Telomere length (TL) shortens with age but telomere dynamics can relate to fitness components independent of age. Immune function often relates to such fitness components and can also interact with telomeres. Studying the link between TL and immune function may therefore help us understand telomere-fitness associations. We assessed the relationships between erythrocyte TL and four immune indices (haptoglobin, natural antibodies, complement activity, heterophil-lymphocyte ratio; n=477-589), from known-aged individuals of a wild passerine (Malurus coronatus). As expected, we find that TL significantly declined with age. To verify whether associations between TL and immune function were independent of parallel age-related changes (e.g. immunosenescence), we statistically controlled for sampling age, and used within-subject centring of TL to separate relationships within or between individuals. We found that TL positively predicted complement activity at the between-individual level (individuals with longer average TL had higher complement activity), but no other immune indices. In contrast, age predicted levels of natural antibodies and heterophil-lymphocyte ratio, allowing inference that respective associations between TL and age with immune indices are independent. Any links existing between TL and fitness are therefore unlikely to be strongly mediated by innate immune function, while TL and immune indices appear independent expressions of individual heterogeneity.

These data were collected as part of the long-term purple-crowned fairy-wren research project established in 2005 by Anne Peters.

The data and code in the accompanying R script file were used to produce the results presented in the main manuscript and supplementary information.

The statistical models coded in the R script are those described in the stastical methods of the main manuscript. Model summaries were extracted to produce the results and tables presented in the manuscript. Code for the figures in the main manuscript is also included.

Individual Band-ID numbers registered with the Australian Bird and Bat Banding Scheme (ABBBS) have been de-identified for online publication. These Band-ID numbers have been replaced with a 6-character alphanumeric string, unique to each individual within each data file. Identities of individual Band-ID numbers are kept on record with Michael J. Roast and Anne Peters and can be re-identified if necessary.