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).