Sexual dimorphism in mean age at reproduction
We found that males contributed more to population growth when they were
older compared to females (Table 1). Male house sparrows in our
metapopulation lived longer and had longer generation times as compared
to females (Table 1). Furthermore, we found that male production of
recruits in their first year was lower compared to females,
corroborating similar results for a larger sample of house sparrow
populations (Stubberud et al. 2017). Due to less costly
production of male gametes, male fitness is hypothesized to be largely
limited by access to fertilizations. Males are thus predicted to invest
more in reproduction through male-male competition at the expense of
survival (Liker & Székely 2005; Bonduriansky et al. 2008). Our
results do not support these predictions; we found that males lived
longer and started reproducing later compared to females.
Sex differences in generation time can be explained by intra-sexual
competition for nest sites or mates. For instance, lower production of
recruits by males during their first breeding year could also be caused
by more males being unpaired in their first breeding year because of a
biased sex ratio. However, we did not find strong systematic bias in the
sex ratio in this metapopulation. Another potential cause of sex
differences in the mean age at reproduction could be associated to
extra-pair reproduction (Hsu et al. 2017; Girndt et al.2018). If older males increased their reproductive output through
extra-pair fertilizations, this would have contributed to the sex
differences in the mean age at reproduction in our population. This is
interpretation is partly supported by findings in this meta-population
where traits that vary among male age classes (body mass, wing length
and badge size; Araya-Ajoy et al. 2019) positively affect the
number of mates (Jensen et al. 2008).