Study system and sampling
The study was performed on the islands of Vega (163 km2, 65°40’N, 11°55’E) and Leka (57 km2, 65°06’N, 11°38’E) off the coast of northern Norway (Fig. S1.1) in the years 2002-2012. The house sparrow is a small sedentary passerine (Anderson, 2006), thus the geographical separation of the islands (52 km) ensured no dispersal between populations (Kvalnes et al., 2017). The average generation time in similar populations is about 2 years (Jensen et al., 2008), and average lifespan is about 2 years (Jensen et al., 2004), but the maximum recorded lifespan is 13 years in the wild (Klimkiewicz & Futcher, 1987). Both study islands are dominated by cultivated land (silage production), heathland, mountains, and sparse forest. The sparrows live closely associated with humans mainly on dairy farms on these islands, where they nest in holes and cavities and have access to food (grain) and shelter all year. During the breeding season (mid-May to mid-August) from 2002-2006 all accessible and active nests (they can lay up to 3 broods per season with on average 4.6±0.06 eggs) were visited at least every 9th day. Laying and hatching date were recorded for each brood (207 broods from 158 nests) and chicks were marked as fledglings (ca. 11 days old, ranging from 5-17 days) with a numbered metal ring and plastic color rings for identification. Mist-netting during the breeding season, in autumn (September-November), and in late winter (February-March) ensured that around 90% of the winter population at both islands were marked at all times during the study (see Kvalnes et al., 2017). For all fledglings and adults, tarsometatarsus (tarsus) length was measured using calipers to the nearest 0.01 mm. Because tarsus length increases with nestling age, we estimated standardized tarsus length as the residuals of a linear regression of tarsus length on age separately for each sex and population. Age-corrected nestling tarsus length is a good predictor of adult tarsus length (Fig. S1.2, r =0.74, for n =220 birds that were recaptured as adults). To minimize disturbance, fledglings were measured only once in the nest, thus, individual growth rates were not directly monitored. However, within species in which growth is seasonally constrained, larger individuals generally grow faster (Metcalfe & Monaghan, 2003). A small blood sample (25 μL) was collected from all nestlings around 11 days (5-17 days) old by puncturing the brachial vein. Blood was stored in 1 ml absolute ethanol at room temperature in the field and subsequently in the laboratory at -20°C until DNA extraction.