Ecological Applications, 16(5), 2006, pp. 1696–1705
Ó 2006 by the Ecological Society of America

CHERNOBYL AS A POPULATION SINK FOR BARN SWALLOWS:
TRACKING DISPERSAL USING STABLE-ISOTOPE PROFILES
A. P. MøLLER,1,5 K. A. HOBSON,2 T. A. MOUSSEAU,3 AND A. M. PEKLO4
1

Laboratoire de Parasitologie Evolutive, CNRS UMR 7103, Universite´ Pierre et Marie Curie, Bât. A, 7e`me e´tage,
7 quai St. Bernard, Case 237, 75252 Paris Cedex 05, France
2
Canadian Wildlife Service, 115 Perimeter Road, Saskatoon, SK S7N 0X4, Canada;
3
Department of Biological Sciences, University of South Carolina, Columbia, South Carolina 29208 USA
4
Zoological Museum, National Museum of the Natural History, National Academy of Sciences,
15 Bohdan Chmelnitsky Street, 01030 Kyiv-30, Ukraine

Abstract. Stable-isotope proﬁles of feathers can reveal the location or habitat used byindividual birds during the molting period. Heterogeneity in isotope proﬁles will reﬂect
heterogeneity in molt locations, but also heterogeneity in breeding locations, because spatial
heterogeneity in molt locations will be congruent with spatial heterogeneity in breeding
locations in species with high connectivity between breeding and molting sites. We used
information on the congruence of spatial heterogeneity in molt and breeding location to study
population processes in Barn Swallows (Hirundo rustica) from a region near Chernobyl,
Ukraine, that has been radioactively contaminated since 1986; from an uncontaminatedcontrol region near Kanev, Ukraine; and from a sample of pre-1986 museum specimens used
to investigate patterns prior to the nuclear disaster at Chernobyl, from both regions. Previous
studies have revealed severe reductions in Barn Swallow reproductive performance and adultsurvival in the Chernobyl region, implying that the population is a sink and unable to sustain
itself. Female Barn Swallows are known to disperse farther from their natal site than males,
implying that female stable-isotope proﬁles should tend to be more variable than proﬁles ofmales. However, if the Barn Swallows breeding at Chernobyl are not self-sustaining, we would
expect males there also to originate from a larger area than males from the control region. We
found evidence that the sample of adult Barn Swallows from the Chernobyl region was moreisotopically heterogeneous than the control sample, as evidenced from a signiﬁcantcorrelation between feather d13C and d15N values in the control region, but not in the
Chernobyl region. Furthermore, we found a signiﬁcant difference in feather d15N values
between regions and periods (before and after 1986). When we compared the variances ind13C values of feathers, we found that variances in both sexes from post-1986 samples from
Chernobyl were signiﬁcantly larger than variances for feather samples from the control
region, and than variances for historical samples from both regions. These ﬁndings suggestthat stable-isotope measurements can provide information about population processes
following environmental perturbations.
Key words: Barn Swallows; Chernobyl, Ukraine; correlation of stable-isotope proﬁles; Hirundo rustica;
sinks; sources; variance in stable-isotope proﬁles.

INTRODUCTION
The mean and variance of the reproductive output of
populations of free-living organisms vary considerably.
Such variance is of considerable importance because
populations that produce a larger and less variable
number of recruits to future generations are consideredless prone to extinction (Lande et al. 2003, Roff 2001).
Two variance components are important contributors tosuch variability. Environmental variance similarly affects all individuals in a population due to variation incommon environmental conditions, whereas demographic variance arises from individual variation in
Manuscript received 8 December 4004; revised 21 November
2005; accepted 14 December 2005; ﬁnal version received 6
February 2006. Corresponding Editor: F. C. James.
5 E-mail: amoller@snv.jussieu.fr

realization of survival and reproduction. Whether apopulation constitutes a net contributor to the maintenance of a globally viable population will depend on
both environmental and demographic variances of eachof the composite subpopulations. Pulliam (1988) originally suggested that populations with positive population growth, r, are source populations, while
populations with negative r are sinks. Immigration
may allow individuals to immigrate into sites where
local recruitment would be insufﬁcient for persistence ofa population (Pulliam 1996). Source populations were
subsequently deﬁned as populations that have a positive
intrinsic growth rate, while sink populations weredeﬁned as being unable to sustain not only their own
populations, but also neighboring populations (Dias
1996, Hanski 1999). Successful identiﬁcation of sourceand sink populations is therefore of considerable

1696

October 2006

VARIATION IN ISOTOPES IN BARN SWALLOWS

importance for conservation purposes, but also for
ecological research. Many studies have found evidence
consistent with predictions for source–sink metapopulations using a number of different ecological andgenetic approaches (Dias 1996, Paradis 1995, Pulliam
1996, Pulliam and Danielson 1991, Stacey et al. 1997).
Here we propose a novel approach to the identiﬁcationand study of source and sink populations. This method
is based on heterogeneity in stable-isotope proﬁlesacross individuals in the different parts of a metapopulation caused by different degrees of admixture in sourceand sink populations (Hobson 2005).
Here we argue that source and sink populations undercertain assumptions can be identiﬁed using heterogeneity in stable-isotope proﬁles. The underlying assumptions are as follows: First, stable-isotope abundance
recorded in the feathers or other tissues reﬂect theisotope contents of food, revealing information about
the habitats and the geographical locations where thetissue was developed (reviews in Hobson 1999, 2004,
Rubenstein and Hobson 2004). In the case of metabolically inactive tissues such as scales, feathers, and hair,isotope proﬁles provide a frozen snapshot of theenvironmental conditions experienced during development of the character. Individuals with speciﬁc characteristics will subsequently carry this signature until thenext molt. As the feathers of birds wear out, they are
replaced in a process of molt, normally at least once a
year; in the Barn Swallow once a year. Second, the ratioof stable isotopes in the environment varies spatially,
showing increasing heterogeneity with increasing area
(e.g., Still et al. 2003, Urton and Hobson 2005). For
example, previous studies of Palaearctic breeding birds
that typically molt ﬂight feathers on their wintering
grounds have indicated that mixtures of wintering
populations can be discerned based on variation infeather isotope proﬁles among individuals on thebreeding grounds (e.g., Chamberlain et al. 2001, Møller
and Hobson 2004). Third, population sinks are deﬁned
as requiring immigration of individuals to sustain
populations, while sources do not (Pulliam 1988). Thisimplies that individuals in population sinks have comefrom a larger area than those from population sources.
Fourth, individuals from a single population will
generally be found in the same region while the tissue
is being grown (Berthold 2001), so that a population ofindividuals that have come from a larger area will tend,by necessity, to have a greater variance in isotope ratio.
Fifth, spatial heterogeneity in a sample of individuals
from the breeding grounds of birds also reﬂects spatial
heterogeneity from the wintering grounds where thetissue has been grown. Sixth, sex differences in heterogeneity in samples from the breeding grounds are
expected to reﬂect spatial heterogeneity from thewintering grounds because female Barn Swallows
generally disperse considerably further than males.
Seventh, in species with migration divides, where one
population takes one migratory route and a second

1697

takes another migratory route (Berthold 2001), single
breeding populations can consist of an admixture ofbirds from different wintering populations. R. Ambrosini, A. P. Møller, and N. Saino (unpublished manuscript)
have analyzed spatial distribution patterns of over 1000
recoveries of Barn Swallows (Hirundo rustica) from theAfrican winter quarters of birds breeding in the Western
Palearctic. These analyses, based on Mantel tests,
revealed a high degree of congruence in spatial patterns
in the winter quarters and the breeding grounds. Thus,
breeding birds from Western Europe winter in Western
Africa, those from Eastern Europe winter in East Africa,
those from Northern Europe winter in Southern Africa,
and those from Southern Europe winter in Northern
Africa. In other words, at large spatial scales, aheterogeneous sample of birds from the winter quarters,
when considering the isotope composition of feathers
caused by incorporation of stable isotopes during theannual molt in winter, also signiﬁes a heterogeneous
sample of birds in terms of previous breeding-groundorigins. Barn Swallows breeding in Europe could be
divided into western and eastern populations thatdiffered in migration routes and wintering areas (R.
Ambrosini, A. P. Møller, and N. Saino, unpublishedmanuscript), with birds from Finland, parts of Sweden,
and eastern Europe migrating to eastern Africa along aneastern ﬂyway, while birds from Norway, Denmark, andwestern Europe use a western ﬂyway. Breeding populations in Denmark consist of an admixture of the two
populations (Møller and Hobson 2004). Thus, ourassumption that heterogeneity in stable-isotope proﬁlescaused by heterogeneity in winter grounds also reﬂects
heterogeneity in terms of breeding origin is supported byempirical evidence. These seven assumptions lead to thecritical test that if we compare variances of isotope
ratios of two or more samples, all things being equal, thesample with the greater variance is a sink to a greater
extent than the one with the smaller variance.
However, there have, to the best of our knowledge,
been no attempts to use stable-isotope techniques toinvestigate such admixture of populations to study
population processes, including identiﬁcation of sources
and sinks. Several studies used stable isotopes to
examine population demographics and population
processes (Hershey et al. 1993, Marra et al. 1998, Kelly
et al. 2002, Rubenstein et al. 2002, Sillett and Holmes
2002). Using isotopic variability in feathers amongindividuals to infer diversity of population origins will
necessarily depend on the degree of variability in stable
isotopes across potential contributing populations(Hobson et al. 2004, Hobson 2005). In general, we
should expect breeding populations that constitute sinks
to reveal greater variance in feather isotope proﬁles than
populations that constitute source populations, asargued in the previous paragraphs. Males often differ
from females in dispersal distance from site of birth tosite of breeding, with female birds generally dispersing
much further than males, while mammals commonly

1698

Ecological Applications
Vol. 16, No. 5

A. P. MØLLER ET AL.

show the opposite pattern (e.g., Greenwood 1980,
Clobert et al. 2001). Such sex differences, if wellestablished in particular species, provide a unique
opportunity to use one sex as a natural control groupand the other sex as the ‘‘experimental’’ group whenstudying sources and sinks. Although it is often difﬁcult
to measure the range of isotopic variation in diet used bybirds on their wintering grounds, where they moltfeathers, in source–sink situations, we would expect
individuals of the more-dispersing sex to show anincrease in the variance in isotope proﬁle due toadmixture of individuals of different origins (potentiallyover hundreds or thousands of kilometers). In contrast,
the more philopatric sex should show less variation. Thisprediction has a strong inference because individuals of
the two sexes would be expected to otherwise be similar
in many respects. Furthermore, if there are temporalcontrols in terms of tissue samples from before and after
the creation of a sink population, strong inferences
could be predicted with respect to temporal changes invariances in isotope proﬁles. Recently, Hobson et al.
(2004) demonstrated that individual isotopic outliers
among male Ovenbirds (Seirus aurocapillus) and American Redstarts (Setophaga ruticilla) in breeding populations in North America represented birds that had
dispersed from other (unknown) areas. These authors
suggested that isotopic variance within breeding populations could be used to establish minimum estimatesof recruitment into populations of interest, provided
that sufﬁcient isotopic heterogeneity occurs among
possible molting areas of source populations. We
adopted a similar approach to examine the structure ofBarn Swallows from two different regions in Ukrainethat differ in sustainability.
The aims of this study were to investigate whether
stable-isotope proﬁles could be used to identify apopulation sink in a wild population of Barn Swallows
from Ukraine (see Møller and Mousseau [2001, 2003] fora description of this system). We can make three
predictions. First, positive correlation between stable
isotopes should break down when population admixture
increases. For several elements, stable-isotope relative
abundance in the food shows gradual clinal variation oncontinental scales (e.g., Still et al. 2003, Bowen et al.
2005). So, we should expect a sample of individuals froma population to be normally distributed in space because
individuals from a given population winter and molt in a
speciﬁc location, with both genetic and environmental
factors contributing to heterogeneity in this spatial
distribution. Such a normally distributed spatial distribution of individuals should cause stable-isotope ratios
for C and N to be positively correlated. Such positive
correlation has previously been found within samples(e.g., Møller and Hobson 2004), but has also been
demonstrated for C and N stable-isotope ratios withinfood webs (Kelly 2000). However, if a sample from a
speciﬁc region shows signiﬁcant admixture of individuals from different breeding and, hence, wintering

populations, positive covariance between stable-isotope
ratios for C and N should no longer exist. The reason
for this claim is that sampling sites should no longer
show a normal distribution along the clinal gradients ofthe different stable isotopes in the environment. Second,males and females should differ in mean and variance ofstable-isotope proﬁles due to sex differences in natal
dispersal, again assuming that spatial heterogeneity inbreeding sites is reﬂected in spatial heterogeneity inmolting sites. Because female Barn Swallows dispersefrom their natal site to their future breeding site morethan twice as far as males, with measured maximum
dispersal distances reaching 700 km (Møller 1994, Glutz
von Blotzheim and Bauer 1994), we would expect thatfemales from the two areas in Ukraine would be moredifferent in means and variances of isotope proﬁles thanmales. This prediction is based on the fact that females
originate from a larger area than males, and that thevariance in distances between the origins of females isgreater than the variance in distances between theorigins of males. Since long-distance dispersal is much
more common in female Barn Swallows than in males(Møller 1994), we should expect the variance in stableisotope proﬁle to be greater in females than in males.
Third, the variance in stable-isotope proﬁle should begreater in population sinks than in population sources.
Capture–mark–recapture analyses of Barn Swallows
from the Chernobyl region has shown that adult survival
is 60% lower than from the Kanev region, and estimatesof reproductive success have shown that annual
fecundity is 20% lower than from the Kanev region(Møller et al. 2005). Such large differences would suggestthat Barn Swallows in the Chernobyl region aresustained by immigration from elsewhere. Such immigration should be particularly obvious in males, because
males from the control region near Kanev were expected
to be relatively homogeneous in isotope composition
mainly due to local recruitment, while the males in the
Chernobyl region should have a larger proportion of
immigrants. We predicted that the patterns of stable
isotopes in Kanev would be the same before 1986 and in
2000, but that the patterns would differ between these
periods at Chernobyl. Explicitly, we would predict anincrease in the stable isotopic variance at Chernobyl,
particularly amongst males.
MATERIALS AND METHODS
Study species
The Barn Swallow is a small (;20 g) passerine that
feeds on insects, mainly Diptera and Hymenoptera
caught on the wing. Barn Swallows arrive in Europe
from the African winter quarters in April–June, andleave in August–October (Møller 1994). Males andfemales build nests inside buildings, where the female
lays a clutch, usually 4–5 eggs. More than 50% lay asecond clutch. Barn Swallows breed solitarily or incolonies that can exceed 120 pairs. Long-tailed males
enjoy a mating advantage, as demonstrated by observa-

October 2006

VARIATION IN ISOTOPES IN BARN SWALLOWS

1699

tions and experiments (Møller 1994). Male Barn
Swallows in Chernobyl have dramatically increased
asymmetry in their elongated outermost tail feathers
and much paler throat coloration compared to males
from a control region near Kanev and to males
predating the Chernobyl accident (Møller 1993). Longtailed males in particular are very pale compared tocontrols (Camplani et al. 1999). Genetic studies havereported increased mutation rates in Barn Swallows
from the radioactively contaminated region around
Chernobyl in Ukraine (Ellegren et al. 1997). Furthermore, the frequency of partial albinism is elevatedcompared to other populations, and such phenotypicdeviants are associated with reduced ﬁtness of Barn
Swallows (Møller and Mousseau 2001). Of the phenotypic traits measured, those that differ the most between
Chernobyl and an uncontaminated control region near
Kanev, are also those that are most strongly associatedwith male mating success (Møller and Mousseau 2003).
Barn Swallows from Ukraine winter in Southern Africa(Fig. 1), where they undergo a complete molt. Barn
Swallows from Chernobyl and Kanev winter in the same
areas (Fig. 1). There is no evidence suggesting that males
and females winter in different areas (R. Ambrosini, A.
P. Møller, and N. Saino, unpublished manuscript).
Study areas
We studied Barn Swallows in a region around
Chernobyl, Ukraine (Shestopalov 1996), by visiting
villages and checking collective farms for the presenceof Barn Swallows (Fig. 2). Once such a farm had been
located, we recorded radiation levels and captured adult
Barn Swallows for measurements and subsequentbanding. Capture was made with mist nets, providing
a random sample of adults from the breeding farms. Our
own ﬁeld measurements of radiation at the ground level
using a hand-held dosimeter (Inspector, SE International, Summertown, Tennessee, USA) revealed levels ofradiation of 0.390 6 0.317 mR/h (milliroentgens perhour; mean 6 SE) at 14 farms in the Chernobyl region.
As a control region, we used Kanev (;150 km southwestof Kiev), which has a relatively low level of contamination (Fig. 2). Mean levels of radiation were 0.025 6
0.002 mR/h at ﬁve farms in this control region. We used
the Chernobyl region for our study because radioactive
contamination arising from the explosion of a nuclear
reactor on 26 April 1986 released vast amounts ofradioactive material into the environment. Most of theradioactive isotopes remain in the environment due totheir long half-life, with serious detrimental effects on
the reproductive success and adult survival of Barn
Swallows (Møller et al. 2005). We studied Barn
Swallows in the two study regions 6–12 June 2000 (A.
P. Møller, T. A. Mousseau). The maximum distance
between farms in the Chernobyl region was ,20 km,
and in the Kanev region it was ,8 km. These distances
are small compared to natal dispersal distances, and we
considered it unlikely that this spatial scale would have

FIG. 1. The map shows connections between the bandingsites of Barn Swallows breeding in Ukraine and winter locationsin Africa.

any effect on variance is stable-isotope proﬁles. The
wintering grounds of Ukrainian-breeding Barn Swallows, where the annual molt takes place, is shown in Fig.
1, based on recoveries of banded Barn Swallows from
Ukraine (during the breeding season May–August) andSouth Africa (during winter December–February).
Information on recoveries of banded birds was kindly
provided by the Ukrainian Ringing Center, EURING(European Union for bird Ringing), and SAFRING(South African Bird Ringing Unit).
As a second control sample from the Chernobyl
region and the region around Kanev, we used samples offeathers from the Zoological Museum, Kiev, Ukraine.
We only used adult birds collected during the breeding
season (June–July) to avoid inclusion of specimens thatmight be migrants from more northern populations.
Specimens attributed to the Chernobyl region derived
from the same locations that are now within thecontaminated areas (Fig. 2), while specimens from asimilar area southeast of Kiev surrounding Kanev wereconsidered to be controls. While long-term storage may
affect samples from museum specimens, any changes

1700

Ecological Applications
Vol. 16, No. 5

A. P. MØLLER ET AL.

FIG. 2. Location of villages with study farms in 2000 and levels of radiation around Chernobyl and Kanev, Ukraine. The
shaded region around Chernobyl demarks the area where .15 curies of 137Cs per km2 were deposited in 1986 (CIA 1986). The
radiation levels in the contaminated areas were 0.390 6 0.317 mR/h (mean 6 SE) at 14 breeding farms in the Chernobyl region. In
the control region near Kanev (;150 km southeast of Chernobyl) radiation levels were, on average, an order of magnitude lower(0.025 6 0.002 mR/h). Approximate locations of villages with study farms are indicated either as black diamonds (in contaminatedareas) or stars (relatively uncontaminated control areas).

should be uniform across samples and so should not
inﬂuence our results in a heterogeneous way. The sample
of museum specimens was collected by shotgun, whereas
the recent samples of live birds were obtained by using
mist nets. However, because the museum samples werecollected with shotgun from breeding populations, wefeel justiﬁed to assume that they also represent a random
sample. Sample sizes for the two sexes, regions, andperiods are reported in Table 1.
Collecting feathers
Upon capture, we collected the two outermost tailfeathers from each adult. The feathers for each bird werestored at room temperature in a plastic bag in complete
darkness until measurements were made. The tailfeathers of adult Barn Swallows are molted in the
African winter quarters (Cramp 1988; A. P. Møller,
unpublished data from South Africa, Namibia, andGhana), and so isotopic variability in feathers represents
variability in the isotopic proﬁle of the prey being
related to the food web from which they come. The
feathers from the museum specimens were the same as
those for the ﬁeld specimens.

Isotope analyses
Feathers were analyzed blindly with respect toinformation on origin and sex. Feathers were ﬁrst
cleaned of surface oils by rinsing several times in a 2:1
chloroform : methanol solution followed by air drying ina fume hood for several days. Feather vanes were thensubsampled and 1 mg was weighed into small tin cups.
These samples were then combusted in a Robo Prep
elemental analyzer interfaced with a 20:20 isotope-ratio
mass spectrometer (Europa Scientiﬁc, Manchester, UK).
Resultant CO2 and N2 gases were measured for theirstable-isotope ratios in d notation relative to Pee Dee
Belemnite (PDB) and atmospheric AIR standards,
respectively, according to the formula presented in
Hobson (1995). Based on thousands of measurements
of internal laboratory standards (egg albumen andbaleen keratin), we estimated measurement error to be
0.1ø and 0.3ø for d13C and d15N measurements,
respectively.
Statistical analyses
We tested for correlation between feather d13C andd N values using the different samples. Heterogeneity15

October 2006

VARIATION IN ISOTOPES IN BARN SWALLOWS1701

TABLE 1. Results from tests for unequal variances with Barlett’s test and for means with Welch’s
ANOVA for d13C and d15N from Barn Swallow feathers in relation to sex, region (Chernobyl
and Kanev, Ukraine), and period (pre-1986, post-1986).
Female

Male

N

SD

Mean absolutedifference tothe meanC
Chernobyl
Pre-1986
Post-1986

6
39

2.79
8.62

2.28
6.36

3
53

1.03
7.34

0.70
4.52

Kanev
Pre-1986
Post-1986

8
29

1.73
2.29

1.44
1.80

21
25

2.37
2.47

1.68
1.83

d 15N
Chernobyl
Pre-1986
Post-1986

6
39

0.94
1.18

0.72
0.95

3
53

0.61
1.43

0.42
1.15

Kanev
Pre-1986
Post-1986

8
29

1.10
1.30

0.75
1.13

21
25

1.04
1.06

0.84
0.78

Sample, site,and period
d

N

SD

Mean absolutedifference tothe mean13

Note: For d 13C, Bartlett’s test, F ¼ 15.07, df ¼ 7, P , 0.0001; Welch’s ANOVA, F ¼ 2.09, df ¼ 7,
26.5, P ¼ 0.08. For d 13N, Bartlett’s test, F ¼ 0.96, df ¼ 7, P ¼ 0.84; Welch’s ANOVA, F ¼ 4.18, df ¼
7, 24.1, P ¼ 0.004.

in Pearson correlation coefﬁcients was tested using theprocedure described in Sokal and Rohlf (1995). We did
not transform data since they did not differ signiﬁcantly
from normal distributions according to the ShapiroWilk statistic. We tested for sex, region, and time effects
on isotope proﬁles with Welch’s ANOVA, which does
not require equality of variances. Means for groups werecompared using Fisher’s protected least mean-squares
test, adjusting means for unequal variances by weighting
by the reciprocal of the sample variances of the groupmeans. Variances in d13C and d15N values werecompared among samples for different periods using
Bartlett’s test for homogeneity of variances (Sokal andRohlf 1995). All statistical tests were made with JMP(SAS Institute 2000).
We used sequential Bonferroni correction to assess thetablewide Type I error rate by adjusting signiﬁcancelevel to the number of tests (Holm 1979, Wright 1992).
Strict application of this method severely reduces thepower of tests (Wright 1992), but such sacriﬁcial loss of
power can be avoided by choosing an experiment-wiseerror rate higher than the accepted 5%. We used 10% asrecommended by Wright (1992) and Chandler (1995).
RESULTS
Frequency distribution of stable-isotope values in feathers
and correlation among stable isotopes
There were clearly unimodal distributions of d13C andd N feather values with distributions not differingsigniﬁcantly from normality. When split among sex,
region, and period categories, none of the distributions
differed signiﬁcantly from a normal distribution after
15

sequential Bonferroni correction for eight statisticaltests.
Correlations between d13C and d15N values for Barn
Swallows from the recent ﬁeld samples differed between
regions. There was a nonsigniﬁcant relationship in
Chernobyl (F ¼ 0.16, df ¼ 1, 90, r2 ¼ 0.002, P ¼ 0.69;
Fig. 3a), but a signiﬁcant positive relationship in Kanev(F ¼ 17.48, df ¼ 1, 52; r2 ¼ 0.25, P , 0.0001, slope 6 SE ¼
0.28 6 0.07; Fig. 3b).
The correlations for the recent ﬁeld samples were as
follows: males from Chernobyl (r ¼ 0.009, t ¼ 0.07, df ¼
51, P ¼ 0.95), females from Chernobyl (r ¼ 0.15, t ¼
0.92, df ¼ 37, P ¼ 0.37); males from Kanev (r ¼ 0.35, t ¼1.81, d.f. ¼ 23, P ¼ 0.08), and females from Kanev (r ¼
0.654, t ¼ 4.49, df ¼ 27, P ¼ 0.0001). The two correlation
coefﬁcients between d13C and d15N values for females
were signiﬁcantly different from each other (t ¼ 3.41, P, 0.001), while the two coefﬁcients for males were notsigniﬁcantly different (t ¼ 0.87, P ¼ 0.39).
Differences in means and variances in stable-isotope
proﬁles between regions
Summary statistics for d13C and d15N values for thevarious sex, age, and region categories are reported inTable 1. Bartlett’s test for unequal variances revealed asigniﬁcant difference for d13C (F ¼ 15.07, df ¼ 7, P ,0.0001). However, a test for equal means for d13C did
not reach signiﬁcance (Welch’s ANOVA, F ¼ 2.09, df ¼
7, 26.48, P ¼ 0.081).
In contrast to the results for d13C, there was no
signiﬁcant difference in variance for d15N (Table 1). A
posteriori tests revealed a signiﬁcant difference in means
between regions, with a higher mean value in Chernobyl

1702

Ecological Applications
Vol. 16, No. 5

A. P. MØLLER ET AL.

both regions (Table 1), there were signiﬁcantly larger
variances in the recent samples from Chernobyl following sequential Bonferroni correction (variance ratio test,
males: F . 9.52, P , 0.01; females: F . 50.72, P ,0.001). None of the sex differences were signiﬁcant.
DISCUSSION

FIG. 3. Values of d13C in relation to d15N in feathers fromadult male and female Barn Swallows trapped in 2000 from (a)Chernobyl and (b) Kanev.

than in Kanev (Fisher’s protected least squares difference test weighted by the sample variances of the groupmeans, P , 0.001). Likewise there was a signiﬁcantdifference in means between periods (Fisher’s protected
least squares difference test, P , 0.001), with lower
values in the museum samples as compared to the recent
samples. The difference between Chernobyl and Kanev
differed between periods, with the difference being small
in the museum samples, but considerable in the recent
ﬁeld samples (Fisher’s protected least squares difference
test, P , 0.001). In contrast, there was no signiﬁcant sex
difference (Fisher’s protected least squares difference
test, P ¼ 0.39).
When we compared isotopic variances for the two
isotopes for each sex between post-1986 samples from
Chernobyl and from Kanev, and between post-1986
samples from Chernobyl and pre-1986 samples from

The main ﬁndings of this study were that (1)correlations between the two isotopes differed signiﬁcantly between regions for females, but not for males; (2)mean values for d15N differed between regions andperiods; and (3) variances in stable-isotope proﬁles for
d13C differed between periods for both sexes. Theseﬁndings were consistent with our initial predictions for a
population sink when individuals of the two sexes differ
in natal dispersal distances, and this difference indispersal depended on the recent environmental disasterin Chernobyl. We will brieﬂy discuss each of theseﬁndings, and their limitations and implications.
Source–sink patterns of natural populations are amajor ﬁnding in population biology, because they haveimportant implications for theoretical and applied
ecology (Pulliam 1988, Dias 1996, Hanski 1999). It is
not straightforward identifying source and sink populations, and a number of different techniques have been
proposed to achieve correct identiﬁcation (Paradis 1995,
Dias 1996, Pulliam 1996, Pulliam and Danielson 1991,
Stacey et al. 1997). Here we proposed that stable-isotope
proﬁles of feathers from migratory birds may provide
information about heterogeneity in breeding populations, and such heterogeneity will reﬂect the net
emigration and immigration rates of different populations. We found evidence consistent with this scenario
based on (1) patterns of correlations between different
isotopes, (2) temporal and spatial patterns of variationin isotope proﬁles, and (3) differences in variances inisotope proﬁles.
We found evidence of weak positive correlations
between the two isotopes in three of four sex and region
categories, and one of these correlations reachedstatistical signiﬁcance. There were statistically signiﬁcantdifferences in the strength of these correlations forfemales, but not for males, implying that the patterns ofcorrelation between feather d13C and d15N values
differed between Chernobyl and Kanev for females.
The signiﬁcantly stronger correlation among females
from Kanev as compared to Chernobyl is consistent
with the prediction that females from Kanev come froma smaller recruitment area than the females from
Chernobyl, while there is no evidence of males fromthe two regions differing in the relationship between
d13C and d15N. How does emigration and immigration
and, hence, differences in migration and wintering areas
among individuals, affect such correlations? If a sample
of individuals with a greater dispersal distance molt theirfeathers in a larger wintering area (as observed in
Palearctic Barn Swallow populations; R. Ambrosini, A.
P. Møller, and N. Saino, unpublished manuscript), then

October 2006

VARIATION IN ISOTOPES IN BARN SWALLOWS

stable-isotope variance will also be greater in thatsample. If these birds then immigrate into a breeding
population, we would predict that the isotopic varianceof the breeding population would increase. Previous
isotopic studies led to the expectation that d13C andd15N values within food webs are positively correlated(Kelly 2000). A previous study of adult Barn Swallows
breeding in Northern Denmark did not reveal signiﬁcant
positive correlations between feather d13C and d15N
values (males: r ¼ 0.10, t ¼ 0.82, df ¼ 71, P ¼ 0.42;
females: r ¼0.05, t ¼ 0.52, df ¼ 94, P ¼ 0.60; Møller andHobson 2004). However, this absence of a positive
correlation depended on this Danish sample of breeding
Barn Swallows having a highly heterogeneous isotope
proﬁle, presumably because at least two and likely threedifferent populations with different winter quarters or
winter habitats bred in the same area. When wereanalyzed this data set by excluding the small numberof individuals with extreme isotope proﬁles, whichcaused bimodality in the distributions, we found asigniﬁcant positive correlation for both sexes (males, r ¼
0.36, t ¼ 3.15, df ¼ 66, P ¼ 0.003; females, r ¼ 0.22, t ¼
2.08, df ¼ 82, P ¼ 0.04), as in the samples from Kanev in
Ukraine. Therefore, weak positive correlations within‘‘isotopic populations’’ seem to be common, with the
exception of females from Chernobyl, which clearly
differed from females from Kanev and from thehomogeneous sample from Denmark. When extreme
isotope proﬁles were included in the samples from
Denmark, correlations also became weak and nonsigniﬁcant. This suggests that, when birds originatingfrom other populations become admixed in a homogeneous sample of individuals, the positive correlation
between d13C and d15N disappears. Such decoupling of
the two isotopes has similarly been evoked as evidence
for different ultimate sources of nutrition to omnivores(Hobson et al. 2000), but for an aerial insectivore likethe Barn Swallow, such a mechanism is not expected.
We found differences in feather d15N values between
regions and periods. Comparison of means from thedifferent regions, periods, and sexes revealed a difference
between Chernobyl and Kanev, between pre- and post1986 samples, and a larger difference between Chernobyl and Kanev in post-1986 than in pre-1986 samples(Table 1). These ﬁndings are consistent with theprediction of a larger recruitment area for Barn
Swallows breeding in the population sink in Chernobyl.
They are also consistent with the expectation that nosuch effect should be visible prior to the nuclear disasterin 1986. That we did not see similar differences in mean
d13C values is not necessarily surprising, because
isotopic distributions across landscapes can be driven
by different processes. For example, in addition toclimatic factors, food web d15N values appear to beinﬂuenced more than d13C values by land-use practices(Rubenstein and Hobson 2004). Such difference between
food web d13C and d15N values underlines the advantage
of using more than one isotope in resolving structure1703

within populations (e.g., Pain et al. 2004, Yohannes etal. 2005).
Variances in feather d13C values differed between
regions, periods, and sexes. Both males and, inparticular, females differed signiﬁcantly between Chernobyl and Kanev in the post-1986 samples. Likewise,
variances were larger in post-1986 samples from
Chernobyl compared to pre-1986 samples from either
region. Some of our samples of museum specimens were
small, despite the Barn Swallows being one of the mostcommon breeding bird species in Ukraine. Obviously,
results based on such small samples should be interpreted with caution. Long-term studies of isotope
proﬁles in swallow food webs have not been conductedat potential wintering sites in Africa. Such changes may
be signiﬁcant in areas of variable climate (Koch et al.
1995). In other areas, ﬁsh have shown constant carbon
isotope contents over decades (Begg and Weidman 2001,
Jamieson et al. 2004), and long-term stability in stableisotope ratios has been demonstrated in tree-ring dataspanning hundreds of years (see data and references in
McCarroll and Loader 2004), showing that temporalchange in isotope composition of food webs is not
ubiquitous. There were no differences in variances for
d15N values, which did show weak sex- and periodspeciﬁc variation between the two regions, but nointeraction effects. The ﬁndings for d13C values are
consistent with our a priori predictions about differencesin variances in isotope contents related to source–sinkdynamics of the Barn Swallows in Ukraine. Variances
may differ among samples for reasons other than ahigher degree of admixture of individuals from different
populations. For example, Pain et al. (2004) suggested
that differences in variances in feather d13C values
among populations of migratory birds wintering in
Africa could be due to the degree of isotopic heterogeneity within molting areas, although they did notprovide any empirical evidence to support this suggestion. More recently, E. Yohannes and K. A. Hobson(unpublished manuscript) determined low isotopic (d13C,
d15N, dD) variance among years for nine species of
Palearctic migrants molting at sites in sub-SaharanAfrica. We ﬁnd the explanation of isotopic heterogeneity
proposed by Pain et al. (2004) unlikely for the Barn
Swallow, given that such heterogeneity within different
molting areas should match the spatial and temporal
patterns of variation recorded. Without any evidence forthis more complex explanation, application of Occam’s
razor would suggest that the more simple explanation
based on the Chernobyl nuclear disaster is more likely tobe the cause of heterogeneity.
Our studies of museum and ﬁeld samples of isotopes
from adult Barn Swallows from Chernobyl and acontrol region near Kanev in the Ukraine showed
evidence consistent with the Barn Swallow sites around
Chernobyl having become a sink following the disaster.
What are the requirements for isotopes providing
information that would allow identiﬁcation of popula-

1704

A. P. MØLLER ET AL.

tion sinks without providing false positives? Clearly, therelative abundance of stable isotopes must vary at ascale that allows identiﬁcation of heterogeneous samples. If different populations of breeding birds were fully
admixed during the period when feathers are molted andhad identical foraging ecology, there would be little orno heterogeneity in isotope proﬁles among samples.
Hence, it seems important that stable isotopes or othermarkers should show variation at a geographical scale
similar to that at which population processes (such as
local recruitment, emigration, and immigration) that
affect local population size are acting (Hobson 2005).
This is not trivial, since we found evidence consistent
with source–sink dynamics for variances in Barn
Swallow d13C, but not for variances in d15N values.
We suggest that this difference between d13C and d15N
values may arise from differences in spatial distributionof stable-isotope ratios in the environment wherefeathers are grown.
In conclusion, we found evidence of isotope proﬁlesand, in particular, variances in isotope proﬁles differing
between areas and sexes in a way that is consistent with
the Chernobyl region having become a sink following
the nuclear disaster. In particular, we found evidence ofsex differences in isotope proﬁles that are consistent withknown sex differences in dispersal distances, but alsowith expected sex differences in dispersal between sourceand sink populations. These ﬁndings emphasize thatstable-isotope proﬁles can provide powerful tools thatcan be helpful in analyzing population processes, whencarefully considering temporal and spatial controls andthe natural history of the study organism.
ACKNOWLEDGMENTS
Patricia Healy assisted with preparation of stable-isotope
samples. Analysis of carbon and nitrogen isotope samples was
conducted at the Department of Soil Science, University of
Saskatchewan, by M. Stocki. We are grateful to G. Milinevski
and E. Pysanets for logistical help and access to feather samples
in the Zoological Museum, Kiev, Ukraine. The Ukrainian
Ringing Center, EURING, and SAFRING kindly provided
information on recoveries of banded birds. T. Szép kindly
provided the map of recoveries. A. P. Møller and T. A.
Mousseau received funding from the CNRS (France), theUniversity of South Carolina School of the Environment, theSamuel Freeman Charitable Trust, the National Science
Foundation, and the National Geographic Society to conduct
this research. K. A. Hobson was supported by grants from The
Canadian Wildlife Service, Prairie and Northern Region. I. L.
Brisbin, Jr., R. McGregor, L. Thomas, and an anonymous
reviewer kindly provided constructive suggestions.
LITERATURE CITED
Begg, G. A., and C. R. Weidman. 2001. Stable delta C-13 anddelta O-18 isotopes in otoliths of haddock Melanogrammus
aegleﬁnus from the northwest Atlantic Ocean. Marine
Ecology – Progress Series 216:223–233.
Berthold, P. 2001. Bird migration: a general survey. Second
edition. Oxford University Press, Oxford, UK.
Bowen, G. J., L. I. Wassenaar, and K. A. Hobson. 2005.
Application of stable hydrogen and oxygen isotopes towildlife forensic investigations at global scales. Oecologia
143:337–348.

Ecological Applications
Vol. 16, No. 5

Camplani, C., N. Saino, and A. P. Møller. 1999. Carotenoids,
sexual signals and immune function in barn swallows from
Chernobyl. Proceedings of the Royal Society of London B
266:1111–1116.
Chamberlain, C. P., S. Bensch, X. Feng, S. Åkesson, and T.
Andersson. 2001. Stable isotopes examined across a migratory divide in Scandinavian willow warblers (Phylloscopus
trochilus trochilus and Phylloscopus trochilus acredula) reﬂect
their African winter quarters. Proceedings of the Royal
Society of London B 267:43–48.
Chandler, C. R. 1995. Practical considerations in the use ofsimultaneous inference for multiple tests. Animal Behaviour
49:524–527.
CIA [Central Intelligence Agency]. 1996. Handbook of international economic statistics. Directorate of Intelligence, Washington, D.C., USA.
Clobert, J., J. D. Nichols, E. Danchin, A. Dhondt, editors.
2001. Dispersal. Oxford University Press, Oxford, UK.
Cramp, S., editor. 1988. The birds of the Western Palearctic.
Volume 5. Oxford University Press, Oxford, UK.
Dias, P. C. 1996. Sources and sinks in population biology.
Trends in Ecology and Evolution 11:326–330.
Ellegren, H., G. Lindgren, C. R. Primmer, and A. P. Møller.
1997. Fitness loss and germline mutations in barn swallows
breeding in Chernobyl. Nature 389:593–596.
Glutz von Blotzheim, U. N., and K. M. Bauer, editors. 1994.
Handbuch der Vögel Mitteleuropas. Volume 9. AULAVerlag, Wiesbaden, Germany.
Greenwood, P. J. 1980. Mating systems, philopatry anddispersal in birds and mammals. Animal Behaviour 28:
1140–1162.
Hanski, I. 1999. Metapopulation ecology. Oxford University
Press, Oxford, UK.
Hershey, A. E., J. Pastor, B. J. Peterson, and G. W. Kling.
1993. Stable isotopes resolve the drift paradox for Baetis
mayﬂies in an Arctic river. Ecology 74:2315–2325.
Hobson, K. A. 1995. Reconstructing avian diets using stableisotope analysis of egg components—patterns of isotopic
fractionation and turnover. Condor 97:752–762.
Hobson, K. A. 1999. Tracing origins and migration of wildlife
using stable isotopes: a review. Oecologia 120:314–326.
Hobson, K. A. 2004. Flying ﬁngerprints: making connections
with stable isotopes and trace elements. Pages 235–246 in P.
Marra and R. Greenberg, editors. Birds of two worlds. Johns
Hopkins University Press, Washington, D.C., USA.
Hobson, K. A. 2005. Using stable isotopes to trace longdistance dispersal in birds and other taxa. Diversity andDistributions 11:157–164.
Hobson, K. A., B. N. McLellan, and J. Woods. 2000. Using
stable-carbon (d13C) and nitrogen (d15N) isotopes to infer
trophic relationships among black and grizzly bears in Upper
Columbia River Basin, British Columbia. Canadian Journalof Zoology 78:1332–1339.
Hobson, K. A., L. I. Wassenaar, and E. Bayne. 2004. Using
isotopic variance to detect long-distance dispersal andphilopatry in birds: an example with Ovenbirds andAmerican Redstarts. Condor 106:732–743.
Holm, S. 1979. A simple sequentially rejective multiple test
procedure. Scandinavian Journal of Statistics 6:65–70.
Jamieson, R. E., H. P. Schwarcz, and J. Brattey. 2004. Carbon
isotopic records from the otoliths of Atlantic cod (Gadus
morhua) from eastern Newfoundland. Canadian Fisheries
Research 68:83–97.
Kelly, J. F. 2000. Stable isotopes of carbon and nitrogen in the
study of avian and mammalian trophic ecology. Canadian
Journal of Zoology 78:1–27.
Kelly, J. F., V. Atudorei, Z. D. Sharp, and D. M. Finch. 2002.
Insights into Wilson’s warbler migration from analyses ofhydrogen stable-isotope ratios. Oecologia 130:216–221.
Koch, P. L., J. Heisinger, C. Moss, R. W. Carlson, M. L. Fogel,and A. K. Behrensmeyer. 1995. Isotopic tracking of change inOctober 2006

VARIATION IN ISOTOPES IN BARN SWALLOWS

diet and habitat use in African elephants. Science 267:1340–
1343.
Lande, R., S. Engen, and B.-E. Sæther. 2003. Stochastic
population dynamics in ecology and conservation. Oxford
University Press, Oxford, UK.
Marra, P. P., K. A. Hobson, and R. T. Holmes. 1998. Linking
winter and summer events in a migratory bird by using
stable-carbon isotopes. Science 282:1884–1886.
McCarroll, D., and N. J. Loader. 2004. Stable isotopes in treerings. Quaternary Science Reviews 23:771–801.
Møller, A. P. 1993. Morphology and sexual selection in thebarn swallow Hirundo rustica in Chernobyl, Ukraine.
Proceedings of the Royal Society of London B 252:51–57.
Møller, A. P. 1994. Sexual selection and the barn swallow.
Oxford University Press, Oxford, UK.
Møller, A. P., and K. A. Hobson. 2004. Heterogeneity in stable
isotope proﬁles predicts coexistence of populations of barnswallows Hirundo rustica differing in morphology andreproductive performance. Proceedings of the Royal Societyof London B 271:1355–1362.
Møller, A. P., and T. A. Mousseau. 2001. Albinism andphenotype of barn swallows Hirundo rustica from Chernobyl.
Evolution 55:2097–2104.
Møller, A. P., and T. A. Mousseau. 2003. Mutation and sexual
selection: a test using barn swallows from Chernobyl.
Evolution 57:2139–2146.
Møller, A. P., T. A. Mousseau, G. Milinevsky, A. Peklo, E.
Pysanets, and T. Szép. 2005. Condition, reproduction andsurvival of barn swallows from Chernobyl. Journal ofAnimal Ecology 74:1102–1111.
Pain, D. J., R. E. Green, B. Giessing, A. Kozulin, A. Poluda, U.
Ottosson, M. Flade, and G. M. Hilton. 2004. Using stable
isotopes to investigate migratory connectivity of the globallythreatened aquatic warbler Acrocephalus paludicola. Oecologia 138:168–174.
Paradis, E. 1995. Survival, immigration and habitat quality in
the Mediterranean pine vole. Journal of Animal Ecology 64:
579–591.
Pulliam, H. R. 1988. Sources, sinks, and population regulation.
American Naturalist 132:652–661.
Pulliam, H. R. 1996. Sources and sinks: empirical evidence andpopulation consequences. Pages 45–70 in O. E. Rhodes, Jr.,R. K. Chester, and M. H. Smith, editors. Population

1705

dynamics in ecological space and time. University of Chicago
Press, Chicago, Illinois, USA.
Pulliam, H. R., and B. J. Danielson. 1991. Sources, sinks andhabitat selection: a landscape perspective on population
dynamics. American Naturalist 137:50–66.
Roff, D. A. 2001. Life history evolution. Sinauer Associates,
Sunderland, Massachusetts, USA.
Rubenstein, D. R., C. P. Chamberlain, R. T. Holmes, M. P.
Ayres, J. R. Waldbauer, G. R. Graves, and N. C. Tuross.
2002. Linking breeding and wintering ranges of a migratory
songbird using stable isotopes. Science 295:1062–1065.
Rubenstein, D. R., and K. A. Hobson. 2004. From birds tobutterﬂies: animal movement patterns and stable isotopes.
Trends in Ecology and Evolution 19:256–263.
SAS Institute. 2000. JMP. SAS Institute, Cary, North Carolina,
USA.
Shestopalov, V. M. 1996. Atlas of Chernobyl exclusion zone.
Ukrainian Academy of Science, Kiev, Ukraine.
Sillett, T. S., and R. Holmes. 2002. Variation in survivorship ofa migratory songbird throughout its annual cycle. Journal ofAnimal Ecology 71:296–308.
Sokal, R. R., and F. J. Rohlf. 1995. Biometry. Third edition.
Freeman, New York, New York, USA.
Stacey, P. B., V. A. Johnson, and M. L. Taper. 1997. Migration
within metapopulations: the impact upon local population
dynamics. Pages 267–292 in I. A. Hanski and M. E. Gilpin,
editors. Metapopulation biology. Academic Press, San Diego, California, USA.
Still, C. J., J. A. Berry, G. J. Collatz, and R. S. DeFries. 2003.
The global distribution of C3 and C4 vegetation: carbon cycle
implications. Global Biogeochemical Cycles 17(1):1006.
Urton, E. J. M., and K. A. Hobson. 2005. Intrapopulation
variation in gray wolf isotope (d15N and d13C) proﬁles:
implications for the ecology of individuals. Oecologia 14:
317–326.
Wright, S. P. 1992. Adjusted P-values for simultaneous
inference. Biometrics 48:1005–1013.
Yohannes, E., K. A. Hobson, D. Pearson, L. I. Wassenaar, and
H. Biebach. 2005. Stable isotope analyses of feathers help
identify autumn stopover sites of three long-distancemigrants in northeastern Africa. Journal of Avian Biology
36:235–241.