Effects of body size and weather on telomere length
Combining data from both populations (n =566), TL was found to be negatively related to tarsus length (model ranked 1:βtarsus =-0.03, CI=[-0.05, -0.00], Tables 2 and 3, Fig. 5a) and positively related to the NAO_30 index (βNAO_30 =0.10, CI=[0.01, 0.19], Fig. 5b). This means that telomeres are shorter in larger individuals and when overall weather conditions are harsh (Appendix S1). In addition, TLs were shorter in females than males (βfemale =-0.08, CI=[-0.14, -0.02]), and TLs were shorter in the low population compared to the highpopulation (βlow =-0.17, CI=[-0.24, -0.10]). The second-best model (∆AICc=0.2) showed weak evidence that the negative relationship between TL and tarsus length tended to be steeper in thehigh population compared to the low population (βlow*tarsus =0.05, CI=[-0.02, 0.11]), suggesting that artificial selection for longer tarsus had a stronger effect on TL compared to selecting for shorter tarsus. There was little evidence for any effect of nestling age on TL (model ranked 3, ∆AICc=1.4, βage =0.01, CI=[-0.01, 0.03], Table 2).