Results
There were no significant effects of the ages of the natal or foster parents on offspring telomeres at hatching or the change in offspring telomere length from hatching to the end of post-natal development (Figure 1, see Tables 2 and S2 for results with age groups, young vs old, and Tables 3 and S1 for results with parental age as a continuous factor). When we entered the parental age squared in the models, it did not change this result (Tables S7-S9). Telomere length was individually repeatable between the first and second sampling, separated by on average 11 days (repeatability r = 0.55, CI= 0.368 to 0.69, p < 0.0001, Figure 2). Chick telomere length between the first and second telomere measurements increased rather than decreased (the estimated change in relative telomere length was 0.08 ± 0.05, p=0.003, Table 2; when one outlier chick with highest increase in telomere length was removed, the results stayed similar, estimated change 0.06 ± 0.04, p = 0.005). The rate of growth during post-natal development was not significantly related to the change in telomere length (measured as change of head size, r < 0.0001; p = 0.97, Figure 3). Chick growth rate was not related to the age of the parents and foster parents (all parental effect p-values > 0.31, Tables S4-S5). The power for detecting potential effects of parental age on telomere length and telomere shortening in our sample is reported in Table S3. The study set up also allowed us to assess wide-sense heritability of telomere length, although these results must be taken with caution, since we did not use a split nest cross-fostering design. Chick telomere length at both first (h2=0.27 CI=0.05 to 0.50) and second (h2=0.30 CI=0.08 to 0.54) measurement as well as the change of telomere length (calculated as telomere length 2 - telomere length 1, h2=0.14 CI=0.03 to 0.29) were heritable.