Study populations and field methods
The study comprises data from eight subpopulations of an insular house
sparrow metapopulation located in the Helgeland archipelago in northern
Norway (66°30′N, 12°30′E; Figure 1). These 8 islands are part of a
larger metapopulation study system covering ca. 1600
km2, where extensive field work during the breeding
seasons (May-August) and autumns (September-October) since 1993 has
ensured that most house sparrows have been individually marked and
followed through their lives from hatching to death (Baalsrud et al.,
2014; Ranke et al., 2021; Saatoglu et al., 2021). From each ringed
individual we collected a small blood sample (~25 μl) to
obtain DNA for genotyping purposes, genetic parentage determination and
pedigree construction (Niskanen et al., 2020). The subpopulations on
islands in this metapopulation are almost solely interconnected by natal
dispersal (breeding dispersal < 0.3%; Altwegg, Ringsby, &
Sæther, 2000; Jensen et al., 2013; Tufto, Ringsby, Dhondt, Adriaensen,
& Matthysen, 2005) with an overall dispersal rate of 22.2% (Saatoglu
et al., 2021). However, dispersal rates also differ among islands and
years, and depend on environmental conditions related to the habitat
type of the subpopulations (Ranke et al., 2021; Saatoglu et al., 2021).
On five islands (Aldra, Gjerøy, Hestmannøy, Indre Kvarøy and Nesøy),
which are also denoted as “farm” islands, house sparrows nest in a
colony-like manner on and near dairy farms, and thus have higher local
densities but are also more sheltered during harsh weather conditions
(Araya-Ajoy et al., 2019). Conversely, in the subpopulations located
farther from the mainland (Myken, Træna and Selvær), denoted
“non-farm” islands, the house sparrows live and breed in nest boxes
and other suitable cavities on the houses in small villages. Non-farm
islands have less sheltering opportunities and probably less stable food
availability during winter-time (Araya-Ajoy et al., 2019). We have
previously shown that the dispersal rate is higher among the non-farm
islands (24.3%) than among the farm islands (9.6%), and that the
dispersal rate is higher from non-farm to farm islands (7.2%) than from
farm to non-farm islands (2.1%; see Saatoglu et al., 2021).