ANIMAL MIGRATION
Magnetic stop signs signal a European songbirdÕs
arrival at the breeding site after migration
Joe Wynn^1 , Oliver Padget^1 , Henrik Mouritsen2,3, Joe Morford^1 , Paris Jaggers^1 , Tim Guilford^1
Although it is known that birds can return to their breeding grounds with exceptional precision, it has
remained a mystery how they know when and where to stop migrating. Using nearly a century’s
worth of Eurasian reed warbler (Acrocephalus scirpaceus) ringing recoveries, we investigated whether
fluctuations in Earth’s magnetic field predict variation in the sites to which birds return. Ringing
recoveries suggest that magnetic inclination is learned before departure and is subsequently used as
a uni-coordinate“stop sign”when relocating the natal or breeding site. However, many locations
have the same inclination angle. Data from populations with different migratory directions indicate that
birds solve this ambiguity by stopping at the first place where the right inclination is encountered on
an inherited return vector.
A
lthough first outbound migration in song-
birds is typically, though not universally
( 1 ), thought to use an inherited clock
and compass (vector) navigation pro-
gram ( 2 – 5 ), additional or alternative
mechanisms must be required to account
for the accuracy with which birds return to the
natal or breeding site (philopatry) ( 6 , 7 ). Ex-
perienced birds could use learnt bicoordinate
maps comprising two or more cue gradients
during philopatry, with cues derived from
Earth’s magnetic field suggested to be prime
candidates for accurate natal or breeding site
return ( 3 , 4 ). Such cues include magnetic in-
clination, or the dip angle between Earth’s
magnetic field and Earth’s surface; magnetic
declination, or the angle between magnetic
and true north; and magnetic intensity, or the
overall strength of Earth’s magnetic field ( 7 ).
Maps comprising these components have been
suggested to underlie navigation in sea turtles
( 8 ), fish ( 9 ), and songbirds ( 10 , 11 ). Previous
studies have suggested that songbirds could
use declination alongside inclination and/or
intensity as a bicoordinate map when com-
pensating for long-distance displacement
( 10 , 11 ), although magnetic gradient cues
(alongside other cues) might also allow song-
birds to locate the natal or breeding site during
philopatry.
Here, we used 17,799 ringing recoveries (from
1940 to 2018) made in consecutive years to
investigate whether the Eurasian reed warbler
(Acrocephalus scirpaceus), a trans-Saharan mi-
gratory songbird ( 12 ), used magnetic informa-
tion during philopatry and, if so, how. Because
Earth’s magnetic field shifts slightly year by
year, the magnetic parameter values charac-
teristic of an individual’s natal or breeding
site will exist in a different location the fol-
lowing year. Hence, if birds used magnetic
parameters to determine the location of their
natal or breeding site, we would expect that
positional changes between years would re-
flect year-on-year changes in the location of
specific magnetic parameters.First, we investigated whether changes in
specific magnetic parameters (inclination, dec-
lination, and intensity) between ringing and
recovery were smaller than would be expected
by chance. This, we reasoned, would test
whether birds were attentive to specific mag-
netic cues. Using randomization analyses, we
compared the changes in magnetic field pa-
rameters experienced by ringed and recov-
ered reed warblers with those that would be
expected if movements were equivalent but
random, parameterizing our null reed warbler
movements using the observed movements.
We found that the median observed change
in inclination (randomization;P< 0.001) (fig.
S3), but not in declination (P> 0.99) or in-
tensity (P= 0.18), was smaller than would be
expected under our null model of philopatry.
We found, however, that the mean changes
in inclination, declination, and intensity were
not significantly smaller than those expected
under our null distribution (randomization;
inclination,P> 0.99; declination,P=0.4;in-
tensity,P= 0.38). Because the distribution of
between-year movements is necessarily long-
tailed and nonnormal ( 13 ), it is perhaps un-
surprising that the mean and median changes
in a given magnetic parameter yield different
results. Further, this disparity could imply that
the processes driving long-distance dispersal
are not closely linked to magnetic secular var-
iation. To test this, we reran our randomiza-
tion analysis with birds from the first percentile
for ringing to recovery distance removed. This
was done for adults and chicks separately,
ensuring that the proportion of adults to chicks446 28 JANUARY 2022•VOL 375 ISSUE 6579 science.orgSCIENCE
Fig. 1. Changes in position
between ringing and
recovery for Eurasian
reed warblers.(Left) Reed
warbler ringing sites,
colored orange for west-
migrating birds and blue for
east-migrating birds.
(Middle) Changes in longi-
tude (eastings in kilometers)
plotted against changes
in latitude (northings in
kilometers) for birds
breeding in northern and
western Europe. (Right)
Changes in longitude plotted
against changes in latitude
for birds breeding in south-
eastern Europe. In all panels,
opacity indicates multiple
overlapping data points.
−10 0 10 20 3035404550556065Longitude (degrees)Latitude (degrees)−500 0 500−1000010002000Delta eastings (km)Delta northings (km)−300 −100 100−2000200400600800Delta eastings (km)Delta northings (km)(^1) Oxford Navigation Group, Department of Zoology, Oxford OX1
3SZ, UK.^2 AG“Neurosensorik/(Animal Navigation),”Carl-von-
Ossietzky Universität Oldenburg, 26111 Oldenburg, Germany.
(^3) Research Centre for Neurosensory Sciences, University of
Oldenburg, 26111 Oldenburg, Germany.
*Corresponding author. Email: [email protected]
(J.W.); [email protected] (T.G.)
RESEARCH | REPORTS