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).