Mean age at reproduction and population growth
The average age of fathers of all the recruits produced in this house sparrow metapopulation was 2.24 years (SD = 1.29), while the average age of the mothers of all the recruits was 2.06 (SD = 1.32). These meta-population generation times are higher than the mean of the individual generation times (Table 1), because the meta-population generation time is heavily influenced by the individuals that reproduced the most. The higher estimates for the meta-population generation time imply that individuals that reproduced when they were, on average, older tended to contribute more recruits to the meta-population.
In years when the mean fitness of the population was lower, the mean age of successfully reproducing individuals was older (Table 4, Figure 2). This suggests that in years when competition was high and/or environmental conditions were bad, and so individuals had on average lower fitness, the successfully reproducing males where amongst the older males. In contrast, when the mean fitness of the population was high, and thus populations are expected to grow, the average age of reproducing males was younger. These effects cannot be solely attributed to differences in age structure, because even after correcting for the mean age of all the adults present, there was evidence that these effects were still different from zero (Table 4, model 1B). Supporting this findings, we also found a trend suggesting that in years when population size was higher than average, the mean age of reproducing males was older (Table 4).