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.