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Migrating Songbirds Recalibrate
Their Magnetic Compass Daily
from Twilight Cues
William W. Cochran,
1
* Henrik Mouritsen,
2
Martin Wikelski
3
Night migratory songbirds can use stars, sun, geomagnetic field, and polarized
light for orientation when tested in captivity. We studied the interaction of
magnetic, stellar, and twilight orientation cues in free-flying songbirds. We
exposed Catharus thrushes to eastward-turned magnetic fields during the twi-
light period before takeoff and then followed them for up to 1100 kilometers.
Instead of heading north, experimental birds flew westward. On subsequent
nights, the same individuals migrated northward again. We suggest that birds
orient with a magnetic compass calibrated daily from twilight cues. This could
explain how birds cross the magnetic equator and deal with declination.
Billions of songbirds migrate between conti-
nents twice each year, but their orientation ca-
pabilities are almost exclusively studied in the
laboratory. We presented birds with experimen-
tally altered orientation cues and followed their
subsequent migratory flights in the wild. Avian
navigation capabilities are very precise (1), with
many individuals returning to the same breed-
ing sites year after year (1–3) after a voyage of
up to 25,000 km (4,5). Migratory songbirds can
orient on the basis of compass information from
the sun and its associated polarized light pat-
terns (6–9), the stars (10–12), the earth’s mag-
netic field (13,14), and the memorization of
spatial cues en route (15,16). However, the
interactions and relative importance of these
cues remain unclear and a source of much
debate (7,17–18). Our knowledge about the
orientation mechanisms of songbirds relies al-
most exclusively on data from cue-manipulated
captive migrants tested in various orientation
cages, on vanishing bearings based on the first
few hundred meters of flight (19,20), and to a
much lesser degree on field data (ringing and
radar and visual observations) from unmanipu-
lated natural migrants (7,15,21).
Testing five hypotheses. On clear eve-
nings, we fitted Catharus thrushes with radio
transmitters and placed them in outdoor cages
in an artificial eastward-turned magnetic field
from about sunset until the sun was 11° or more
below the horizon when they were set free (22).
We then radio-tracked (22–28) them in flight to
obtain heading data. Because Catharus thrushes
do not compensate for wind drift but individu-
als maintain nearly constant preferred headings
from night to night (26), we used measured
headings for orientation analyses (22).
Our data on the birds’ headings enabled us
to distinguish between the five most likely mi-
gratory orientation hypotheses: Catharus
thrushes use either (i) a magnetic compass or
(ii) a star compass, which dominate over all
other cues [e.g., (19,20,29,30)]. Alternatively,
(iii) Catharus thrushes may use twilight cues
provided by the sunset direction and/or the
associated polarization patterns (when avail-
able) for selecting a migratory direction and the
stars (when available) for maintaining that di-
rection during nocturnal flight (7), that is, a star
compass calibrated daily from twilight cues. As
another possibility, (iv) the birds could use twi-
light cues (when available) for selecting a mi-
gratory direction and a geomagnetic compass
for maintaining that direction during nocturnal
flight (23), that is, a magnetic compass calibrat-
ed daily from twilight cues. Finally, (v) Catha-
rus thrushes could use magnetic cues for select-
ing a migratory direction during the twilight
period and a star compass or dead-reckoning
strategy for maintaining that direction during
nocturnal flight (19,31), in other words, a
visual compass calibrated daily from magnetic
cues. This hypothesis becomes particularly
plausible if birds can only sense the geomag-
netic field when sufficient amounts of light are
available (32,33). Figure 1, A to C, illustrates
the predicted outcomes of our experiment based
on each of the five hypotheses.
Treatment shifts free-flight direction.
Natural migrating gray-cheeked thrushes, C.
minimus, show very consistent northerly head-
ings (mean ⫽357°, r⫽0.973, P⬍0.001, and
95% and 99% confidence intervals for mean of
268° to 320° and 260° to 328°, respectively).
Birds taking off after being exposed to clockwise
changes in the magnetic field (70° to 90°, mean ⫽
77°, r⫽0.990, and n⫽8) headed in westerly
directions (mean ⫽294°, r⫽0.871, n⫽8,
P⬍0.001; 95% and 99% confidence interval
for mean 268° to 320° and 260° to 328°, respec-
tively). These headings are in very close agree-
ment with the mirror-image directions (mean of
283°) predicted if the birds had calibrated a
magnetic compass from sunset-related twilight
cues (Fig. 1C). The difference in headings be-
tween the magnetically treated birds and the nat-
ural migrants is highly significant (compare col-
umns 1 and 3 in Fig. 1D) {Watson-Williams test:
F⫽54.41, F
1,30[␣⫽0.001(2)]
⫽15.2, P⬍0.001}.
Five gray-cheeked thrushes that were exposed to
the changed magnetic field did not migrate the
night they were treated, but flew to nearby woods,
and, 1 to 6 nights later (controls, Figs. 1D, column
4, and 2A), headed in the normal northerly migra-
tory direction (mean ⫽358°, r⫽0.972, n⫽5,
P⬍0.01, and 95% and 99% confidence inter-
vals for mean of 341° to 16° and 336° to 21°,
respectively). Their headings were significantly
different from the birds that took off during the
night of treatment (compare columns 3 and 4, Fig.
1D) {F⫽16.52, F
1,11[␣⫽0.005(2)]
⫽15.2, P⬍
0.005; 99% confidence intervals do not overlap}
but not different from the headings of the natural
migrants (compare columns 1 and 4, Fig. 1D)
{F⫽0.07, F
1,27[␣⫽0.50(2)]
⫽1.38, P⫽0.79}.
Because control birds were in the wild during
the evening before their departure, they experi-
enced the normal unchanged geomagnetic field
during the last twilight period before departure.
We therefore suggest that gray-cheeked thrush-
es recalibrate their magnetic compass from twi-
light cues on a daily basis.
The wide spread of headings (west through
north to northeast, Fig. 1E, column 1) of natu-
rally migrating Swainson’s thrushes, C. ustula-
tus, meant that we had to use each individual
bird as its own control. Therefore, in addition to
headings obtained on the night of experimental
release, we also measured headings for the
same Swainson’s thrushes on at least one sub-
sequent night of migration. All of these second
(control) flights occurred under clear skies that
provided solar, twilight, as well as stellar cues.
Headings of naturally migrating individual
Swainson’s thrushes followed during several
consecutive migratory flights (26) (Figs. 1E,
column 4, and 2B and fig. S2) were very con-
sistent between nights (first night’s heading
relative to second night’s heading: mean ⫽
358°, r⫽0.990, n⫽5, P⬍0.002, and 95%
and 99% confidence intervals for mean of 348°
to 8° and 345° to 12°, respectively). In contrast,
headings of Swainson’s thrushes followed on
the night when they had been exposed to clock-
1
Illinois Natural History Survey, 607 East Peabody
Drive, Champaign, IL 61820, USA.
2
Volkswagen Nach-
wuchsgruppe Animal Navigation, Institute of Biology,
University of Oldenburg, D-26111 Oldenburg, Ger-
many.
3
Department of Ecology and Evolutionary Bi-
ology, Guyot Hall 303, Princeton University, Prince-
ton, NJ 08544, USA. E-mail: Sparrow@springnet1.com
(W.W.C.); Henrik.mouritsen@uni-oldenburg.de (H.M.);
Wikelski@princeton.edu (M.W.)
*Present address: 1204 West Union Street, Cham-
paign, IL 61821, USA.
RESEARCH ARTICLES
www.sciencemag.org SCIENCE VOL 304 16 APRIL 2004 405
wise changes in the magnetic field toward the
east (70°to 90°, mean ⫽86°,r⫽0.991, and
n⫽5) were oriented in directions turned 55°to
103°counterclockwise toward the west relative
to their heading during the following migratory
flight (first night’s heading relative to second
night’s heading standardized to 0°: mean ⫽
281°,r⫽0.950, n⫽5, P⬍0.005, and 95%
and 99% confidence intervals for mean of 258°
to 304°and 251°to 311°, respectively) (Figs.
1E and 2B). The headings on the first night
relative to the second night are significantly
different between the treated and the nontreated
birds {F⫽57.49, F
1,8[␣⫽0.001(2)]
⫽31.6, P⬍
0.001; 99% confidence intervals do not overlap;
compare columns 3 and 4 in Fig. 1E}, and the
headings of the treated birds are significantly
different on the night of treatment than on
following nights [P⬍0.01, because 99% con-
fidence interval (251°to 311°) for orientation
on the first night relative to the second night
does not include 0°]. The results fit the predic-
tions (Fig. 1C) only if Swainson’s thrushes used
a magnetic compass that was calibrated from
sunset-related twilight cues.
Magnetic cue during flight. The fact
that the birds’headings in the treated groups
were deflected implies that the magnetic field
was sensed by the birds while stationary in a
cage (for the calibration) and also in flight
after departing from the coil system (for en
route orientation). We therefore suggest that
Catharus thrushes use their geomagnetic
compass not only before takeoff (7) but also
as the primary cue during nocturnal migrato-
ry flight after takeoff (23,34). Our experi-
mental Catharus thrushes continued flying in
westerly (inappropriate) directions in spite of
their opportunity to reorient on the clearly
visible stars. Thus, our experimental birds
seem to have either ignored the stars as an
orientation cue altogether, or they may have
calibrated them from the magnetic field after
takeoff. Our data suggest that the time it takes
Catharus thrushes to determine a magnetic
compass direction while aloft must be rela-
tively short, because we recorded several
headings of treated birds within a few min-
utes after takeoff and all of these headings
were already deflected. Furthermore, if mag-
netoreception in Catharus thrushes is light-
dependent (32,33), the amount of light need-
ed for magnetic orientation was minimal: At
our release site, light available at the time of
release on starlit nights was measured to be
0.0003 to 0.002 lux. Considering the high
homogeneity of our experimental magnetic
fields (22) and the symmetrical nature of any
minor artifacts, it is highly unlikely that our
results could be caused by systematic mag-
netic map effects (22,35).
Preflight solar calibration. Our results
indicate that free-flying, naturally migrating
Swainson’s and gray-cheeked thrushes use a
magnetic compass as primary orientation
Fig. 1. (Ato C) Predicted orientation responses of birds with respect to potential orientation mechanisms and
their interactions. (Dand E) The actual orientation responses of free-flying gray-cheeked and Swainson’s
thrushes, respectively. (A) If the stars or the magnetic field show simple domination, or if the stars are
calibrated from twilight cues, there should be no effect of the magnetic treatment after release (third and
fourth column). (B) If birds use their magnetic compass to calibrate a celestial compass at sunset, they should
head east on the first night after treatment and then in the normal northerly migratory direction. (C) If
twilight cues are used to calibrate the magnetic compass on a daily basis, birds experiencing a magnetic field
turned toward the east should orient toward the west during the same night after release. On subsequent
nights they should return to their normal northerly migratory direction. (A to C) The four thin parallel arrows
indicate the horizontal direction of the magnetic field lines experienced by the birds. The thick arrow indicates
the expected orientation of the birds. The star indicates the unchanged directional information potentially
available from celestial cues. (D) Each dot at the circle periphery indicates the measured true heading of one
free-flying gray-cheeked thrush during natural migration (left column), after treatment (middle column), or
after flying to nearby woods after treatment and migrating on a subsequent night (right column). The arrows
indicate the length and direction of the group mean vectors. The inner and outer dashed circles indicate the
radius of the group mean vector needed for significance (P⬍0.05 and P⬍0.01, respectively) according to
the Rayleigh test (42). mN, true magnetic north. (E) Each dot at the circle periphery indicates the heading of
one free-flying Swainson’s thrush during natural migration (left column) on the night of treatment, relative
to its heading on its subsequent flight (standardized to 0°) from an unmanipulated environment (middle
column), or during natural migration, relative to its heading on its subsequent natural night’s flight,
standardized to 0° (right column). Open dots indicate four additional relative headings of one individual
followed for 6 nights, but only the first data point for that individual was used for statistical analysis to avoid
pseudoreplication. Relative mN is magnetic north on first night relative to magnetic north on second night.
Note that the directional spread in natural headings in (D) is narrow (column 1), thus control birds (column
4) suffice to document an effect of magnetic treatment (column 3), whereas in (E), Swainson’s thrushes show
wide variation in natural migratory headings (column 1), forcing us to use each individual as its own control
during its subsequent migratory flight initiated under natural conditions (column 3).
RESEARCH ARTICLES
16 APRIL 2004 VOL 304 SCIENCE www.sciencemag.org406
mechanism in flight (28). This magnetic
compass, however, does not seem to be based
on a fixed magnetic heading relative to mag-
netic north. Instead, the magnetic heading
used during migration seems to be calibrated
relative to the solar azimuth during the sunset
and/or twilight period. A calibration could be
accurate within a degree or so for several
days because solar twilight azimuths change
slowly with time. Thus, well-oriented flights
initiated after overcast days (23,26) could be
guided by a previously calibrated magnetic
compass. The calibrating twilight cue could
be the setting sun itself and/or the polarized
light patterns in the overhead sky. Overhead
polarized skylight is the most likely cue be-
cause it seems to be sufficient and preferred
over the sun itself in several orientation cage
experiments (7,9,36 –38).
Discussion. Birds calibrating their mag-
netic compass from a solar azimuth reference
would be unsusceptible to changes in declina-
tion, which can be as much as –20°to ⫹30°
within the North American continent. Similar-
ly, breeding in a magnetic anomaly would be
unproblematic for the birds’orientation. A
twilight-calibrated magnetic compass could
also explain one of the enigmas of bird migra-
tion, namely how migratory songbirds, known
to have a magnetic inclination compass (13,
14), can cross the magnetic equator without
becoming disoriented (34,39). Birds using a
sunset-calibrated magnetic compass are predict-
ed to follow curved tracks because the sunset
direction varies up to ⫾25°with latitude and
time of year. Such a feature may prove prob-
lematic to some species and advantageous to
others. The clockwise changes implicit with the
twilight-calibrated magnetic compass in spring
will in general produce a crescent route through
Central America or over the Caribbean Sea and
Gulf of Mexico, avoiding the more direct but long
over-water trans-Atlantic route. It is unclear
whether a twilight-calibrated magnetic compass
would possess similar adaptive advantages for
migration from Africa to Europe or elsewhere.
Differences in orientation mechanisms of
birds between continents could be one reason why
our findings are inconsistent with the results of
many orientation funnel experiments, which sug-
gest that migrants rely on a dominating uncali-
brated magnetic compass or on a magnetically
calibrated stellar (or other visual) compass for
orientation [e.g., (19,29,31,40,41)]. It is also
conceivable that birds use different cue calibra-
tions under different ecological scenarios. For ex-
ample, a pure reliance on the geomagnetic field
without reference to the sun when facing a major
ecological barrier could explain dissimilar results
for Swainson’s thrushes that were tested in orien-
tation cages (Gulf of Mexico) (29). Alternatively,
our data allow the possibility that not all hypoth-
eses about orientation mechanisms deduced in
orientation cages can be generalized to free-flying
birds migrating under natural conditions, where all
cues are continuously available in their natural
form. The fact that birds orient in their appro-
priate migratory direction in orientation cages
even after months in captivity [e.g., (32)] sug-
gests that orientation cage results are valuable
for elucidating the basic orientation capabilities
of migratory birds. Cue-conflict experiments in
cages [e.g., (7,17–19,31,40)] show that birds
can, in principle, transfer information between
specific orientation cues. Nevertheless, migra-
tory restlessness in a cage is not identical to
natural mid-air flight, and orientation cages pro-
vide birds with simplified cue environments.
Particularly in complex cue-conflict experi-
ments, previous experience of the birds, non-
present but normally relevant cues, time spent
in captivity, and small variations in the experi-
mental setup could alter delicate interactions
between the cues. We therefore support Moore’s
(7) assertion that “it is essential that results of
orientation cage studies be interpreted in the light
of field observations of migratory behavior and
experiments with free-flying migrants.”We sug-
gest that the simple yet reliable twilight-calibrated
magnetic compass may be used by many other
species of night migratory birds in the wild.
References and Notes
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Fig. 2. Tracks of free-flying (A) gray-cheeked thrushes and (B) Swainson’s thrushes. Arrows indicate the
direction and ground track of flights if the flights were conducted under no-wind conditions (22). Data
are depicted differently in (A) and (B) because for gray-cheeked thrushes experimental and control birds
are different individuals, whereas in Swainson’s thrushes the same experimental individuals were
followed for at least two successive nocturnal migrations (because of the large spread in natural
headings) (Fig. 1). Connected arrows show flights of the same individual during successive nights.
Arrows depict natural migratory flights in black; experimental birds for which the magnetic field was
turned east before takeoff, red; subsequent night flights of experimental birds, yellow; and experimental
birds that did not migrate on the night of magnetic treatment but did so 1 to 6 days later, white. Broken
lines indicate that birds were lost during tracking at the site where the broken lines start.
RESEARCH ARTICLES
www.sciencemag.org SCIENCE VOL 304 16 APRIL 2004 407
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43. We are indebted to A. Raim for lifelong help; E. Gwinner for
inspiration; and G. Swenson, G. Swenson, and J. Brugge-
mann for help with light measurements and thank M.
Bowlin, N. Sapir, A. Medina, W. Cochran, J. Cochran, J.
Mandel, and S. David for their help, support, and under-
standing during this intensive project and W. Wiltschko for
constructive comments on the manuscript. Supported by
the National Geographic Society (M.W.), Princeton Univer-
sity (M.W.), Volkswagen Stiftung (H.M.), Oldenburg Uni-
versity (to H.M.) and NSF (GB 3155 and 6680 to W.W.C.).
Supporting Online Material
www.sciencemag.org/cgi/content/full/304/5669/405/
DC1
Materials and Methods
Figs. S1 and S2
Tables S1 and S2
20 January 2004; accepted 10 March 2004
Southern Ocean Iron Enrichment
Experiment: Carbon Cycling in
High- and Low-Si Waters
Kenneth H. Coale,
1
* Kenneth S. Johnson,
2
Francisco P. Chavez,
2
Ken O. Buesseler,
3
Richard T. Barber,
4
Mark A. Brzezinski,
5
William P. Cochlan,
6
Frank J. Millero,
7
Paul G. Falkowski,
8
James E. Bauer,
9
Rik H. Wanninkhof,
10
Raphael M. Kudela,
11
Mark A. Altabet,
12
Burke E. Hales,
13
Taro Takahashi,
14
Michael R. Landry,
15
Robert R. Bidigare,
16
Xiujun Wang,
1
Zanna Chase,
2
Pete G. Strutton,
2
Gernot E. Friederich,
2
Maxim Y. Gorbunov,
8
Veronica P. Lance,
4
Anna K. Hilting,
4
Michael R. Hiscock,
4
Mark Demarest,
5
William T. Hiscock,
7
Kevin F. Sullivan,
10
Sara J. Tanner,
1
R. Mike Gordon,
1
Craig N. Hunter,
1
Virginia A. Elrod,
2
Steve E. Fitzwater,
2
Janice L. Jones,
5
Sasha Tozzi,
8,9
Michal Koblizek,
8
Alice E. Roberts,
6
Julian Herndon,
6
Jodi Brewster,
1
Nicolas Ladizinsky,
1,6
Geoffrey Smith,
1
David Cooper,
1
David Timothy,
12
Susan L. Brown,
16
Karen E. Selph,
16
Cecelia C. Sheridan,
16
Benjamin S. Twining,
17
Zackary I. Johnson
18
The availability of iron is known to exert a controlling influence on biological produc-
tivity in surface waters over large areas of the ocean and may have been an important
factor in the variation of the concentration of atmospheric carbon dioxide over glacial
cycles. The effect of iron in the Southern Ocean is particularly important because of
its large area and abundant nitrate, yet iron-enhanced growth of phytoplankton may
be differentially expressed between waters with high silicic acid in the south and low
silicic acid in the north, where diatom growth may be limited by both silicic acid and
iron. Two mesoscale experiments, designed to investigate the effects of iron enrichment
in regions with high and low concentrations of silicic acid, were performed in the
Southern Ocean. These experiments demonstrate iron’s pivotal role in controlling
carbon uptake and regulating atmospheric partial pressure of carbon dioxide.
The Southern Ocean exerts a major control on the
partial pressure of carbon dioxide ( pCO
2
)inthe
atmosphere. Because rates of photosynthesis and
biological carbon export are low in Antarctic wa-
ters, macronutrients are largely unused, and up-
welled CO
2
entering the atmosphere (1,2) sus-
tains the relatively high interglacial atmospheric
pCO
2
of the present day (3).
Southern Ocean surface waters contain ex-
tremely low iron concentrations (4,5), and the
low rates of primary production have been at-
tributed to iron deficiency. Recent open-ocean
iron enrichment experiments demonstrate the
validity of this hypothesis in the Southern
Ocean (6,7). Martin (8) proposed that natural
variations in the atmospheric iron flux ultimate-
ly regulate primary production in the Southern
Ocean and influence the pCO
2
of the atmo-
sphere, thereby potentially affecting the radia-
tive balance of the planet. Syntheses of models,
field observations, and paleoceanographic data
(3,9,10,11,12) support a role for iron-
regulated changes in Southern Ocean macronutri-
ent use. Indeed there is a strong inverse correlation
between iron-rich dust, marine production, and
atmospheric pCO
2
over the past four glacial cy-
cles as recorded in Antarctic ice cores (13). These
observations support the “iron hypothesis”as pro-
posed by Martin (8), yet the magnitude of the iron
enrichment effect on marine production and atmo-
spheric pCO
2
remains uncertain.
Although all Southern Ocean surface waters
have high concentrations of nitrate and phos-
phate, silicic acid concentrations differ marked-
ly from north to south. Subantarctic waters
north of the Antarctic Polar Front Zone (APFZ)
have low Si concentrations (1 to 5 M), where-
as high Si (⬎60 M) is found to the south (fig.
S1). Diatoms, which require Si for growth, are
believed responsible for much of the carbon
export from the surface to the deep sea (14). In
1
Moss Landing Marine Laboratories, 8272 Moss Landing
Road, Moss Landing, CA 95039–9647, USA.
2
Monterey
Bay Aquarium Research Institute, 7700 Sandholdt Road,
Moss Landing, CA 95039, USA.
3
Department of Marine
Chemistry and Geochemistry, Woods Hole Oceano-
graphic Institution, Woods Hole, MA 02543, USA.
4
Nich-
olas School of the Environment and Earth Sciences, Duke
University, 135 Duke Marine Lab Road, Beaufort, NC
28516, USA.
5
Marine Science Institute and the Depart-
ment of Ecology, Evolution, and Marine Biology, Univer-
sity of California, Santa Barbara, CA 93106, USA.
6
Rom-
berg Tiburon Center for Environmental Studies, San
Francisco State University, 3152 Paradise Drive, Tiburon,
CA 94920–1205, USA.
7
Rosenstiel School of Marine and
Atmospheric Research, University of Miami, 4600 Rick-
enbacker Causeway, Miami, FL 33149–1098, USA.
8
En-
vironmental Biophysics and Molecular Ecology Program,
Institute of Marine and Coastal Sciences and Depart-
ment of Geology, Rutgers University, 71 Dudley Road,
New Brunswick, NJ 08901–8521, USA.
9
Virginia Insti-
tute of Marine Science, Route 1208 Greate Road,
Gloucester Point, VA 23062, USA.
10
Atlantic Ocean-
ographic and Meteorological Laboratory/National
Oceanic and Atmosphereic Administration, 4301
Rickenbacker Causeway, Miami, FL 33149, USA.
11
University of California at Santa Cruz, 1156 High
Street, Santa Cruz, CA 95064, USA.
12
School for Ma-
rine Science and Technology, University of Massachu-
setts, 706 South Rodney French Boulevard, New Bed-
ford, MA 02744–1221, USA.
13
College of Oceanic and
Atmospheric Sciences, Oregon State University Cor-
vallis, OR 97331, USA.
14
Lamont-Doherty Earth Obser-
vatory, Columbia University, 61 Route 9W, Palisades,
NY 10964–1000, USA.
15
Scripps Institution of Ocean-
ography, University of California, San Diego, 9500 Gil-
man Drive, La Jolla, CA 92093–0227, USA.
16
Depart-
ment of Oceanography, University of Hawaii at Manoa,
1000 Pope Road, Honolulu, HI 96822, USA.
17
State
University of New York, Stony Brook, NY 11794, USA.
18
Massachusetts Institute of Technology, 48-336A MIT,
15 Vassar Street, Cambridge, MA 02139, USA.
*To whom correspondence should be addressed. E-
mail: coale@mlml.calstate.edu
RESEARCH ARTICLES
16 APRIL 2004 VOL 304 SCIENCE www.sciencemag.org408
SUPPORTING ONLINE MATERIAL
Migrating Songbirds Recalibrate their Magnetic Compass
Daily from Twilight Cues
by
William W. Cochran, Henrik Mouritsen and Martin Wikelski
Materials and Methods
Experimental birds and magnetic field manipulations
The 30 to 42 g Catharus Thrushes we observed were netted during stopover in
wooded areas near Champaign-Urbana, Illinois, USA (c. 40.1 0N , 88.2 0W). Transmitters
(0.7-1.0 g, Sparrow Systems, Fisher, Illinois, USA) were attached to their backs using the
method of Raim (S1). Release at the net site provided data for naturally initiated
migratory flights (N = 44; S2, S3). Experimental birds (N=18) were placed in cages on
platforms centered within Helmholtz coils located in open fields providing an almost
completely unobstructed view of the horizon and sky.
Two coil-platform arrangements were used. Prior to 2003, 25 cm diameter x 25
cm (H) cages were mounted 2 m above ground inside 1 m diameter coil pairs. In 2003, a
four-compartment 60 cm (L) x 60 cm (W) x 30 cm (H) cage was placed 80 cm above
ground inside 2 m diameter coil pairs. Both arrangements used the same principles for
altering the horizontal magnetic field using a single Helmholtz coil pair (S4).
The plane of the coils was plumbed vertical so that there would be no change in
the vertical component of the geomagnetic field. The parallel coil planes were angled
with respect to magnetic north by half the desired experimental direction. With this
configuration, when the current is adjusted to give the experimental direction (measured
by a Suunto Professional Compass, Suunto, Helsinki, Finland; accurate to <0.5°), the
natural horizontal field is cancelled and a resultant horizontal field of equal intensity is
produced in the experimental direction (e.g. coils planes at 045° M with the proper
current will produce an 090° M (East) horizontal field). Precision current regulated power
supplies and regular checks and fine adjustments further assured that the desired field
was maintained. Since the space covered by the cages, and thereby the possible positions
of the birds, in both setups, remained within the central 60% of the radius of the coils, the
heterogeneities of all our artificial fields were <1% of the applied field strength, that is
<200 nT (S5). Thus, the variations in our artificial magnetic fields were no greater than
the natural daily variations of the Earth’s magnetic field, which are in the order of 200-
500 nT.
If, for unknown reasons, the birds would take note of any small irregularities in
the experimental magnetic fields, such variations (being inherently symmetric around the
center of the coil system) would be in opposite directions in different parts and/or
compartments of the cages. Thus, any possible influences on a hypothetical magnetic
map would have been in opposite directions for different individual birds and could
therefore not have led to the systematic orientation behavior of the birds (all going
approximately westward). In addition, there exists no convincing evidence from birds
showing that young or even adult experienced migrants can determine their east-west
position from any global cue (S6, S7).
Release Procedure
The eight releases prior to 2003 were made from a blind, using a string and pulley
arrangement to lift the cage slowly off the platform, thereby freeing the bird. Although
gently raised, cage movement appeared to initiate escape behavior in the May 16, 18, and
20, 1978 and the May 30, 1984 birds (Tables S1, S2). The other four pre-2003 birds
remained calm and flew some minutes after the cage was raised. The ten birds released
in 2003 were physically tossed vertically into the air from inside the coil system (S4).
Thus, in both cases, we can be sure that all birds (Tables S1, S2) embarked on their
experimental migratory flights directly from within the coil system and thus from within
the changed magnetic field. All releases took place after most stars were visible and after
all traces of polarized overhead skylight were absent. We tested the latter by viewing the
overhead sky through a polaroid filter. When the sun was 8° or more below the horizon,
no intensity changes could be detected while rotating the filter. To be certain that all solar
cues were absent, all experimental birds were released after the sun was 11° or more
below the horizon. At this time, the natural horizon skylight is uniform in all directions,
providing no directional cue. However, horizon skyglow from man-made light
overwhelmed natural skylight in some directions. Treatment and releases prior to 2003
were conducted 3 km south of Champaign-Urbana, where scattered light from the city
filled the sky from WNW to NE, quite intense to at least 30° above the horizon.
Similarly, almost all natural flights (Fig. 1D, 1E column 1) began from near the city,
about half from near the experimental release site and the other half from 4 km NE of the
city, with city glow centered toward SE. All releases during 2003 were from open
farmland 10 km SSE of the cities and 3 km WSW of the airport, where man-made
skyglow was less intense but still substantial from NNE to ENE. Due to the consistency
of control flights independent of release site, and because both experimental and controls
were released from the same locations, we can safely assume that man-made light had no
measureable effect on naturally migrating Catharus thrush orientation during and after
departure (S2), and also no noticeable effect on the natural or experimental flights we
report here.
Radio-tracking procedures and how to measure a bird’s heading in mid-air
When the birds took off, we radio tracked them during their nocturnal migrations
using cars with turnable roof-mounted direction-finding antennas. A detailed description
of special tactics and equipment for obtaining the track vector and altitude of migrating
birds, and three methods of determining their true headings in mid-air is given in Cochran
& Kjos (S2), from which we very briefly summarize methods used to measure headings:
The vector method requires measuring the track vector and the wind vector at the
bird’s altitude, which must also be measured. The heading is the track vector minus the
wind vector. The vector method can be accurate to 2 or 3 degrees if wind variation with
height is small and if the winds are light, but with the typical wind speeds similar to bird
air speeds and high sheer, errors can be 10° or even 20°. The head-null and cross-
polarization methods are more reliable and become possible because the affixed radio
transmitter’s antenna is parallel with the bird’s body and therefore also with the bird’s
heading, and because the transmitter’s antenna has a null (transmits no energy) along its
axis. For example, an observer driving rapidly across the projected path ahead of a bird
can monitor the decrease and increase of signal as he thus passes through the null. The
bearings taken during the null passage will average as the reciprocal of the bird’s heading
to a typical accuracy of about 5°. The cross-polarization method requires the observer to
maneuver approximately underneath the bird, as determined by polar direction finding.
Then, with the receiver antenna pointed up at the bird, it is rotated for a null, which
occurs when the plane of its elements is at right angles to the horizontal direction of the
bird’s transmitter antenna. At the null, the heading is plus or minus 90° from the
horizontal plane of the receiving antenna elements. The null is quite sharp and accuracy
is typically 2° or 3°.
All three methods were variously used in measuring the headings reported in this
paper. Often two or even all three methods were used on a single flight. For all the flights
reported in Table S1 and S2, the vast majority of heading measurements were based on
the more reliable cross-polarization and head/tail null methods, whereas none were based
exclusively on the vector method. We made as many heading measurements during a
flight as the situation permitted. Three to five determinations of heading were typical and
the error of their mean for one night’s flight was usually less than 10°.
Headings versus ground tracks
The distinction between a bird’s heading and ground track is important in these
experiments because individual Catharus thrushes maintain a consistent preferred
heading during spring migratory flight over Central North America (S1), whereas their
ground track directions often vary markedly from flight to flight due to lateral wind drift
(S2). These flight-to-flight variations of ground track directions make them unsuitable for
orientation experiments on free-flying Catharus thrushes. However, lateral wind drift
over long migratory journeys tends to cancel out (S2), eg., the 312° preferred heading of a
Swainson’s thrush (S2) was only 8° less than the 320° azimuth from the start to end of its
monitored journey. Because Catharus thrushes do not compensate for wind drift but keep
consistent headings (S2), headings are the relevant measure for Catharus thrush
orientation. To easily visualize orientation behavior, we therefore used measured
headings, airspeeds and durations of flight to calculate and map a flight’s ground track as
if there were no wind. For example, the long serpentine flight track in Figure S2 is
presented in Figure 2B as it would have been without wind.
References
S1. A. Raim. Bird Banding 49, 326-332 (1978).
S2. W. W. Cochran, C. J. Kjos. Illinois Nat. Hist. Survey Bull. 33, 297 (1985).
S3. W. W. Cochran, M. Wikelski, In: Birds of Two Worlds, P. Marra, R. Greenberg, Eds.
(Johns Hopkins Press, Baltimore, 2004), in press.
S4. R. Sandberg, F.R. Moore, J. Bäckman, M. Löhmus, Auk 119, 201 (2002).
S5. H. Mouritsen, Anim. Behav. 55, 1311 (1998).
S6. H. Mouritsen, in Avian Migration, P. Berthold, E. Gwinner, E. Sonnenschein, Eds.
(Springer, Berlin, 2003, pp. 493-513).
S7. H. Mouritsen, O. Mouritsen. J. Theor. Biol. 207, 283-291 (2000).
S8. W. W. Cochran, Anim. Behav. 35, 927 (1987).
Tables
Table S1. Gray-cheeked thrushes
a) Individuals that were treated in an experimentally changed magnetic field and released, but which did
not migrate on the first night. Instead, these birds migrated during a subsequent night (thus after exposure
to natural twillight cues without experimental treatment).
Treatment
date
Shift in
magnetic
field
Sun’s angle
below the
horizon at
time of
departure
Minutes of
exposure to
sun *
Departure
date
Heading Predicted
shift in
heading
5-14-1972 70° -22° 0 5-15-1972 000° 0°
5-20-1978 70° -8° 61 5-22-1978 010° 0°
5-13-2003 80° -31° 87 5-16-2003 356° 0°
5-13-2003 80° -25° 87 5-19-2003 011° 0°
5-25-2003 90° -25° 100 5-26-2003 337° 0°
b) Individuals that were released after being treated in an experimentally changed magnetic field and which
migrated on the night of release. A negative ‘minute of exposure to sun’ means that the bird was put into
the experimental magnetic field after sunset. A negative value in the columns “predicted shift in heading”
and “measured shift in heading relative to magnetic North” indicates a counter clockwise shift.
Treatment
Date
Shift in
magnetic
field
Sun’s angle
below
horizon at
release
Minutes of
exposure to
sun *
heading Predicted
shift in
heading
Measured
shift in
heading&
5-16-1978 70° -12° -8 297° -70° -60°
5-18-1978 70° -12° 65 355° -70° -2°
5-15-1979 70° -11° -13 300° -70° -57°
5-28-1979 70° -13° 57 291° -70° -66°
5-12-2003 80° -25° 98 235° -80° -122°
5-12-2003 80° -17° 98 300° -80° -57°
5-19-2003 90° -16° 125 285° -90° -72°
5-20-2003 90° -15° 186 290° -90° -67°
* minutes in experimental magnetic field prior to sunset
& relative to the mean true heading of the controls (357o, column 1 in Fig. 1D).
Table S2. Swainson’s thrushes
Individuals were treated in an experimentally changed magnetic field, released and subsequently followed
throughout a complete, natural nocturnal migratory flight. The same individuals then made a natural
stopover and were followed during their subsequent natural migratory flight. We group the flights by
individual, i.e. the first row of a continuous record is always the experimental flight, whereas subsequent
row(s) represent natural flight(s) of the same individual. A dashed line (---) indicates that these data are not
relevant. A negative value in the columns “predicted shift in heading” and “measured shift in heading”
indicates a counter clockwise shift.
Date Shift in
magnetic
field
Sun’s angle
below the
horizon
Minutes of
exposure to
sun *
heading Predicted
shift in
heading ^
Measured
shift in
heading
5-21-1978 70° -14°§ 68 311° --- ---
5-25-1978 natural -9°
# --- 029° -73° -78.5°
5-30-1984 88° -14°§ -5 206° --- ---
06-1-1984 natural -8°
# --- 304° -88° -98°
06-3-1984 natural -8°
# --- 305° +3° +5°
06-4-1984 natural -7°
# --- 310° +3° +2°
06-5-1984 natural -6°
# --- 313° +1° +3°
5-20-2003 90° -15°§ 186 280° --- ---
5-21-2003 natural -5°# --- 335° -91.5° -55°
5-20-2003 90° -18°§ 186 325° --- ---
5-25-2003 natural –12°# --- 035° -93° -70°
5-21-2003 90° -14°§ 150 270° --- ---
5-22-2003 natural –8°# --- 013° -91.5° -103°
§ at time of release
# at time of natural departure
* minutes in experimental magnetic field prior to sunset
^ shift in magnetic field plus clockwise change in sun azimuth between date and latitude of the release site
and date and latitude of the second night’s flight.
Fig. S1. Plot of deviations of track directions and estimated headings, for individual
thrushes, from the respective mean value of tracking direction and estimated heading.
Large dots are from one Swainson’s thrush followed for six consecutive nights. Reprinted
from S2, with permission.
Track direction – Deviation from mean (degrees)
Heading estimate – Deviation from mean (degrees)
+ = Clockwise
- = Counterclockwise
Track direction – Deviation from mean (degrees)
Heading estimate – Deviation from mean (degrees)
+ = Clockwise
- = Counterclockwise
Fig. S2.
Solar azimuth
317o
313o
312o
309o
308o
307o
Heading
314o
310o
308o
306o
304o
302o
Path
307o
313o
283o
325o
300o
294o
Complete
overcast
Lost
Winnipeg
Minneapolis
Complete
overcast
St. Louis
Solar azimuth
317o
313o
312o
309o
308o
307o
Heading
314o
310o
308o
306o
304o
302o
Path
307o
313o
283o
325o
300o
294o
Complete
overcast
Lost
Winnipeg
Minneapolis
Complete
overcast
St. Louis
Solar azimuth
317o
313o
312o
309o
308o
307o
Heading
314o
310o
308o
306o
304o
302o
Path
307o
313o
283o
325o
300o
294o
Complete
overcast
Lost
Winnipeg
Minneapolis
Complete
overcast
St. Louis
Fig. S2. Migration behavior of a single Swainson’s Thrush between 13 and 20 May 1973,
as followed for 1512 km. Left panel: The flight path is indicated by lines (solid line:
heading information available; dotted line: no heading information). The six stopover
sites are depicted by circles along the path. The bird kept a constant heading throughout
its flight even when flying under completely overcast skies (nights 2 and 5) and landed
280
290
300
310
320
330
300 310 320
Solar azimuth (degrees)
r2=0.98
Headin g
Flight path
when it hit cold fronts, such as during 13/14 May (first night) and 15/16 May (third
night). The heading of the bird (second right column) always showed a constant
relationship toward the solar azimuth (left column), whereas the realized flight path (right
column) showed no relationship to the solar azimuth. Right column: We depict the
relationship between solar azimuth, heading, and flight path, showing an almost complete
congruence of the bird’s heading and the solar azimuth (heading was always 3-40 less
than the solar azimuth). Note that solar azimuth changes with latitude, as did the bird’s
heading. Modified after S3 and S8.
