Phylogenetic patterns in life-history traits
For our 53 species of birds, life history traits were largely
represented on three orthogonal axes: body size (PC1),
reproductive and longevity features along a slow-fast continuumof the pace of life (PC2), and post-fledging parental care(PC3). Our interpretations of life history axes followed well accepted
principles based on previous studies of vertebrate species
(Gaillard et al., 1989;
Harvey & Purvis, 1999;
Stearns, 1992). The first axis of the PCA
clearly reflected the magnitude of collinear life-history traits,
usually interpreted as reflecting variation among species in body size.
This conclusion is supported by positive loadings over PC1 for adult
structural size and body mass, as well as egg and hatching mass. Some
body size-related traits that reflect duration of life also had strong
positive loadings on PC1, such as incubation duration, age at fledging,
age at sexual maturity, and maximal lifespan
(Speakman, 2005a). The second axis of the
PCA appeared to reflect the slow-fast continuum . Eigenvectors
built within a PC analysis are orthogonal to each other, thus the
species-specific trade-off between reproduction (clutch size, a positive
factor loading) and survival (age at sexual maturity and maximal
lifespan, negative factor loadings) on PC2 were statistically
independent of the species’ adult body sizes (the latter reflected by
loadings on PC1). As investment in reproduction decreases, age at
maturity increases and lifespan increases, an interspecific pattern that
is consistent with a fundamental life-history trade-off. Longevity thus
has two components, that associated with body size and a residual
component associated with the pace of life. Finally, the third axis of
the PCA had a strong factor loading only for the duration of
post-fledging parental care that is the number of days parents care for
their offspring once they have left the nest. This axis was
statistically independent of both body size and the slow-fast continuum.
Unsurprisingly, variance among species for the three PC axes of life
history traits were strongly associated with the species’ phylogenetic
histories. Associations of life history with phylogeny are well
documented in vertebrate animals (de
Magalhaes, Costa, & Church, 2007; Dobson
& Jouventin, 2007; Dobson & Oli,
2007). As “body plans” are associated traits that change over
evolutionary time, life-history traits are expected to coevolve
(Bennett & Owens, 2002;
Brown & Sibly, 2006;
Roff, 2002;
Stearns, 1992). In mammals, a relatively
strong trade-off is found between growth and longevity, long-lived
species showing slower post-natal growth rates. In birds, there is
strong selection for a rapid growth after hatching, in order to quickly
reach emancipation (thermal and nutritional)
(de Magalhaes et al., 2007). While we
found that embryonic growth rate showed the expected negatively related
pattern of variation with lifespan along the slow-fast continuum,
post-hatching growth rate was relatively low for the largest species and
not associated with the slow-fast continuum. This suggests that
embryonic growth rate may have a larger effect on lifespan evolution
than post-hatching growth rate. Short-lived birds were characterized by
faster embryonic development, a relationship previously characterized
for vertebrates in general (Ricklefs,
2006). Such observations need to be remembered when further focusing at
how life-history axes relate to Adult TL (see below), since the
mechanisms of the delayed cost of rapid growth in terms of impaired
somatic maintenance and shortened lifespan are not well understood
(Monaghan & Ozanne, 2018).