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Supplemental material and methods
Alternative methods of kinship reconstruction
Multiple methods have been used to estimate sibling relationships within a population using
molecular data [1-2]. Previous studies of dispersal in avian cooperative breeders have used the
sibling reconstruction ‘Descending Ratio’ algorithm in Kingroup v2 [3] to identify sibling
coalitions within yearly cohorts of immigrants [2]. Our attempts to apply this method to a multiyear dataset of superb starlings were unreliable. We first separated group population inputs by
year, but this inconsistently grouped individuals as siblings across years. When we used the
entire population to estimate r-values (irrespective of year), we were unable to confirm high
relatedness when we visually inspected sibships (mean = 0.38; range = 0.10 to 0.72).
Given the apparent inconsistencies of this sibling reconstruction algorithm with our
multi-year dataset, we therefore identified female sibships using the 97.5th percent confidence
interval threshold described in the main text (figure S1, table S1). Ultimately, we believe that this
approach is more appropriate for our analysis because (i) the mean full-sibling r-values were
closer to 0.5 than for the ‘Descending Ratio’ sibling reconstruction algorithm (0.52 versus 0.38),
and (ii) it uses the allele frequencies of each group over the course of 12 years to estimate the
relatedness between immigrant females, many of whom immigrated at different times. We are
confident that immigrant females with pairwise r-values close to 0.5 are sisters and not motherdaughter pairs because long-term focal monitoring suggests that birds do not immigrate after
breeding. Instead, females that complete their breeding tenure often act as (grandparent) helpers
in the group. Although we recognize that this method has the potential to misidentify sister pairs,
we believe that the 97.5th percent confidence interval threshold is more likely to have
underestimated sibling pairs in the immigrant population rather than to have inaccurately paired
siblings. In other words, this conservative method was more likely to generate false negatives
than false positives. To account for the possibility of false negatives in our data set, subsequent
analysis of kin substructure made comparisons only between females identified as part of
sibships, and not between those in sibships and those who are not. Such an approach also allows
us to account for other potentially confounding variables that could influence a female’s
likelihood to breed (e.g. habitat quality or individual differences). Thus, we believe that the
Pollack and Rubenstein: Supplemental Material
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97.5th percent confidence interval threshold method was able to accurately and conservatively
identify immigrant female sibships in our dataset.
Low frequency of chance events
Although we are unable to estimate the exact probability that a female immigrates into a social
group with a sibling by chance, the probability of this occurring randomly is likely to be small
because of a number of factors associated with our study system: the landscape at our study site
in central Kenya is saturated with social groups, birds can disperse long distances, annual
recruitment into the population is low, and few pairs fledge multiple female offspring. Superb
starlings are extremely common on this landscape, and groups can occupy most areas with mixed
grassland and trees (i.e. savanna woodland). Marked social groups are also surrounded on all
sides by other superb starling groups. Moreover, birds can disperse more than 5 km from the
groups in which they were born [4], suggesting that individuals are likely to have the opportunity
to settle in a large number of social groups each year. Although we have recovered only a few
individuals who have dispersed from their natal groups during the course of the long-term study,
the majority of those individuals have not settled in neighboring groups. Instead, they have
typically dispersed a few kilometers away. Furthermore, most breeding pairs are unlikely to
fledge large number of females siblings in successive years because predation rates are high;
between 2002-2012, 36.3% (86/237) of breeding pairs fledged offspring, with only 16.0%
(38/257) fledging more than one female across a 3-year window. Thus, given the combination of
these factors, we believe that the observed frequency of kin-biased dispersal is higher than what
is likely to occur by chance alone.
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Supplemental discussion
Parental care
In this population of superb starlings, over 90% of nests have at least one auxiliary helper who
aids in nestling provisioning [5]. Females in kin subgroups could benefit from having a sister
present due to added parental care from siblings, as has been shown in cooperatively breeding
long-tailed tits [2]. Unfortunately, because of high predation rates in this population, we lack
enough behavioral observations to directly test this hypothesis. From 2001-2013, there were only
23 recorded observations of females in sibships performing provisioning behavior at nests that
were not their own. There were only five observations of females provisioning during a breeding
season their sister had an active nest (i.e. when they had an opportunity to help a sister). In two
of these observations the female helped her sister, and in three of these observations the female
helped at a nest that was not her sister’s. However, our observations during a nesting period are
limited to brief time windows, and this does not mean she did not help during the nearly three
week nestling stage. We did observe the sister near the nest during many of the focal observation
periods, which suggests that she is likely to help provision offspring or may play an important
role in nest defense from predators. Instead, to determine whether added provisioning behavior
from siblings benefits females, we looked at whether breeding females were more likely to
fledge offspring when a sister was present versus absent (see main text and Figure S1), since
added provisioning should theoretically increase the likelihood of fledging. We found that
breeding females were not more likely to fledge offspring when a sister was present, which is
consistent with our hypothesis that female benefit from kin substructure through better access to
breeding positions rather than increased care for hatchlings.
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Supplemental references
1. Buston PM, Fauvelot C, Wong MYL, Planes S. 2009 Genetic relatedness in groups of the
humbug damselfish Dascyllus aruanus: small, similar-sized individuals may be close kin.
Molec Ecol 18, 4707–4715. (doi:10.1111/j.1365-294X.2009.04383.x)
2. Sharp SP, Simeoni M, Hatchwell BJ. 2008b Dispersal of sibling coalitions promotes helping
among immigrants in a cooperatively breeding bird. Proc R Soc Lond B 275, 2125-30.
(doi:10.1098/rspb.2008.0398)
3. Konovalov DA, Manning C, Henshaw MT. 2004 KINGROUP: a program for pedigree
relationship reconstruction and kin group assignments using genetic markers. Mol Ecol
Notes 4, 779–782. (doi:10.1111/j.1471-8286.2004.00796.x)
4. Rubenstein DR 2015. Superb starlings: cooperation and conflict in an unpredictable
environment. In Cooperative Breeding: Studies of Ecology, Evolution, and Behavior
(Koenig WD and JL Dickinson, eds). Cambridge University Press, Cambridge. In press.
5. Rubenstein, DR 2007. Temporal but not spatial environmental variation drives adaptive
offspring sex allocation in a plural cooperative breeder. Am Nat 170, 155–165.
(doi:10.1073/pnas.1100303108)
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Pollack and Rubenstein: Supplemental Material
Supplemental tables
Table S1. Group demographic and genetic structure for superb starling groups from 2002 to
2013. The mean group size (and numbers of males and females separately) and mean ± SD
pairwise relatedness values (r) for both sexes are given for each of the nine social groups.
group
male
r ± SD
number
of males
female
r ± SD
number
of females
male and female
r ± SD
group size
CF1
CF2
DRT1
DRT2
MRC1
MRCV
SRB1
SRB2
WDB
0.15 ± 0.06
0.12 ± 0.03
0.13 ± 0.08
0.13 ± 0.04
0.02 ± 0.05
0.15 ± 0.02
0.07 ± 0.08
0.13 ± 0.03
0.14 ± 0.07
12
14.36
9.36
7.82
19.82
18.55
13.27
10.45
9.18
0.06 ± 0.06
0.21 ± 0.06
0.11 ± 0.05
0.01 ± 0.07
0.01 ± 0.02
0.04 ± 0.02
0.02 ± 0.04
-0.04 ± 0.04
0.16 ± 0.06
10.27
10.64
11.36
7.82
23.73
15.18
17.45
14.09
9.09
0.11 ± 0.05
0.13 ± 0.03
0.12 ± 0.05
0.08 ± 0.04
0.01 ± 0.03
0.09 ± 0.01
0.05 ± 0.05
0.03 ± 0.02
0.14 ± 0.05
22.18
25
20.73
15.64
43.55
33.73
28.82
24.55
18.27
mean ± SD
0.12 ± 0.04
12.76 ± 4.19
0.07 ± 0.08
13.29 ± 4.97
0.08 ± 0.05
25.83 ± 8.57
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Pollack and Rubenstein: Supplemental Material
Table S2. Percentiles for the 97.5th percentile threshold method of determining kin substructure,
and relatedness values (mean, SD, min, and max) of those sibships identified in each social
group. We determined the presence of sibships among female immigrants by determining those
pairs whose r-values were above the 97.5th percentile of all immigrant female r-values within
their respective social groups (see figure S1).
group
CF1
CF2
DRT1
DRT2
MRC1
MRCV
SRB1
SRB2
WDB
mean
immigrant r values
2.5th percentile
97.5th percentile
-0.48
0.36
-0.42
0.38
-0.40
0.39
-0.31
0.37
-0.41
0.37
-0.41
0.38
-0.38
0.41
-0.45
0.39
-0.30
0.38
-0.40
0.38
mean r
0.58
0.41
0.47
0.53
0.48
0.54
0.62
0.52
0.51
0.52
sibship pairwise r
SD r
min r
0.11
0.48
0.00
0.41
0.08
0.42
0.00
0.53
0.06
0.40
0.13
0.45
0.07
0.52
0.00
0.52
0.00
0.51
0.05
0.47
max r
0.69
0.41
0.52
0.53
0.59
0.72
0.67
0.52
0.51
0.57
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Supplemental figures
Figure S1. Frequency distributions of pairwise r-values of immigrant females from each of the
nine social groups in the study population.
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Figure S2. Breeding success of immigrant females in sibships when their sister was present and
when their sister was absent from the group. Females were categorzied as those that bred
successfully if they fleged at least one chick (black bars), or as those that bred unsuccessfully if
they fledged no chicks (white bars).