Growth rates and other life history measurements
As the expected hatch date was approaching, we intensified nest
monitoring to multiple times a day in order to accurately identify what
is conventionally defined as day 0 of the nestling stage. Once the
nestlings emerged from the egg we painted their tarsi using an
individual combination of non-toxic markers to allow measurement of
individual growth trajectories. On the same day we also started weighing
each nestling using a portable electronic scale (Nuweigh; ±0.01 g).
Measurements proceeded every second day at the same time (16:00 ± 1h)
until fledging. We estimated the nestling growth rate constant
(K ) using logistic regressions (Sofaer et al., 2013; Remeŝ and
Martin, 2002; Ricklefs, 1968). This method allows us to obtain
asymptotic body mass at fledge date, and to calculate the slope of the
line tangent to the growth curve at inflection point (K ). This is
a dimensionless and mass-independent metric of growth speed that is not
confounded by variation among individuals in size at hatch. These
measurements allowed us to test whether fast rates of somatic growth
(Vedder et al. 2017; Monaghan & Ozanne 2018), and offspring mass
(Ringsby et al. 2015) affect telomere length and attrition.
We also recorded brood size, and kept track of hatching order, which are
both known to influence telomere dynamics (Reichert et al. 2014;
Costanzo et al. 2016; Noguera et al. 2016). Finally, when the
offspring produced in our experiment reached 60 days ,of age we recorded
their sex, on the basis of plumage, to account for gender differences in
telomeres (Noguera et al. 2015).