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.