Life-history traits and Principal Component Analysis
Among the 53 avian species, life-history traits exhibited several medium
to strong inter-correlations (ESM 4). In particular, measures of body
size and mass were associated with most variables. In order to deal with
these strong multiple collinearities, we conducted a principle
components analysis. The first principle component (PC1) accounted for
nearly 60% of the variance among life-history traits, and as expected
was strongly associated with adult female mass and other variables that
reflect the body size of the species (Table 1). The second principle
component (PC2) accounted for about 11% of the variance, and positively
reflected reproductive and pre-hatching development variables, and a
negative association with lifespan. With reproduction and lifespan
negatively associated and statistically independent of body size, PC2
likely reflected the “slow-fast continuum” or pace of life syndrome
(reviewed by (Dobson & Oli, 2007).
Scores of the species on PC2 were ordered from relatively fast life
histories (greater reproduction and faster pre-hatching growth and
shorter lives) to relatively slow life histories (longer lives, lower
reproduction and lower pre-hatching growth). The third principle
component (PC3) accounted for about 10% of the variance and primarily
reflected the number of days post-fledging chicks stay with their
parents. We interpreted PC3 as post-fledging parental investment in
offspring.