Population differences in the length and early‐life dynamics of telomeres among European pied flycatchers

Telomere length and shortening rate are increasingly being used as biomarkers for long‐term costs in ecological and evolutionary studies because of their relationships with survival and fitness. Both early‐life conditions and growth, and later‐life stressors can create variation in telomere shortening rate. Studies on between‐population telomere length and dynamics are scarce, despite the expectation that populations exposed to varying environmental constraints would present divergent telomere length patterns. The pied flycatcher (Ficedula hypoleuca) is a passerine bird breeding across Eurasia (from Spain to western Siberia) and migrating through the Iberian Peninsula to spend the nonbreeding period in sub‐Saharan Africa. Thus, different populations show marked differences in migration distance. We studied the large‐scale variation of telomere length and early‐life dynamics in the pied flycatcher by comparing six European populations across a north–south gradient (Finland, Estonia, England and Spain) predicting a negative effect of migration distance on adult telomere length, and of nestling growth on nestling telomere dynamics. There were clear population differences in telomere length, with English birds from midlatitudes having the longest telomeres. Telomere length did not thus show consistent latitudinal variation and was not linearly linked to differences in migration distance. Early‐life telomere shortening rate tended to vary between populations. Fast growth was associated with shorter telomeres in the early life, but faster nestling growth affected telomeres more negatively in northern than southern populations. While the sources of between‐population differences in telomere‐related biology remain to be more intensively studied, our study illustrates the need to expand telomere studies at the between‐population level.