Generation time determines the pace of key demographic and evolutionary
processes. Quantified as the weighted mean age at reproduction, it can
be studied as a trait that may evolve and change in response to
ecological conditions. We combined quantitative genetic analyses of
individual projection matrices with age- and density-dependent models to
study generation time variation in a bird metapopulation. We found that
males have longer generation times than females and that it is a
heritable trait. Individuals with longer generation times contributed to
population growth later in life, lived longer, produced fewer recruits
per year, had greater lifetime reproductive success, but not necessarily
a higher expected individual growth rate. As predicted by
density-dependence theory, generation times were shorter when
populations were growing, and longer when populations were closer to
equilibrium or declining. These results support classic theory
predicting that competitive regimes are key determinants of the pace of
life-history strategies.