vertebrate Genome Project, coordinated by EDJ. OF coordinated the genome assembly; BH and JM were responsible for sample processing and generated the genome sequence data; CC, GF and MU-S for the genome assembly; WC, JW and KH curated the final reference genome. BSM and SB wrote the manuscript with input from ML, MM and EDJ. All authors commented on drafts of the manuscript.

Tables and Figures

Table 1. Result of a Generalised Linear Mixed Model with binomial error distribution testing whether variation in autosomal methylation level is explained by sex (males as a reference category) and the between- (average age) and within-individual (delta age) components of age. Provided are parameter estimates and associated 95% credible intervals (95% CI). Significant effects (p-value < 0.05 and 95% CI which do not overlap with zero) are presented in bold.