Study populations and data collection
The study was performed in four insular house sparrow populations off
the coast of northern Norway (Fig. S1.1 in Appendix S1). The study
periods differed between the populations with data from Hestmannøy
(66°33’N, 12°50‘E) in the years 1994-2013, Træna (Husøy island, 66°30’N,
12°05‘E) in the years 2004-2013, and Leka (65°06’N, 11°38’E) and Vega
(65°40’N, 11°55’E) both in the years 2002-2006. Hestmannøy and Træna
were unmanipulated natural populations and are included in the primary
analyses. The populations of Leka and Vega underwent artificial size
selection (see Kvalnes et al., 2017; Pepke et al., 2021,submitted ) and were analyzed separately in a set of secondary
analyses as replications of the primary analyses. All four islands are
characterized by heathland, mountains, and sparse forest. The sparrows
live closely associated with humans and within the study area they are
found mainly on dairy farms (Hestmannøy, Vega and Leka), where they have
access to food and shelter all year, or in gardens and residential areas
(Træna), where they may be more exposed to weather conditions
(Araya-Ajoy et al., 2019). Natural nests inside barns or artificial nest
boxes were visited at least every 9th day during the
breeding season (May-August) to
sample fledglings (5-14 days old, with a median of 11 days). All
individuals were ringed using a unique combination of a metal ring and
three plastic color rings. Fledged juvenile sparrows and unmarked adults
were captured using mist nets during the summer and autumn
(September-October). These procedures ensured that approximately 90% of
all adult birds were marked on all islands during the study period
(Jensen, Steinsland, Ringsby, & Sæther, 2008; Kvalnes et al., 2017).
For most fledglings, we measured tarsometatarsus (tarsus) length
using digital slide calipers to
nearest 0.01 mm and body mass to nearest 0.1 g with a Pesola spring
balance (see details in Appendix S1).
For 234 nestlings, no nestling
morphological measurements were available. Following Schulte-Hostedde,
Zinner, Millar, and Hickling (2005) nestling body condition was
calculated as the residuals of a linear regression of mass on tarsus
length (both log10-transformed). To avoid collinearity
in models where both nestling age and tarsus length were included as
covariates, we age-corrected tarsus length by using the residuals
from a regression of tarsus length
on age and age squared (to account for the diminishing increase in
tarsus length with age). A blood sample (25 μL) was collected from all
individuals, which was stored in 96% ethanol at room temperature in the
field and subsequently at -20°C in the laboratory until DNA extraction.
Molecular sexing and
pedigree construction
DNA extraction is described in Appendix S1. Sex of most fledglings
(n =2641) was determined using amplification of the CHD-gene
located on the avian sex chromosomes as described in Griffiths, Double,
Orr, and Dawson (1998). 21 individuals were sexed exclusively based on
their phenotype as adults and 84 nestlings could not be sexed. We used
individual genotypes on 13 polymorphic microsatellite markers scored
using the GeneMapper 4.0 software (Applied Biosystems) to assign
parentage in CERVUS 3.0 (Kalinowski, Taper, & Marshall, 2007), as
detailed in Rønning et al. (2016). Briefly, for each nestling, CERVUS
calculates a LOD‐score (log‐likelihood ratio) for all putative parents,
which is compared to the critical values generated by the simulated
parentage analyses, resulting in a 95% parentage assignment confidence.
Nestlings within the same clutch were assumed to have the same mother.
Nestlings with missing (unassigned) parents were assigned dummy parents,
assuming that nestlings within the same clutch were full siblings and
thus had the same (unassigned) parents. The dummy parents were included
in the pedigree as founders. We calculated individual inbreeding
coefficients (F ) based on the microsatellite pedigree using the R
package ‘pedigree’ (Coster, 2012). Pedigrees were ordered using the R
package ‘MasterBayes’ (Hadfield, Richardson, & Burke, 2006) and pruned
to only contain informative individuals. The pruned pedigrees included
4118 individuals (3093 maternities and 3130 paternities) in the natural
populations, and 1057 individuals in artificially selected populations.
Maximum pedigree depth was 13 generations, the number of equivalent
complete generations (the sum of the proportion of known ancestors
across all generations, Wellmann, 2021) was 1.510, and mean pairwise
relatedness was 0.003.