Methods in Ecology and Evolution 2016
doi: 10.1111/2041-210X.12695
Ecological forensics: using single point stable isotope
values to infer seasonal schedules of animals after two
diet switches
Jeltje Jouta*,1,2, Maurine W. Dietz2, Jeroen Reneerkens2, Theunis Piersma1,2,
Eldar Rakhimberdiev1,3, Gunnar T. Hallgrımsson4 and Ido Pen2
1
Department of Coastal Systems and Utrecht University, NIOZ Royal Netherlands Institute for Sea Research, P.O. Box 59,
1790 AB Den Burg, Texel, The Netherlands; 2Groningen Institute for Evolutionary Life Sciences (GELIFES), University of
Groningen, P.O. Box 11103, 9700 CC Groningen, The Netherlands; 3Department of Vertebrate Zoology, Lomonosov Moscow
State University, 119991 Moscow, Russia; and 4Institute of Biology, University of Iceland, Sturlugata 7, 101 Reykjavik, Iceland
Summary
1. Animals adjust to seasonal challenges in physical, behavioural and spatial ways. Such adjustments are commonly associated with diet changes that often can be characterised isotopically.
2. We introduce the ‘double diet switch model’, with which the occurrence and timing of two subsequent diet
switches of an individual animal can be traced with a single sample assayed for stable isotopes. We demonstrate
the model for Sanderling, Calidris alba, a small shorebird that migrates from the Nearctic tundra breeding
grounds to the intertidal flats of the Wadden Sea; during this migration some birds may stage in the North Atlantic areas.
3. The ‘double diet switch model’ successfully predicted the occurrence and timing of two diet switches in 59 Sanderlings captured in the Wadden Sea in July–September. Excluding birds that likely had over-summered at
North Atlantic staging areas, the model predicted that Sanderlings departed from the Arctic on 13 July (range:
9–17 July), had a staging duration of 186 days in the North Atlantic, and arrived in the Wadden Sea on 1 August
(31 July–1 August).The estimated mean Arctic departure dates coincided with the mean hatching date, suggesting that many individuals failed to produce young or left the care to a partner. Estimated mean arrival date
matched the main arrival period in the Wadden Sea obtained from observation data. In this study we did not use
lipid-free tissues, which may bias model predictions. After correcting for lipid components, the estimated departure date was 11 days later and the staging duration 85 days shorter, while arrival date was similar.
4. The ‘double diet switch model’ successfully identified the occurrence and timing of two subsequent diet
switches. The ‘double diet switch model’ will not only apply to switches between three isotopic levels (as in the
case study on Sanderling) but also to scenarios where the second switch reverses to the initial isotopic level. Due
to this general applicability, the model can be adapted to a wide range of taxa and situations. Foreseeable applications include changes in habitat and food type, ontogenetic development or drastic phenotypic changes such as
the metamorphosis in insects and amphibians.
Key-words: Calidris alba, dietary change, migration, seasonal patterns, shorebirds, stable isotopes,
staging duration, timing, trophic change, Wadden Sea
Introduction
Animals adjust to seasonal challenges by movements and by
physical and behavioural changes (Piersma & van Gils 2011).
Quite commonly, these adjustments are associated with diet
changes that can be isotopically characterised (Hobson 1999;
Caut, Angulo & Courchamp 2009). The accompanying shifts
in isotopic value enables researchers to illuminate seasonal
phenomena such as migration, metamorphosis or (temporary)
increasing or declining food availability (Phillips & Eldridge
*Correspondence author. E-mail: jeltje.jouta@nioz.nl
2006; Karasov & Martınez del Rio 2007; Schwemmer et al.
2016). No surprise that ‘ecological forensics’ is thriving (Dawson & Siegwolf 2007).
Stable isotope analyses can track the occurrence and timing
of diet switches based on differences in (i) isotopic values generated by foraging on isotopically distinct food sources and (ii)
incorporation times of an isotope in distinct consumer tissues
(e.g. plasma and red blood cells: Hobson 1999; Klaassen et al.
2010). After a diet switch, the isotopic incorporation of the
new diet in a consumer’s tissues follows a first order kinetics
model, mostly described by an exponential decay function.
This model can estimate the time since a single diet switch
using stable isotope values of one, or preferably two, tissue
© 2016 The Authors. Methods in Ecology and Evolution © 2016 British Ecological Society
2 J. Jouta et al.
types (Phillips & Eldridge 2006; Klaassen et al. 2010; Oppel &
Powell 2010). For animals that change their foraging location
or diet more than once over relatively short time spans, we here
describe a ‘double diet switch model’. This model can deal with
three successive isotopically distinct diets based on a single
assessment of isotopic values in two tissues with distinct turnover rates in one individual and gives estimates of the timing of
the two consecutive diet switches.
To demonstrate the functionality of the model, we estimate
the timing of post-breeding migration of Sanderlings Calidris
alba upon their arrival in the Dutch Wadden Sea. After a
breeding season in the High Arctic, these long-distance migratory shorebirds depart from the tundra where they fed on terrestrial arthropods (Wirta et al. 2015). Before arrival in the
Wadden Sea, where they mainly feed on Brown Shrimp Crangon crangon (JR pers. comm.), Sanderlings may or may not
make refuelling stops in coastal habitats in the North Atlantic
where soft-bodied marine invertebrates comprise the diet
(Reneerkens et al. 2009).
Materials and methods
THE DOUBLE DIET SWITCH MODEL
The isotopic change in body tissues after a diet switch typically follows
a first-order kinetic response which is generally well described by a negative exponential function (Tieszen et al. 1983; Phillips & Eldridge
2006; Klaassen et al. 2010). Specifically, consider a focal animal on a
diet A, with a corresponding isotope ratio dA1 in tissue 1. If at time
t = 0 the animal switches from diet A to diet B, then after tB days on
the new diet, its tissue-specific isotope ratio is given by the formula:
dðtB1 Þ ¼ dB1 þ ðdB1
dB1 Þe
k 1 tB
eqn 1
;
where dB1 is the characteristic isotope ratio of diet B in issue 1, and k1 is
the tissue-specific turnover rate (1 per day) of the isotope. Given estimates of dA1, dB1 and k1, this ‘single diet switch model’ allows estimation of tB, the amount of time since the diet switch occurred (Phillips &
Eldridge 2006; Klaassen et al. 2010).
Here we expand this ‘single diet switch model’ to one which
describes two diet switches: the ‘double diet switch model’. Suppose
that at time t = tB, our focal animal switches once again, from diet
B to diet C, the latter having characteristic isotopic ratio dC1 in tissue 1. After tC days on diet C, at time t = tB + tC, the animal’s isotope ratio is now given by
dðtÞ ¼dC1 þ ½dðtB Þ
dC1 e
¼dC1 þ ½dB1 þ ðdA1
k1 tC
dB1 Þe
k1 tB
dC1 e
eqn 2
k1 tC
;
where we substituted the right-hand side of formula (1) for d(tB) in the
first line. Note that this formula is not very useful by itself, since any
observed value of d(t) within the range spanned by dA1, dB1 and dC1 is
typically consistent with infinitely many combinations of tB and tC.
However, if a sample is taken simultaneously from a second tissue with
a different turnover rate k2, then we have a system of two equations for
the two unknowns tB and tC:
d1 ðtÞ ¼ dC1 þ ½dB1
d2 ðtÞ ¼ dC2 þ ½dB2
dC1 þ ðdA1
dC2 þ ðdA2
dB1 Þe
k1 tB
dB2 Þe
k2 tB
e
k1 tC
e
k2 tC
eqn 3
Geometrically, the two equations correspond to two curves in
the tB tC plane, and solutions to the two equations occur if and
where the curves intersect. As we shall see below, these solutions
are precisely the maximum likelihood (ML) estimates of tB and
tC, provided that d1(t) and d2(t) are normally distributed around
their predicted values. Solving both equations for tC gives explicit
formulas for the two curves:
1
dC1 dB1 þ ðdB1 dA1 Þe k1 tB
tC ¼ ln
k1
dC1 d1 ðtÞ
eqn 4
1
dC2 dB2 þ ðdB2 dA2 Þe k2 tB
tC ¼ ln
k2
dC2 d2 ðtÞ
Equating both right-hand-sides yields an equation in tB, which
does not have closed-form solutions, but which may be solved by
standard numerical routines. If a solution is found, it can be put
back into either of the right-hand sides of (4) to give a corresponding solution for tC.
Thus, the ‘double diets switch model’ allows estimation of seasonal scheduling of animals with three subsequent diets, such as
migrant birds consuming isotopically distinct diets before the start
of migration, during a staging episode and after arrival to final destination, respectively, or grizzly bears (Ursus arctos) switching temporarily from a diet with mainly whitebark pine (Pinus albicaulis) to
a diet with mainly elk (Cervus elaphus) (Schwartz et al. 2014). The
conditions for the use of the ‘double diet switch model’ are presented in Table 1. In the next section we describe a statistical
method to estimate tB and tC.
THE LIKELIHOOD MODEL
We use a ML approach to estimate the parameters tB and tC in the nonlinear model (3), given estimates of all other parameters and the
Table 1. An overview of the required conditions for the ‘double
diet switch model’ to estimate the timing of two consecutive diet
switches
Conditions for using the ‘double diet switch model’:
a) Stable isotope analysis (eg. d13C) of individuals of the
study species should be measured while on the third
diet. This should be done for two tissue types: one
with a relatively high turnover rate such as plasma
and one with a relatively low turnover rate such as
RBC). Both tissues should be sampled at the same
moment. Tissue sampling needs to be performed before
the individual has reached isotopic adaptation of the
new equilibrium of the third diet.
b) At all three stages (or locations), the stable isotope values of
the study species itself or that of its food are known (plus a
discrimination factor; but see Data S1) and sufficiently distinct from each other
Ideally, stable isotope values are known for both separate
tissue types at all three stages.
c) Turnover rates of the two tissues are known for the study
species (or can be estimated sufficiently accurately)
d) Preferably, sampling dates of the tissue types are known
With this information durations of the use of a diet can be
transferred to dates instead of number of days.
e) Diet uniformity among individuals
f) The sampling moment is important, since there should not
have been enough time to approach equilibration to diet C
Besides, the animal’s staging duration should be shorter
than the time to approach the equilibration to diet B.
© 2016 The Authors. Methods in Ecology and Evolution © 2016 British Ecological Society, Methods in Ecology and Evolution
Using isotopes to infer seasonal schedules
measured values of d1 and d2. We assume that measurement errors have
a normal density:
1
1
2
2
pðd1 ; d2 jtB ; tC Þ ¼
exp
ððd
l
Þ
þ
ðd
l
Þ
Þ
1
2
1
2
2pr2d
2r2d
eqn 5
Here r2d is the variance, assumed known and identical for both tissues, whereas l1 and l2 are the expected values of d1 and d2 according
to model (3):
l1 ðtB ; tC Þ ¼ dC1 þ ½dB1
l2 ðtB ; tC Þ ¼ dC2 þ ½dB2
dC1 þ ðdA1
dC2 þ ðdA2
dB1 Þe
k1 tB
dB2 Þe
k2 tB
e
k1 tC
e
k2 tC
The log-likelihood is then, up to a constant term:
1
lðtB ; tC Þ ¼
ðd1 l1 Þ2 þ ðd2 l2 Þ2
2
2rd
eqn 6
eqn 7
The score, the partial derivatives of the log-likelihood with respect to
both parameters is then given by
ol
1
ol
ol
¼ 2 ðd1 l1 Þ 1 þ ðd2 l2 Þ 2
otB rd
otB
otB
eqn8
ol
1
ol1
ol
þ ðd2 l2 Þ 2
¼ 2 ðd1 l1 Þ
otC
otC
otC rd
Clearly the score vanishes if l1 = d1 and l2 = d2, which shows that
the ML estimates of tB and tC are indeed the solutions to the system of
equations (4). We used the function uniroot in R version 3.3.0 (R Core
Team 2016) to find numerical solutions. All R scripts are available as
online appendices to this paper.
The Hessian matrix of second order derivatives, evaluated at the candidate ML estimates, is
3
2 2 2
2 2
3
ol1
ol2
ol1 ol1
ol2 ol2
o l
o2 l
2
1
otB otC
otB
otB
otB otC
otB otC 7
ot
6
5¼
H ¼ 4 o2Bl
2 2 5
4
o2 l
ol1 ol1
ol2 ol2
ol1
ol2
r2d
ot ot
ot2
B
C
C
otB otC
otB otC
otC
otC
eqn 9
The Hessian has two uses here: first, to verify that candidate ML
solutions are indeed maxima of the likelihood, and secondly, to provide
approximate standard errors for the ML estimates. A local maximum
is verified if tr(H) = H11 + H22 < 0, which is easily seen to be true,
and if det(H) = H11 H22 H12 H21 > 0, which is also true since det
2
ol1 ol2
1 ol2
[ 0. Approximate standard errors and
(H) = ol
otB otC
otC otB
covariances for the ML estimates t^B and t^C follow from
"
#
r2t^B rt^B t^C
1
H
rt^B t^C r2t^C
eqn 10
The matrix H is called the information matrix, since the inverse of
information is uncertainty, as quantified by standard errors. To evaluate H we need to evaluate the partial derivatives for tissues i = 1, 2:
oli
¼ ki ðdAi dBi Þe ki ð^tB þ^tC Þ
otB tB ¼^tB ;tC ¼t^C
eqn 11
oli
ki ^t
^
¼ ki ðdBi dCi þ ðdAi dBi Þe ki tB Þe C
otC tB ¼^tB ;tC ¼t^C
Plugging these into (9) clearly shows that the uncertainty about t^B
and t^C increases exponentially with their estimated mean values. Specifically, according to the first equation in (11), information regarding tB
decays exponentially if either tB or tC grows large, whereas according to
the second equation information regarding tC is especially sensitive to
large tC but not tB values. Thus, unless turnover rates are very low, it is
3
clearly preferable to sample not too long after the second diet switch,
nor should the time between diet switches be too long.
We have attempted to take a full Bayesian approach to estimate tB
and tC, but the ML approach was superior. Simulations indicated (results not shown) that even weakly informative priors produced considerable bias in estimates. The use of flat priors is ruled out for our model
since the likelihood does not converge to zero as tB and tC go to infinity,
rendering the corresponding posterior distribution non-integrable.
SENSITIVITY ANALYSIS
The model has eight parameters: for each tissue i = 1, 2 and diet j = A,
B, C the equilibrium isotope ratios are denoted by dij and turnover rates
by ki. For the Sanderling data, the diet-and tissue-specific isotope ratios
and associated standard deviations were estimated directly from blood
and indirectly from prey items (Table 2, Data S1, Supporting Information). No direct information about turnover rates was available for the
Sanderling. Instead values for ki were predicted on the basis of interspecific allometric regressions, whereas standard deviations were
obtained as averages of intraspecific standard deviations (Table S1).
To assess the sensitivity of model predictions to uncertainty in the 8
parameters, for each bird in our data set we drew 10 000 random normal deviates for each of the 6 isotope ratios and for the logarithms of
the turnover rates (which must be positive), based on our estimates of
mean values and standard deviations. For the isotope ratios we used
independent draws, whereas for turnover rates we allowed for a positive correlation between tissues since it seems plausible that variation in
metabolic rate affects turnover rates in the same direction. For each of
the draws we attempted to obtain ML estimates for tB and tC by solving
system (4). When we obtained a candidate solution, we calculated the
Hessian to verify it corresponded to a maximum and to estimate standard errors for the parameter estimates. Thus, for each bird we
obtained 1000 distributions, one for each successful random draw,
which we approximated as a mixture of 10 000 gamma distributions to
avoid negative values in the tails of the distributions. The mixture was
stored as a ‘posterior distribution’ from which we calculated mean values and 89% highest posterior density intervals.
As an alternative to our simulation approach to sensitivity analysis,
parameter likelihoods may also be incorporated into an overall likelihood for all model parameters, in addition to tB and tC, and corresponding confidence levels calculated. Such an extended likelihoodapproach would have to be tailored to the study-specific way the additional parameters were estimated.
THE CASE: TIMING OF SOUTHWARD MIGRATION IN
SANDERLING
Using the ‘double diet switch model’, we reconstructed the timing of
southward migration by Sanderlings from the tundra breeding grounds
(where they ate diet A) and subsequently flew, with or without staging
in the North Atlantic (diet B), to the Wadden Sea (diet C). In July–
September 2011 and 2012, 65 adult Sanderlings were captured with
mist-nets during new moon nights near high-tide roosts in the western
Dutch Wadden Sea (53°N, 4–5°E). In addition, 10 adult Sanderlings
were caught on their nests in Greenland (Zackenberg, 74°300 N,
21°000 W) in the second half of June 2009. Blood samples of these latter
birds were used to determine the d13C value of red blood cells (RBC)
and plasma of birds on the initial diet in the Arctic (diet A; see Data
S1). Immediately after capture, all 75 Sanderlings were (colour)ringed,
weighed and aged based on plumage criteria (Prater, Marchant &
Vuorinen 1977), and a blood sample (~300 lL) for stable isotope
© 2016 The Authors. Methods in Ecology and Evolution © 2016 British Ecological Society, Methods in Ecology and Evolution
4 J. Jouta et al.
Table 2. Summary of all general input variables of the ‘double switch model’ to estimates individual schedules in migrating Sanderling
Diet tissue type
Arctic†
Staging area‡
Wadden Sea
Turnover ratek
Calc. (prey + DiF)*
Plasma
RBC
Plasma
RBC
Plasma
RBC
Tissue type
Plasma
RBC
1829
1762
1456
1390
Mean
0303
0056
024&
024&
009&
009&
N
True (bird blood)
N
10
2599
2533
1828
1794
1454
1394
25
20
029&
029&
014§&
030§&
016¶&
013¶&
4
6
t-test
t(27)
t(27)
t(24)
t(24)
=
=
=
=
002, P
053, P
016, P
023, P
=
=
=
=
099
060
091
082
SD
0033
0012
Presented are the d13C values of Sanderling in equilibrium with the diets on the three locations along southward migration (mean SE). The d13C
values were calculated in two ways and shown in two columns: obtained from Sanderling blood (True) and a calculated value with help of d13C values of prey and a discrimination factor (Calc.). The results of the two methods did not differ significantly (see t-test in last column and Data S1). Bold
values were used in the model.
*See Data S1 for details about indirect calculations of the d13C signal of Sanderlings. DiF = discrimination factor.
†
Based on blood of Sanderlings caught in northeast Greenland.
‡
North Atlantic staging area.
§
Blood of Sanderlings caught in Wadden Sea in summer with d13C values of plasma and RBC that both represented the staging location (d13Cplasma
minus d13CRBC <023&). These birds were suspected to have just arrived in the Wadden Sea after using a staging area somewhere in the North
Atlantic.
–
Blood of Sanderlings caught in September in the Wadden Sea with d13C values of plasma and RBC that both represented the Wadden Sea
(d13Cplasma minus d13CRBC <023&).
k
See Data S2 for calculation of the turnover rate of d13C in plasma and RBC of Sanderlings.
analysis was drawn from the brachial vein into heparinised capillaries.
Note that second calendar year Sanderlings cannot be distinguished
from older Sanderling based on their plumage after their first basic
moult in spring (Prater, Marchant & Vuorinen 1977; Lemke, Bowler &
Reneerkens 2012). Immediately after sampling, the blood was centrifuged in Eppendorf cups in a haematocrit centrifuge (microfuge
Sigma 1–13, 6 min on 5000 rpm). Plasma and RBC were pipetted in
separate glass vials and stored in a freezer ( 20 °C) until analysis.
The Sanderling data set serves all conditions for the ‘double diet
switch model’, as described in Table 1: (i) Stable carbon isotope analysis were performed on plasma and RBC of Sanderlings caught in the
Wadden Sea. (ii) The d13C values of plasma and RBC of Sanderlings
differed between all three locations along the migration route (Table 2).
North Atlantic staging areas were assigned based on eight re-sightings
of colour-ringed Sanderlings (2007–2014) recorded within the same season of southward migration at both a North Atlantic staging area and
the Wadden Sea (Fig. 1). The isotope values of Sanderling’s RBC and
plasma at locations A and C were obtained from Sanderling blood
samples, whereas the isotope values of RBC and plasma at the North
Atlantic staging location (location B) were estimated via prey tissues
and a discrimination factor (see Data S1). (iii) The turnover rates for
plasma (kplasma = 0303 0033 SD) and RBC (kRBC = 0
056 0012 SD) were estimated for an average adult Sanderling (see
Data S2). (iv) The tissue sampling dates of all Sanderlings captured in
the Wadden Sea were known. (v) There is no indication for non-uniformity in diet between individual Sanderlings under any of the three diets.
Besides, it is unlikely that individual diet specialisation alters the average stable isotope signature of the diet, because we took all important
prey species into account, intra-diet variation was within the limits of
inter-diet variation, and the consumed prey species differed between
the three sites. (vi) Samples were collected in the period shortly after the
mean arrival period in the Wadden Sea. The ten samples that were collected in late summer, some weeks after the arrival period, indeed
showed that the majority of these birds were already adapted to the
Wadden Sea diet (Fig. 2).
Figure 2 shows the predictions of the ‘double diet switch model’
for Sanderlings with different staging durations. The steepness of
the slopes of the model predictions increases with turnover rate of
the tissue, showing that plasma d13C values (dashed lines) adapt
more quickly to the new diet than RBC d13C values (solid lines).
The model is based on the combined differences in values for
d13Cplasma, d13CRBC and the difference between plasma and RBC
isotope values (d13Cplasma minus d13CRBC) over time (tB and tC).
Therefore, the seasonal schedule of an individual Sanderling can
be predicted using a single time point measurement of the stable
isotopic value of two tissues. Birds with an Arctic isotopic value
in both RBC and plasma are still in equilibrium with the Arctic
diet and must have flown directly to the Wadden Sea. Birds with
a very short staging period in the North Atlantic staging area
and recently arrived in the Wadden Sea will also show a predominantly Arctic signature. Birds with Wadden Sea isotopic values in
both RBC and plasma are birds that have been long enough in
the Wadden Sea for both tissues to achieve equilibrium with the
Wadden Sea diet. We expect that the ‘double diet switch model’
cannot assign a staging duration to Sanderlings that are already
isotopically resident in the Wadden Sea (cf. Hobson 1999). Birds
with intermediate values might have been in the Wadden Sea for
some time, but not long enough to be in equilibrium with the
Wadden Sea diet, and/or may have staged in the North Atlantic
region.
Note that migratory flights from the Arctic breeding area in
Greenland to the Wadden Sea, which we expect to last approximately 2 days (65 km h 1 ground speed for the whole flight of
approximately 2850 km; Zwarts et al. 1990), are not taken into
account in the model. Although this could potentially affect the
biological interpretation of departure dates from the Arctic, the
time in flight is short in comparison with the mean error term of tB
(91 days, N = 52). We assumed (i) that a diet switch started upon
arrival at a new location and (ii) uniform isotopic diets in the three
reference areas are representative for the different regions (Arctic,
© 2016 The Authors. Methods in Ecology and Evolution © 2016 British Ecological Society, Methods in Ecology and Evolution
Using isotopes to infer seasonal schedules
5
Fig. 2. Predicted changes in d13C values in plasma and RBC of Sanderlings with different staging durations during southward migration. The
horizontal bars for plasma (light grey) and RBC (dark grey) represent
d13C values in equilibrium with diets used in the Arctic breeding area,
the North Atlantic staging area and in the Wadden Sea. The isotopic
changes of d13Cplasma (dashed lines, turnover rate of 0303) and
d13CRBC (solid lines, turnover rate of 0056) are given for staging durations of 0, 5, 10 and 20 days. Black lines show a migration without a
stopover. Green lines show migrations with a stopover in the North
Atlantic staging area, with colour-darkness corresponding with ascending staging durations.
STABLE ISOTOPE ANALYSIS
Fig. 1. Arctic breeding areas (white), North Atlantic staging areas
(striped) and the Wadden Sea (black) used by Sanderlings visiting the
Wadden Sea in late summer. Known wintering areas are shown in dark
grey, but the Wadden Sea area (black) is a wintering area too. The
coastal North Atlantic staging areas were determined based on observations of eight colour-ringed Sanderlings (white dots in striped area)
that were observed in the Dutch Wadden Sea a few days later.
North Atlantic staging areas and Wadden Sea) used by Sanderlings
during southward migration to the Wadden Sea. Note also that
output dates were reconstructed from termination of ‘day of the
year’ of 2011, since most birds were caught in that year, while the
day of the year differs 1 day between 2011 and 2012.
To evaluate the seasonal schedules of Sanderlings estimated by our
‘double diet switch model’, we compared our model data with observation data of seasonal schedules of Greenlandic breeding Sanderlings
migrating southwards. In 2007–2014, Sanderling nests were annually
searched for in northeast Greenland (Reneerkens et al. 2014). Dates of
hatch were often exactly known or, in case of clutch predation, estimated based on egg flotation (Hansen et al. 2011). For families found
post-hatch, a body mass growth curve based on local data was used to
estimate the hatching date. In total we determined hatching dates of
417 clutches and broods (annual range 25–77). The timing of southward migration of Sanderlings was determined based on sightings of
individually colour-ringed birds. More than 5600 Sanderlings were
individually marked in 12 countries produced over 58 000 unique
observations along the East Atlantic flyway collected by us and many
volunteers. This data set was used to extract information of birds
sighted in the North Atlantic region and the Wadden Sea within the
same season of southward migration.
All bird plasma, RBC and prey items were stored at 20 °C before
analysis. The samples were freeze-dried before grinding them with a
mortar and pestle. We used a microbalance (Sartorius CP2P) to weigh
04–08 mg of the sample material in 5 9 8 mm tin capsules. The d13C
values were determined with a Thermo Flash 2000 elemental analyser
coupled to a Thermo Delta V isotope ratio mass spectrometer. Isotope
values were calibrated to a laboratory acetanilide standard (d13C
261& calibrated on NBS-22) and corrected for blank contribution.
72% of the plasma and RBC samples were analysed in duplicate. The
results are reported on the per mill scale with respect to Vienna Pee Dee
Belemnite [VPDB]. The replicate error on the standard, acetalinide,
ranged between 003 and 008, using one standard every 43 to 7 bird
samples.
ELIMINATION OF BIRDS OVERSUMMERING IN THE
NORTH ATLANTIC REGION
Out data set on stable isotope profiles appeared to contain Sanderlings
that probably over-summered in the North Atlantic ‘staging area’ and
did not migrate to the Arctic tundra. The estimated staging duration of
these individuals was so exceptionally long that if they would have
arrived from the Arctic they would have had to depart unrealistically
early (as early as 14 May, when Sanderlings are still on northward
migration to the Arctic). The ‘double diet switch model’ cannot eliminate birds that over-summered in the North Atlantic, but simply predicts that these birds have exceptionally long staging durations. To
eliminate the birds that may have over-summered in the North Atlantic, we excluded birds with a d13CRBC that fell within or was higher than
the d13C of the North Atlantic staging area and also had a d13Cplasma
© 2016 The Authors. Methods in Ecology and Evolution © 2016 British Ecological Society, Methods in Ecology and Evolution
6 J. Jouta et al.
that was still not yet adapted to the Wadden Sea diet (7 birds; see
Fig. 3a).
Results
The d13C values of RBC and plasma of Sanderlings caught in
the Wadden Sea varied from 2432&, which is close to a signature of bird’s blood in equilibrium with a diet on the Arctic
terrestrial arthropods, to 135&, which is a signature for
bird’s blood in equilibrium with the Wadden Sea diet (Fig. 3a).
Whereas most birds captured in late summer showed Wadden
Sea diet type isotopic values in both RBC and plasma, birds
captured in the main arrival period (23 July to 2 August)
showed a variety of patterns ranging from almost purely Arctic
signatures, North Atlantic isotopic signatures, intermediate
isotopic values, to Wadden Sea diet signatures (Fig. 3a).
Fig. 3. d13C values of Sanderlings caught in the Wadden Sea after southward migration and their corresponding estimated staging duration along
North Atlantic coasts. For clarity, individuals are sorted along the X-axis according to raw d13C values. Depicting individuals in chronological order
of arrival caused many overlaying points because multiple birds were mist-netted per day. Birds in the yellow bar were caught in late summer and
represented separately to show the high number of birds that are adapted to the Wadden Sea diet in late summer. (a) Measured values of d13Cplasma
(triangles) and d13CRBC (dots) of all 65 individual Sanderlings. Although the model was able to fit a tB and tC for all birds, only birds that had been in
the Arctic breeding area, indicated with black symbols, were taken into account for further interpretations (N = 52). Birds with a d13CRBC within or
above the d13C of the diet of the North Atlantic staging area and a d13Cplasma that was not already adapted to the Wadden Sea (red symbols) were
considered to have oversummered and not used for further interpretations of the migration schedule of Sanderlings. For individuals that were
already resident to the Wadden Sea (Late summer, N = 6, Table 1), the model could (and should) not fit tB and tC. (b) The staging duration of all
individuals not yet adjusted to the Wadden Sea diet (N = 59) as calculated by the ‘double diet switch model’. The confidence limits of the staging
duration (tB) for each individual bird are expressed with standard deviation bars. Again, red symbols indicate birds that likely oversummered in the
North Atlantic staging area and therefore were left out for further interpretations of the migration schedule (N = 7). For visualisation, we distinguished between birds caught in the main arrival period in summer (23 July–2 August) and birds caught after the main arrival period in late summer
(20 August–1 September; yellow bar) in the graphs.
© 2016 The Authors. Methods in Ecology and Evolution © 2016 British Ecological Society, Methods in Ecology and Evolution
Using isotopes to infer seasonal schedules
Based on the ‘double diet switch model’ we assessed the individual seasonal schedules of the Sanderlings (Fig. 3). Sanderlings had a wide range of migration strategies with staging
periods along North Atlantic coasts ranging from 22 to
376 days (Fig. 3b). Sanderlings departed from the Arctic on
average on 13 July (range: 9–17 July, N = 52, Fig. 4), to arrive
in the Wadden Sea on 1 August (31 July–1 August, N = 52,
Fig. 4). When we include the seven birds that over-summered
in the North Atlantic staging areas, the mean arrival date
remained 1 August (range: 31 July–1 August, N = 59, Fig. 4).
Departure dates from the Arctic and arrival dates in the Wadden Sea for all individual birds are presented in Fig. 4b.
Discussion
Here we developed a new inferential statistical tool to estimate
the timing of movements between distinct habitats on the basis
of chemical markers in animal tissues. Ecological forensic
problems by their nature are particular and specific, and for
this reason we will discuss the Sanderling case before zooming
out to the wider range of situations to which our new tool can
be applied.
7
Interestingly, with the help of the ‘double diet switch model’,
we are the first to describe the timing of southward migration
of Sanderlings. Our results shows that Sanderlings that spend
the summer in the Arctic, as well as those which over-summered in the North Atlantic, arrive simultaneously in the Wadden Sea, matching the main arrival date obtained by
observations (Loonstra, Piersma & Reneerkens 2016). As surmised by Reneerkens et al. (2009), the ‘double diet switch
model’ revealed that Sanderlings show large temporal variation in the autumn migration schedules. Contrary to the work
of Dietz et al. (2010) who, with the help of a ‘single diet switch
model’ found that Red Knots Calidris canutus do not stage in
the North Atlantic during southward migration, we show that
Sanderlings stage for variable lengths of time in the North
Atlantic before moving on the Wadden Sea. The mean staging
duration in coastal areas between Greenland and the Netherlands of southward migrating Sanderlings was estimated to last
186 days. The mean departure date from the Arctic was estimated as 13 July. This coincides with the mean hatching date
in northeast Greenland (13 July). The majority of clutches fails
due to depredation (Reneerkens et al. 2014) and Sanderlings
often leave their partner with the care of eggs (Reneerkens
(a)
(b)
Fig. 4. Migration schedule of Sanderlings, shown as departure dates from the Arctic and arrival dates in the Wadden Sea. (a) The distribution of the
departure date from the Arctic (thick line) and arrival date in the Wadden Sea (thin line), for birds that likely arrived from the Arctic breeding area
and thus completed the entire migration (N = 52). The mean departure date from the Arctic is 13 July, the mean arrival date in the Wadden Sea is 1
August. (b) Individual migrating schedules of all 59 Sanderlings with the estimated departure date from the Arctic (filled dots) and the arrival date in
the Wadden Sea (open circles), both given as mean SD. Black symbols represent birds that likely arrived from the Arctic (N = 52), whereas red
symbols represent birds that likely over-summered in the North Atlantic (N = 7). Grey and white alternating zones refer to months. Bird ID shown
on the Y-axis of this figure, correspond with Bird ID of Fig. 3.
© 2016 The Authors. Methods in Ecology and Evolution © 2016 British Ecological Society, Methods in Ecology and Evolution
8 J. Jouta et al.
et al. 2011). When clutches are incubated by two adults, one of
the partners always leaves the other parent with the chicks, as
soon as they hatch (Reneerkens et al. 2014). This would
explain the early departures from the Arctic tundra by the
majority of assayed birds. The seven individuals that seemed to
have over-summered in the North Atlantic were most likely
second calendar year birds (Summers, Underhill & Pr^
ys-Jones
1995). The proportion over-summering Sanderlings in the
North Atlantic (12%) is comparable to an earlier study by
Lemke, Bowler & Reneerkens (2012) who estimated the percentage of juveniles in a wintering population in Scotland to be
6–9%.
At time of our isotope analyses, it was not common practice to use lipid-free tissues. It is clear now that lipids may
influence isotopic values substantially, also in blood tissue
(e.g. Rode et al. 2016). Specifically, high lipid contents in tissue biases d13C values downwards, whereas lipid contents
may vary between individual and tissue type. Although our
case study with Sanderlings clearly demonstrates the applicability of the double diet switch model, the estimated migration schedule may be biased for not using lipid-free tissues.
To explore this possible bias, we corrected for lipid contents
following the method of Post et al. (2007), who suggested to
use C:N ratios of the sampled tissue to correct for lipid contents by adding a correction term to the estimated d13C values, and we reran the model with the ‘lipid-free’
approximate d13C values (of all tissues, from Sanderlings
and prey). Using the ‘lipid-free’ data, the model did not converge for 14 birds (while all 59 birds converged when using
incorrected values), indicating that corrections were inconsistent with the model. Using the approximated ‘lipid-free’ data
of the remaining birds, resulted in an estimated departure
date from the Arctic that was later than when using uncorrected data (24 July [CI 20–26 July], rather than 13 July), a
shorter estimated staging duration (101 days [CI 76–149],
rather than 186 days), but a similar arrival date in the
Wadden Sea (31 July [29 July–2 August] compared with 1
August) (N = 45). The model estimates using the ‘lipid-free’
data matched better with our expectations on the timing of
southward Sanderling migration.
As it is likely that Sanderlings show moderate intraspecific
variation, we used distributions of the input parameters rather
than the mean values, for two reasons. First, individual dietary
preferences cause stable isotopic values to vary slightly among
individuals. Moreover, the discrimination factor that may be
used to distinguish between diet and consumer may vary
between individuals as well (Data S1; Caut, Angulo & Courchamp 2009). Second, intraspecific variation in turnover rates
is rather large and poorly understood (Martınez del Rio et al.
2009; Hahn et al. 2012). More accurate information about
intraspecific variation in turnover rates is needed for more
accurate estimations of individual seasonal scheduling. As the
conditions for using the ‘double diet switch model’ can be met
rather easily on the basis of a single time point stable isotope
measurement of the target species (Table 1), the ‘double diet
switch model’ allows a relatively simple way to assess seasonal
schedules.
Fig. 5. A special case of the double diet switch model, the ABBAswitch. This is a simplified representation of a ‘switch-switching back’
situation, from diet A1 to B and from B back to A2 describing how the
isotopic values of two tissues, one with a fast turnover rate (striped
black line) and a slow turnover rate (solid black line), adapt from diet
A to(wards) diet B back to(wards) diet A. The two grey lines (line A
and B) represent the isotopic signature of the tissue in equilibrium with
the two diets. The two arrows indicate the time of the two diet switches.
We encourage future use of our model for estimation of
seasonal schedules of animals and emphasise that other isotopes than carbon can also be used (e.g. nitrogen or sulphur).
The ‘double diet switch model’ might be particularly interesting in deciphering the timing and occurrence of migration in
other migratory animals, animals with changes in food availability during a season (e.g. an animal that follows the food
peak of different prey species), or in the timing of ontogenetic development of animals (e.g. from egg to juvenile to
adult). Although not tested here, the ‘double diet switch
model’ might not be limited to studies with switches between
three isotopic levels, i.e. with diet switches from diet A to B
to C, but might also be applicable to scenarios where the second switch reverses to the initial isotopic level, so a double
diet switches from diet A1 to B and from B back to A2. We
call this an ‘ABBA switch’ (see Fig. 5). An ABBA switch
may occur under temporary changing conditions such as,
e.g. breeding, drought, frozen foraging surfaces (no access to
regular food) or injuries of the animal that restricts regular
prey consumption. The ABBA switch could, theoretically, be
studied with the regular formula of the ‘double diet switch
model’ (see eqn. 2), where diet A2 can be interpreted in the
model as diet C. The model is thus generally applicable, and
can be adapted to a wide range of taxa and situations in
which animals use two or three distinct diets within a short
period of time.
Authors’ contributions
J.J., I.P., M.D. and E.R. conceived the ideas and designed methodology; J.J., J.R,
G.H. and M.D. collected the data; J.J., M.D., I.P., J.R. and T.P. analysed the
data; J.J. (and MR and IP) led the writing of the manuscript. All authors contributed critically to the drafts and gave final approval for publication.
Acknowledgements
We thank Bernard Spaans, Allert Bijleveld, Anne Dekinga and others for
helping us catch birds in the Wadden Sea. We are grateful to the crews of
© 2016 The Authors. Methods in Ecology and Evolution © 2016 British Ecological Society, Methods in Ecology and Evolution
Using isotopes to infer seasonal schedules
the RV Navicula (NIOZ), RV Stern (NIOZ) and MS Phoca (Dutch ministry
of Economic Affairs) for bringing us to the catching location. J.R. thanks the
Zackenberg logistical team at the Department of Bioscience–Roskilde, Aarhus
University, for providing logistics at the research station at Zackenberg,
northeast Greenland. Obeying the Dutch laws, field work was carried out
under animal welfare (DEC) protocol NIOZ-10.04 amendment 1. Funding
from World Wildife Fund (the Netherlands) and INTERACT (project
INTERPRED) under the European Community’s Seventh Framework Programme (grant number 262693) to J.R. and an International Polar Year
grant (NWO) to T.P. and J.R., made the fieldwork in Greenland possible.
We thank Stefan Schouten and Kevin Donkers for help with the isotope
analysis. We thank the reviewers for their valuable contributions that
improved our manuscript. This study was carried out within the projects
‘Waddensleutels’ (WF203930, J.J. and T.P.) and ‘Metawad’ (WF209925, J.R.,
E.R. and T.P.), both funded by Waddenfonds.
Data accessibility
Data are deposited in the Dryad Repository http://dx.doi.org/10.5061/
dryad.t72b0 (Jouta et al. 2016).
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Received 1 June 2016; accepted 10 October 2016
Handling Editor: John Reynolds and Robert Freckleton
Supporting Information
Additional Supporting Information may be found online in the supporting information tab for this article:
Data S1. Estimating diet-specific d13C values.
Data S2. Estimating d13C turnover rates in Sanderlings.
Data S3. R-scripts.
© 2016 The Authors. Methods in Ecology and Evolution © 2016 British Ecological Society, Methods in Ecology and Evolution