Proc. R. Soc. B (2007) 274, 1443–1448
doi:10.1098/rspb.2007.0005
Published online 27 March 2007

Birds prefer to breed in sites with low
radioactivity in Chernobyl
A. P. Møller1,* and T. A. Mousseau2
1

Laboratoire de Parasitologie Evolutive, CNRS UMR 7103, Université Pierre et Marie Curie, Bât. A, 7ème étage,
7 quai St Bernard, Case 237, 75252 Paris Cedex 05, France
2
Department of Biological Sciences, University of South Carolina, Columbia, SC 29208, USA
Low-level radioactive contamination may affect choice of breeding site and life-history decisions if (i)radioactivity directly affects body condition or (ii) it affects resource abundance that then secondarily
influences reproductive decisions. We tested the effects of radioactive contamination on nest-site choice
and reproduction in a community of hole nesting birds by putting up nest boxes in areas differing in levelsof background radiation. Great tit Parus major and pied flycatcher Ficedula hypoleuca significantly avoided
nest boxes in heavily contaminated areas, with a stronger effect in flycatchers than in tits. These preferences
could not be attributed to variation in habitat quality or resource abundance, as determined by analyses ofhabitat use and the relationship between radiation and life-history characters. Likewise, none of theseeffects could be attributed to individuals of a specific age breeding in the most contaminated areas. Laying
date and clutch size were not significantly related to dose rate in either species. Hatching success wasdepressed by elevated radioactive contamination, interacting with habitat in the great tit and with laying
date in the pied flycatcher. Interspecific differences in effects of radiation on nest-site choice suggest thatspecies respond in a species-specific manner to radiation, perhaps related to differences in migratory
habits. We suggest that individual body condition rather than secondary effects of radiation on resource
abundance account for the effects on nest box use and hatching success.
Keywords: clutch size; dose rate; hatching success; laying date

1. INTRODUCTIONRadioactivity occurs naturally or as a consequence of
release of radioactive material by humans into theenvironment. The effects of low-level radiation on plants
and animals, including humans, have attracted considerable attention by scientists since the Hiroshima andNagasaki nuclear bombs in 1945. However, the final
report on mutations in children of atomic bomb survivors
showed little or no effect (Neel et al. 1988). Likewise, a
recent report on the biological consequences of theaccident at the Chernobyl nuclear power plant in 1986
has suggested effects on excess deaths in humans that are
well below those predicted after the accident (Chernobyl
Forum 2005). However, the main conclusion remaining
after 20 years of research of the biological consequences of
the Chernobyl disaster is that our current state ofknowledge is extremely limited, with little or no information on ecological or evolutionary consequences(Mousseau et al. 2005; Møller & Mousseau 2006).
The Chernobyl accident contaminated more than
5000 km2 in Ukraine and Belarus to an extent that theseareas have been completely depopulated (Anonymous
1996). Immediately following the explosion at the
Chernobyl reactor, the main sources of radiation were131
I, 137Cs and 90Sr (Shestopalov 1996). Owing to its
short half-life (8 days), radioactive 131I is no longer a
problem. However, both 137Cs and 90Sr are strong
radiation emitters with half-lives of 30 and 29 years,
respectively (Shestopalov 1996), and they will thus be* Author for correspondence (amoller@snv.jussieu.fr).
Received 2 January 2007
Accepted 12 March 2007

present in significant amounts for many hundred years tocome. Vast areas outside the exclusion zone are affected bylower levels of radiation that greatly exceed natural
background levels and such contamination may haveconsequences for survival and reproduction of humans
and other organisms. Surprisingly, there have been only
few and scattered efforts to quantify the effects ofcontamination on ecological phenomena such as theabundance and distribution of plants and animals.
Likewise, no study has so far investigated how plants andanimals under field conditions respond to variation in lowlevel radiation when selecting a site for reproduction(Møller & Mousseau 2006). If such phenotypic responses
to levels of background radiation could be demonstrated,
this would raise questions about potential mechanisms. We
propose three different mechanisms for such a response.
First, individuals prospecting for a future site of reproduction might use the presence of conspecifics or theirreproductive success as a cue to suitable sites (Boulinier &Danchin 1997; Danchin & Wagner 1997). If low-level
radiation has negative effects on reproductive success, as
shown in at least one study (Møller et al. 2005a), we would
expect lower levels of recruitment to such sites. Second, if
low-level radiation has negative effects on the abundance ofresources, avoidance of contaminated sites would simply
reflect an ideal free distribution of individuals in relation tofood abundance (Fretwell & Lucas 1970). Third, individuals living in contaminated areas may suffer physiologicalconsequences in terms of reduced levels of antioxidants(Chaialo et al. 1991; Bazhan 1998; Ben-Amotz et al.
1998; Ivaniota et al. 1998; Neyfakh et al. 1998a,b;

1443

This journal is q 2007 The Royal Society1444 A. P. Møller & T. A. Mousseau

Nest-site choice and radioactivity

Lykholat & Chernaya 1999; Kumerova et al. 2000; Møller
et al. 2005b), and such reductions can have importantconsequences for reproductive decisions, including
whether or not to reproduce, clutch size and investment
in eggs (Møller et al. 2005a). Therefore, individuals
with reduced levels of antioxidants may be less likely tosettle in a given site than individuals with adequatelevels, creating a correlation between condition and background dose rate.
The objective of this study was to test if free-living
animals choose sites for reproduction related to the local
level of background radiation. To this end, we investigatednest box choice by small passerine birds in an area near
Chernobyl with a very large spatial heterogeneity inbackground radiation levels. Therefore, we predicted anegative relationship between dose rates at nest boxes andprobability of occupation, if radiation level affected nest
box choice. A secondary objective of the study was to test
the potential mechanisms accounting for an association
between nest box use and radiation. More specifically, we
predicted that the previous presence of conspecifics would
be an unlikely mechanism in areas where boxes have neverpreviously been supplied, thus expecting an effect of year,with conspecific presence being more important in thesecond and the third year when compared with the firstyear. If resource abundance were a determining factor, we
would expect measures of reproductive success to be
negatively related to radiation level, because reduced foodavailability would cause partial reproductive failure.

2. MATERIAL AND METHODS(a) Study sites
We investigated nest box use and reproductive parameters in a
250 ha area in the Red Forest, Chernobyl, where levels ofradioactive contamination reach a level that is greater than2000 times the natural background level. In 2002–2003, weerected 232 nest boxes of a standard size (20 cm!20 cm!
40 cm) with a 32 mm diameter hole. All boxes were placed at
a distance of at least 50 m between boxes at a height of
1.5–2.0 m. High levels of spatial heterogeneity in radiationlevels (Shestopalov 1996; figure 1) provided an ideal setting
for investigating nest box choice and reproduction.
The habitat was relatively open forest with a mixture of
deciduous and coniferous trees and shrubs. The oldest trees
were less than 100 years old. S. Gaschak classified the habitat
surrounding all nest boxes with respect to four different axes,
and the involvement of a single person assured that there was
no inter-observer variability: (i) tree species (birch Betula sp.;birch with aspen Populus sp. and alder Alnus sp.; pine Pinussp.), (ii) age of stands (young trees without natural holes;
older trees with natural holes), (iii) undergrowth (absent;
sparse; dense)), and (iv) habitat humidity (dry; humid; wetwith open water).
(b) Recording occupancy and nest box contents
S. Gaschak visited nest boxes during April–May 2003–2005
to check for occupancy. He identified species from the adults,
eggs and nests present. He estimated laying date from thepresence of clutches that had not been completed, assumingthat one egg was laid per day (Cramp & Perrins 1988–1993).
Likewise, he used the age of nestlings to estimate date oflaying of the first egg, assuming an incubation period of 13.9
and 14.75 days, respectively, for great tits Parus major
Proc. R. Soc. B (2007)

N

Chernobyl reactor number 4

0.5 kmFigure 1. Location of nest boxes in the Chernobyl site withradiation levels indicated for each box.
and pied flycatchers Ficedula hypoleuca, and a nestling periodof 18.9 and 14.6 days, respectively, for the same species(Cramp & Perrins 1988–1993).
He recorded clutch size as the number of eggs in complete
clutches and brood size as the number of young in the nest
after hatching and before fledging, respectively.
Hatching success was estimated as brood size at hatching
divided by clutch size, and fledging success brood size atfledging divided by brood size at hatching.
In May–June 2006, we captured 36 adult great tits andpied flycatchers at nest boxes and determined their age using
patterns of moult of wing coverts to distinguish yearlings fromolder individuals (Svensson 1984).
(c) Recording background radiation levels
at nest boxes
As an estimate of background radiation levels, we relied on adataset of air-gamma-survey of the Chernobyl zone. Thisdataset includes geo-positioned data of 137Cs and 90Sr
deposition in a grid of 100 m!100 m as of 1 January 1992.
Using corresponding software (MapInfo Prof.), we sampledfrom the database all data that were within a radius of 500 m
from each nest box. Figure 1 shows the position of nest boxes
and their radiation level. The choice of a radius of 500 m wasbased on the assumption that most birds remained within anarea with a radius of 500 m. This assumption is supported by
the size of the home range of tits and flycatchers during
breeding (Perrins 1979; Lundberg & Alatalo 1992). We crossvalidated these data by using our own field measurements ofbackground radiation at the ground level using a hand-heldInspector (model: Inspector, SE International, Inc., Summertown, TN, USA) dosimeter at sample of 28 nest boxes during
May–June 2006. We found a strong positive relationshipbetween these two series of measurements (FZ210.99, d.f.Z
1, 26, r2Z0.89, p!0.0001, slope (s.e.)Z1.15 (0.08)).
(d) Statistical analyses
We used partial Mantel tests (Smouse et al. 1986) toinvestigate relationships between dose rate, nest box occupation and reproductive variables, using the software XLSTAT2006 with 1000 permutations in each test (Addinsoft 2006).
Hatching success, fledging success and breeding success weresquare-root arcsine-transformed, while dose rate was logtransformed to meet requirements for normally distributed
data (none of the variables differed significantly from normal
distributions after transformation).
We modelled occupation rate (in great tit never occupied,
occupied once or occupied twice; in pied flycatcher neveroccupied or occupied once) in relation to log-transformed

A. P. Møller & T. A. Mousseau

Nest-site choice and radioactivity
(a)

1445

(b) 0.7
0.7

0.6

0.6
box occupation rate

0.5
once
twice

0.5

once

0.4

0.4
0.3

0.3

0.2

0.2

0.1

0.1
0

5

15
10
dose rate (mR h–1)

20

0

5

15
10
dose rate (mR h–1)

20

Figure 2. Nest box choice (proportion of boxes occupied) in relation to dose rate (mR hK1) at nest boxes for (a) great tits Parus
major and (b) pied flycatchers Ficedula hypoleuca. The number of boxes for the 20 radiation intervals were 59, 7, 15, 13, 8, 10, 8,
14, 19, 21, 19, 7, 14, 10, 2, 3, 1, 0, 0 and 0, respectively.
dose rate of boxes, using logistic regression with a logit linkfunction. These models included year as a factor in order tocontrol for differences in abundance among years, and year bydose rate interactions in order to test for differences in effectsof dose rate among years.
We modelled laying date, clutch size, hatching success andfledging success by using year, dose rate, habitat variables andtheir two-way interactions as predictor variables, using
Akaike’s information criterion to develop best-fit models forthese three reproductive variables (Burnham & Anderson
2002). Habitat variables were used as a factor. In addition,
because clutch size may depend on laying date, we included
laying date and its interaction with dose rate in the analyses.
For hatching and fledging success, we included laying dateand clutch size as additional variables.
We determined whether age of breeding birds was
associated with level of background radiation at nest boxes
by assessing whether log-transformed radiation level differed
between yearlings and older individuals, and whether thiseffect differed between great tits and pied flycatchers by
testing for an interaction between age and species.
Data were missing for some variables due to complete nest
failure, causing sample sizes to differ among tests. Variables
reported are means (s.e.).

3. RESULTS(a) Choice of breeding site and rate of radiation
Dose rates for nest boxes ranged from 0.54 to 16.7 mR hK1,
with a mean (s.e.) of 6.25 (0.30), NZ231. For pied
flycatchers, but not for great tits, we found a significantly
decreasing relationship between occupation rate and dose
rate (figure 2; table 1). There was also a significant year
effect for tits, but not for pied flycatchers (table 1, models 1
and 3). Interactions between year and dose rate were nonsignificant. Partial Mantel tests for occupation in relation todose rate after controlling for distance were statisticallysignificant for both species (great tit: rZK0.268,
p!0.0001; pied flycatcher: rZK0.396, p!0.0001).
Therefore, occupation rate was negatively related to dose
rate after adjusting for distance.
We checked the robustness of this conclusion byentering habitat variables as additional predictors in themodels. For great tits, there was a significant interaction
Proc. R. Soc. B (2007)

Table 1. Nest box use by great tits and pied flycatchers in
relation to year and dose rate (models 1 and 3) and dose rateand habitat (models 2 and 4). (The statistics of the models
were for model 1: c26Z26.62, p!0.0001, model 2: c26Z14.01,
pZ0.030, model 3: c23Z38.20, p!0.0001 and model 4:
c21Z34.32, p!0.0001.)
variabletitsmodel 1year
log (dose rate)
model 2age of standlog (dose rate)log (doserate)!age of
standpied flycatcher
model 3year
log (dose rate)
model 4log (dose rate)

d.f.

wald

p

slope (s.e.)

4
2

13.95
4.24

0.00740.12

K0.70 (1.04)

2
2
2

3.11
9.85
5.13

0.21
0.073
0.037

2
1

3.57
14.82

0.17
!0.0001

K3.59 (0.93)

1

16.03

!0.0001

K3.78 (0.94)

K1.49 (0.64)

between age of stands and dose rate (table 1, model 2). We
also entered age of stand in the partial Mantel test for greattits, but this did not affect the conclusions. For pied
flycatchers, only dose rate was entered as a significant
predictor (table 1, model 4). The effect of dose rate
differed significantly between the two species, withthe effect being stronger in flycatchers than in great tits
(FZ22.79, d.f.Z1, p!0.0001). Therefore, boxes withpied flycatchers had significantly lower dose rates thanboxes with great tits (pied flycatcher: 1.03 (0.18), NZ16;
great tit: 4.88 (0.47), NZ71; tZ11.85, d.f.Z85,
p!0.0001).
There was no difference in level of radiation between
age classes (FZ0.07, d.f.Z1, 34, pZ0.79; mean (s.e.)
for yearlings: 0.04 (0.33), NZ24; adults: 0.16 (0.16),
NZ12). Furthermore, there was no significant interaction
between age and species (FZ0.13, d.f.Z1, 32, pZ0.88),
while the effect of species was significant, as already shown
above (FZ4.57, d.f.Z1, 32, pZ0.019). There was no age

1446 A. P. Møller & T. A. Mousseau

Nest-site choice and radioactivity

Table 2. Summary statistics for reproductive variables of greattits and pied flycatchers in Chernobyl. ( Values are means(s.e.))

laying dateclutch size
hatching success (%)
fledging success (%)
Ngreat tit

pied flycatcher

2 May (1)9.56 (0.29)
72.55 (4.23)
82.82 (4.20)
105

19 May (2)5.94 (0.17)
83.39 (0.06)
97.42 (0.01)
32

difference in sex of captured birds (33% males of 24
yearlings and 33% males of 12 older birds).
(b) Laying date, clutch size and reproductive
success and rate of radiation
Summary statistics for reproductive variables are reported
in table 2. Laying date was related to undergrowth as the
only significant predictor (table 3).
In analyses of clutch size, we entered laying date as acovariate because clutch size decreased with laying date(table 3). For neither species did dose rate enter as apredictor of clutch size, while there was a significant year
effect for great tit and a significant effect of undergrowthfor pied flycatcher (table 3).
In the great tit, 13.3% of nests were deserted, 2.9%
destroyed by dormice (Eliomys quercinus) and 2.9% by ants
(NZ105). In the pied flycatcher, 6.2% of nests weredeserted (NZ32). Hatching success in the great tit
differed between years, tree species (lower success inpine) and humidity classes (lower success in humid
habitats; table 3). In addition, there was a significant
interaction between undergrowth and tree species(implying that hatching success increased with increasing
undergrowth for a given tree species) and between
undergrowth and dose rate (implying that hatching
success increased with increasing undergrowth for agiven dose rate; table 3). Hatching success decreased
with laying date in pied flycatchers (table 3). In addition,
there was a significant interaction between dose rate andlaying date, implying that hatching success decreased
more with laying date for high levels of radiation (table 3).
Fledging success was not significantly related to dose ratein either species, while there were significant effects of theinteraction between age of stands and undergrowth in thegreat tit and a significant effect of date in the pied
flycatcher (table 3). We performed partial Mantel tests
between reproductive variables and dose rate after
controlling for distance among boxes. Only the correlations between hatching success and dose rate were
statistically significant (great tit: rZK0.351, p!0.0001;
pied flycatcher: rZK0.271, p!0.0001). Entering theother habitat variables in table 3 into these partial Mantel
tests did not change the conclusions. Therefore, hatching
success was negatively related to dose rate after adjustingfor geographical distance.

4. DISCUSSION
The main findings of this study were that (i) the
occupation rate of nest boxes decreased with increasing
dose rate across a range of background radiation of twoorders of magnitude with a stronger effect in pied
Proc. R. Soc. B (2007)

flycatchers than in great tits; (ii) this effect was notconfounded by individuals of a specific age breeding at themost contaminated boxes; (iii) laying date and clutch size
was not related to dose rate; and (iv) hatching successdecreased with dose rate, interacting with laying date inpied flycatchers and with density of undergrowth in greattits. These findings constitute the first empirical evidence
consistent with the suggestion that animals choose sites ofreproduction dependent on background radiation levels.
The current study of occupancy of nest sites by holenesting birds suggested that great tits and pied flycatchers
discriminate against nest sites with high dose rates,
thereby avoiding radioactively contaminated areas assites of reproduction. These effects were present indifferent years. The effects were present even whencontrolling statistically for the potentially confounding
effects of habitat differences covarying with levels ofradiation, and there was no evidence that yearlings usedboxes with levels of radiation that differed from that usedby older birds. The effect for pied flycatchers wasconsiderably stronger than for great tits. To the best of
our knowledge, there are no previous study investigating
the relationship between radiation and distribution andabundance of plants or animals (Chernobyl Forum 2005;
Møller & Mousseau 2006).
Nest-site choice dependent on radioactive contamination begs the question what are the underlying
mechanisms? The effect cannot arise from limited ability
to disperse because the size of the study area was small
when compared with geometric mean natal dispersal
distances of 0.8 and 14.3 km in great tits and pied
flycatchers (Paradis et al. 1998). We can also dismiss thepossibility that this effect was mediated by attraction toconspecifics because the findings were similar in both
years (the interaction between year and dose rate was notstatistically significant for either species). This implies thatthe effect was already present when the boxes had neverbeen used and hence did not have signs of previous
occupation. We can also dismiss the explanation that theeffect was mediated by resource abundance because suchan effect would assume that food abundance decreased
with increasing dose rate. If that were the case, we would
expect clutch size and brood size to decrease with doserate, which was clearly not observed (table 3). This leaves
the possibility that the effect was mediated by differences
in individual body condition. We have previously shownfor barn swallows Hirundo rustica that individuals in highly
contaminated areas have reduced levels of antioxidants inplasma and liver (Møller et al. 2005a). Such individuals
have reduced body condition, reproduce less frequently,
and produce reduced clutch and brood size whencompared with individuals breeding in highly contaminated areas (Møller et al. 2005b). Therefore, wehypothesize a similar mechanism for hole nesting species,
with individuals breeding in highly contaminated areas
suffering from reduced antioxidant levels, thereby reducing the frequency of reproduction. If individuals differ incondition, with late breeding individuals suffering morefrom poor condition than early breeders, then we would
expect dose effects to be more severe late during thebreeding season (predicting an interaction between laying
date and dose rate for reproductive variables). Indeed, we
found such effects for hatching success in the pied
flycatcher (table 3), consistent with this interpretation.

Nest-site choice and radioactivity

A. P. Møller & T. A. Mousseau

1447

Table 3. Models of reproductive variables of great tits and pied flycatchers in relation to dose rate, laying date, habitat, year andtheir two-way interactions. (The statistics for the models were FZ8.64, d.f.Z1, 81, r2Z0.10, pZ0.004, FZ29.62, d.f.Z2, 80,
r2Z0.43, p!0.0001, FZ3.91, d.f.Z9, 55, r2Z0.39, pZ0.0007, FZ6.39, d.f.Z2, 49, r2Z0.21, pZ0.0034 and FZ4.40, d.f.Z4, 47, r2Z0.27, pZ0.0042 for great tit, and FZ4.25, d.f.Z1, 28, r2Z0.13, pZ0.049, FZ23.41, d.f.Z2, 27, r2Z0.63,
p!0.0001, FZ9.36, d.f.Z4, 23, r2Z0.62, p!0.0001 and FZ4.82, d.f.Z1, 20, r2Z0.19, pZ0.04 for pied flycatcher.)variable
great titlaying date:undergrowth
clutch size:
date
year
hatching successyear
tree species
age of standundergrowth
humiditydatelog (dose rate)
undergrowth!tree species
undergrowth!log (dose rate)
fledging success:age of standundergrowth
age of stand!undergrowthyear
pied flycatcher
laying date:undergrowth
clutch size:
dateundergrowthhatching success:datelog (dose rate)
date!log (dose rate)year
fledging success:
date

d.f.

SS

F

p

slope (s.e.)

11089.618.65

0.0005

K2.00 (0.56)

1
1233.98
50.63

49.81
10.78

!0.0001
0.0015

K0.144 (0.020)

1
1
1
1
1
1
1
1
1

2.47
0.91
0.59
0.52
1.26
0.16
0.59
1.09
1.90

16.62
6.13
3.96
3.50
8.51
1.06
3.94
7.31
12.75

!0.0001
0.016
0.051
0.067
0.005
0.31
0.052
0.009
0.0007

1
1
1
1

0.04
0.00
1.14
1.79

0.22
0.00
6.50
10.20

0.64
0.99
0.010
0.00251

397.34

4.25

0.049

K5.79 (2.81)

1
1

16.93
2.39

46.82
6.62

!0.0001
0.016

K0.080 (0.012)
K0.48 (0.19)

1
1
1
1

1.93
0.22
1.18
0.35

22.56
2.56
13.73
4.03

!0.0001
0.12
0.00120.06

K0.030 (0.006)

1

0.10

4.82

0.040

The two species differed in response to dose rate withthe pied flycatcher showing a stronger relationshipbetween occupation rate and dose rate than the great tit.
Therefore, pied flycatchers bred at sites that weresignificantly less contaminated than did great tits,
reducing the possibility of finding significant effects ofradiation on life-history variables in pied flycatchers.
Neither laying date nor clutch size was significantly related
to dose rate. In contrast, hatching success decreased withdose rate, with a significant interaction with undergrowthcover in the great tit and with laying date in the pied
flycatcher. Dose rate did not predict fledging success ineither species, suggesting that radioactive contamination
did not limit food supply. The two species differ in a
number of respects including foraging ecology (great tits
mainly eat caterpillars, pied flycatchers by catching flying
insects in the air) and migration strategy (tits are residents,
while pied flycatchers are long-distance migrants wintering in sub-Saharan Africa). Differences in relationships
between occupation rate and dose rate could be due todifferences in levels of contamination of food. However,
our estimates of whole body dose rates for the two species
have revealed very similar mean dose rates (S. Gaschack,Proc. R. Soc. B (2007)

K0.37 (0.15)

K0.32 (0.11)

0.47 (0.18)
0.89 (0.25)

0.39 (0.15)

K0.219 (0.059)

K0.009 (0.004)

A. P. Møller and T. A. Mousseau unpublished data),suggesting that this is an unlikely explanation. In addition,
the lack of an association between fledging success anddose rate contradicts that explanation. Alternatively,
strong effects in pied flycatchers may arise from thisspecies being migratory arriving on spring migration to thebreeding grounds with depleted reserves of antioxidants(Ninni et al. 2004). This depletion of antioxidant reserves
may be further exacerbated by extensive maternal
deposition of carotenoids and vitamins A and E into
eggs (Blount et al. 2000; Blount et al. 2003). Therefore,
pied flycatchers would be expected to have lower rates ofreproduction than great tits, but also to be selected toshow stronger avoidance of breeding sites with low
availability of antioxidants.
This study has implications for future studies. First, it
would be crucial to determine the association between
dose rate and condition of hole nesting passerines in orderto test for the hypothesized mechanism. Second, we
consider that it would be important to establish amonitoring scheme for a number of common invertebrates
and vertebrates to assess to which extent elevated levels ofradioactive contamination are associated with reduced

1448 A. P. Møller & T. A. Mousseau

Nest-site choice and radioactivity

population density. Finally, it would be important to
combine such studies of distribution and abundance withstudies of population dynamics to assess the extent towhich apparently high population densities are maintainedby source–sink dynamics (Møller et al. 2006). If thatwere the case, we would expect a larger fraction of
immigrant juveniles in the most contaminated areas. The
lack of significant association between age of breeders
and radiation level and the lack of interaction between
species and age suggests that neither great tits nor
flycatchers have populations in Chernobyl maintained bysource-sink dynamics.
We are grateful to G. Milinevski, S. Gaschak, A. M. Peklo,
E. Pysanets and M. Bondarkov for logistic help. We received
funding from CNRS (France), University of South Carolina
School of the Environment, Bill Murray and Samuel Freeman Charitable Trust, National Science Foundation andNational Geographic Society for this research.

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