Study system
This study was carried out in eight of the island populations in a metapopulation of house sparrows inhabiting eighteen islands in northern Norway (66°N 13°E; see map in Baalsrud et al. 2014). The high recapture rates (~74%) in these eight study populations, combined with extensive sampling effort, allows close monitoring of the life-histories of most of the individuals inhabiting these islands. Each time a bird was handled, as nestling, fledged juvenile or adult, we took a blood sample for genetic analyses. A pedigree based on 605 highly polymorphic and independent single nucleotide polymorphisms (SNPs; Niskanen et al. 2020) was generated and then used for estimating the number of recruits produced per individual. We used specific data sets for the different analyses, but in general this metapopulation has been monitored continuously since 1993. However, systematic SNP genotyping of all adults started in 1998 in some islands and in 2003 for others. Therefore, the maximum possible time period of data included in these analyses differed between populations (Table S1).