Statistical analyses
The correlation between
tarsus length and telomere length
We first tested the phenotypic correlation between TL and tarsus length
(as a proxy for body size) within 2462 house sparrow nestlings from
Hestmannøy and Træna. TL (response variable) was
log10-transformed
and linear mixed-effects models
(LMMs) were fitted with a Gaussian error distribution (R package
‘lme4 ’, Bates, Mächler, Bolker, & Walker, 2015). Sex differences
in TL are known for house sparrows (Pepke et al., 2021,submitted ). Thus, models included sex, (continuous) fledgling age
at sampling, hatch day (ordinal date mean centered across years), and
island identity as fixed effects. We fitted random intercepts for brood
identity and year to account for the non-independence of nestlings from
the same brood and year. Because our study populations are known to be
affected by inbreeding depression (Niskanen et al., 2020), we included
the inbreeding coefficient (F ) as a fixed effect (Reid & Keller,
2010). We then compared models with and without (age-standardized)
tarsus length using Akaike’s information criterion corrected for small
sample sizes (AICc , Akaike, 1973; Hurvich & Tsai, 1989), and
Akaike weights (w ) and evidence ratios (ER ) to determine
the relative fit of models given the data (Burnham & Anderson, 2002).
Models were validated visually by diagnostic plots and model parameters
are from models refitted with restricted maximum likelihood (REML).
Estimates and 95% confidence intervals (CI) are reported.