Data collection
Complete data and references list are presented in the Electronic
Supplementary Material (ESM) 1.
We first identified published articles in peer reviewed journals that
reported telomere length or rate of telomere erosion in birds. We (i)
screened our own reference lists, (ii) backward searched references
sections of selected articles from the list, and (iii) collected
publications using Google Scholar with the following terms: “telomere”
OR “telomere shortening” OR “telomere erosion” AND “birds” OR
“avian.” For each retrieved paper, methods were individually appraised
by one author (FC) to restrict the selection to studies using the
telomere restriction fragment (TRF) method to assess telomere length or
erosion. Our meta-analysis therefore only relies on published telomere
length data expressed in absolute length (kilobases, kb). Using absolute
values of telomere length (TL) allowed us to do interspecific
comparisons of telomere length relationships with other life history
traits, based on the calculation for each species of a mean adult
telomere length (hereafter Adult TL ), mean chick telomere
length (hereafter Chick TL ) and mean telomere length rate
of change over the lifecycle (hereafter TROC ), allowing
inter-specific comparisons. We primarily used studies that sampled
erythrocytes, with one exception of a study that used tissue attached to
feathers. Data extraction is detailed in ESM 2.
Data on life history traits were compiled for species for which TRF
measurements of Adult TL were available. ESM 1 references the
principal sources used to collect TRF values and 12 life-history
variables for each species. We included publications in peer reviewed
journals, on-line peer-reviewed data bases, and books. The protocol for
collecting life history data is described in ESM 2. In particular, we
examined (i) species body mass and size, (ii) longevity (age at maturity
and lifespan), (iii) reproductive patterns (e.g. using egg or
clutch sizes), (iv) growth patterns (pre- and post-hatching growth
rates), and (v) post-fledging parental care.
We adjusted statistical analyses for sample sizes using the standard
procedure for meta-analyses (Liberati et
al., 2009)). We could not control for the mean age at which telomeres
were measured in adult individuals of each species. However, age is
likely to be related to life history variables included in our analysis
(e.g., body size, longevity) and likely to scale with body mass. As
such, risks of multicollinearity of variables were high. Thus, we used
principle components analyses as a variable reduction method (see
details below). We conducted a separate analysis of Chick TL to check
for specific relationships between life history traits and telomere
length early in the species lifecycles. In addition, we analysed
associations of TROC and life-history traits. As in previous studies,
TROC was estimated as the slope of the linear regression between TL and
age over life (i.e. including chick and adults life stages,
(Haussmann et al., 2003;
Tricola et al., 2018).