Data from: Lifespan and reproductive cost explain interspecific variation in the optimal onset of reproduction

Fitness can be profoundly influenced by the age at first reproduction
(AFR), but to date the AFR-fitness relationship only has been investigated
intraspecifically. Here we investigated the relationship between AFR and
average lifetime reproductive success (LRS) across 34 bird species. We
assessed differences in the deviation of the Optimal AFR (i.e., the
species-specific AFR associated with the highest LRS) from the age at
sexual maturity, considering potential effects of life-history as well as
social and ecological factors. Most individuals adopted the
species-specific Optimal AFR and both the mean and Optimal AFR of species
correlated positively with lifespan. Interspecific deviations of the
Optimal AFR were associated with indices reflecting a change in LRS or
survival as a function of AFR: a delayed AFR was beneficial in species
where early AFR was associated with a decrease in subsequent survival or
reproductive output. Overall, our results suggest that a delayed onset of
reproduction beyond maturity is an optimal strategy explained by a long
lifespan and costs of early reproduction. By providing the first empirical
confirmations of key predictions of life-history theory across species,
this study contributes to a better understanding of life-history
evolution.