Electronic supplementary material (ESM) for Charmantier, Keyser & Promislow
Study species and population
This study is based on a population of western bluebirds (Sialia mexicana) in northwestern
Oregon, breeding in nest boxes and monitored by volunteers from the Prescott Bluebird
Recovery Project. For full details on the study area, covering more than 120,000 ha, see
Keyser et al. (2004). Volunteers from the Recovery Project monitor nearly 2000 nest boxes,
with visits at least once per week from late March through mid-September, recording all basic
reproduction traits (dates of nest initiation, laying, hatching and fledging, number of eggs laid,
hatched and fledged). All nestlings and captured adults are banded with an aluminium US
Fish and Wildlife Service band for unique identification.
The western bluebird is a socially monogamous passerine and the pedigree used in our
quantitative genetics models is based on observation and capture of social pairs. Egg dumping
is very rare in this species (Dickinson & Akre 1998), but extra-pair young are common
(around 19% of extra pair young were found in a Californian western bluebird population,
Dickinson & Akre 1998). The presence of extra-pair young will create errors in the pedigree
where social fathers are assumed to be genetic fathers; however, the effect of these errors on
animal model estimations is limited (Charmantier & Réale 2005). In any case, the effect in
our study is conservative since it should result in underestimated additive genetic variances
and heritabilities.
Pairs have up to three broods per year, but more typically two broods, with the first hatching
in mid-April and the second in late June. Average clutch size is five eggs (see Keyser et al.
2004 for annual descriptive statistics on reproductive success, and Table 1 in the main text).
Maximum lifespan is 6 years, with survival probabilities of 63% and 48% for adult males and
females respectively, and 25% for juveniles (Keyser et al. 2004).
A supplemental feeding study was conducted between 1997 and 2002. Over this period, 1,083
first broods were supplemented with five mealworm larvae (Tenebrio molitor) per bird per
day, during the nestling period (~21 days). The mealworms were placed in a feeding station
within 2 m of the nest box, hence the breeding birds could either eat them or feed their
nestlings. Birds that were supplemented as nestlings have increased survival but they pay a
cost in terms of subsequent reproductive output unless they are “rescued” by further
supplemental feeding (Keyser et al. submitted). Here we have controlled for the effect of this
experiment on helping behaviour by investigating whether food supplementation at the
nestling stage (for chicks in the supplemented broods) or the adult stage (for parents of
supplemented broods) affected the studied traits.
Heritability estimated with an animal model - advantages regarding potential
environmental covariance
To partition phenotypic variance and estimate additive genetic variance and heritability for
cooperative breeding behaviour, we used the so-called ‘animal model’ of quantitative genetics
run with the software ASReml (Gilmour et al. 2002). A detailed outline of methods involved
in this restricted estimation of maximum likelihood effects (REML) and of the advantages of
the animal model can be found in Kruuk (2004). One point worth emphasising in our study is
that unlike traditional quantitative genetic analyses (e.g. parent-offspring or half-sib
regressions), the REML animal model makes use of all the relatedness information from a
multi-generational pedigree and estimates variance and covariance components across the
pedigree. The comparison of individuals that are related via a vast spectrum of routes through
the pedigree is a major improvement, limiting the confounding effects of environmental
covariance due to the common nest environment shared by sibs. This is especially useful in
our study, where the sample size did not allow us to run models with additional random
factors such as parental effects. In the pedigree of western bluebirds used for these analyses,
mothers had up to 12 offspring, and fathers up to 11 offspring. However, there were also
many inter-generational links in the pedigree, spreading over a maximum of seven
generations (Figure S1).
Quantitative genetics of fitness parameters
We estimated heritability of several fitness traits (lifespan, lifetime number of eggs, lifetime
number of hatchlings, lifetime number of fledglings, and lifetime number of recruits). As with
the analyses on helping behaviour, we first investigated the fixed effects influencing these
traits before performing pedigree-based animal models to estimate genetic variance and
heritability. However, the sample size of this analysis was small , n = 355 individuals, since
we had to restrict the initial dataset to birds that were considered dead, i.e. were recovered
dead or had not been seen in the last two years of the study period (2001 and 2002). None of
the traits displayed any significant additive genetic variance (VA) or heritability (Table S1).
This result, along with the small sample size, prevented us from exploring genetic covariances
between these traits and the propensity to receive help.
Table S1. Variance components and heritability estimated on fitness traits using animal
models on 355 birds which were dead by the end of the study period.
fixed effects
VR (s.e.)
VA (s.e.)
heritability
(s.e.)
lifespan
received help (yes/no)
0.24 (0.00)
0.00 (0.00)
0.04 (0.11)
lifetime number of eggs
-
38.71 (4.48)
0.00 (3.35)
0.00 (0.09)
lifetime number of hatchlings
-
32.51 (3.46)
0.00 (2.36)
0.00 (0.08)
lifetime number of fledglings
-
26.82 (2.89)
0.00 (2.03)
0.00 (0.08)
lifetime number of recruits
received help, year, adult
1.64 (0.18)
0.00 (0.14)
0.00 (0.08)
food supplementation
Figure S1. Distribution of the number of individuals in each generation of the pedigree of
breeding western bluebirds.
Number of individuals
800
600
400
200
0
0
1
2
3
4
5
Generation in pedigree
6
7
8
References
Charmantier, A. & Réale, D. 2005 How do misassigned paternities affect the estimation of
heritability in the wild? Molecular Ecology 14, 2839-2850.
Dickinson, J. L. & Akre, J. J. 1998 Extrapair paternity, inclusive fitness, and within-group
benefits of helping in western bluebirds. Molecular Ecology 7, 95-105.
Gilmour, A. R., Gogel, B. J., Cullis, B. R., Welham, S. J. & Thompson, R. 2002 ASReml User
Guide Release 1.0. Hemel Hempstead, HP1 1ES, UK: VSN International Ltd.
Keyser, A. J., Conroy, M. J. & Promislow, D. E. L. submitted Supplemental feeding increases
early survival but decreases subsequent fecundity.
Keyser, A. J., Keyser, M. T. & Promislow, D. E. L. 2004 Life-history variation and
demography in Western Bluebirds (Sialia mexicana) in Oregon. Auk 121, 118-133.
Kruuk, L. E. B. 2004 Estimating genetic parameters in natural populations using the 'animal
model'. Philosophical Transactions of the Royal Society of London Series BBiological Sciences 359, 873-890.