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).