Life Nature Magazine

NEW IN SCIENCE FROM DUSK UNTIL DAWN AUTUMN ISSUE
Magnetic eyes and iron beaks:
Image: Lauren Stevens, 2013; Diagrams: Georgia Cass, 2013.
Finding your way in the dark is always a challenge. Whether you’re a bat
catching insects in a dense tropical rainforest, or a badger digging your sett
in an English woodland, sensitivity in darkness is essential to your survival.
Exploitation of the darkest hours has led
to a huge range of adaptations designed
to increase an animal’s ability to
navigate at night. Purely nocturnal creatures
tend to sport more obvious adaptations to
living in darkness; the giant eyes of the bush
baby are a prime example. However, for
those who need to be active night and day,
migrating birds for example, the transition
can be tricky.
Initial research suggested that birds, such as
homing pigeons, use landmarks imprinted
Other birds like European robins are known
to use a combination of landmarks and the
position of the sun and stars as a guide.
Recent studies have had some fascinating
insights into how birds on the wing navigate
at night. It has now been suggested that
the retinas in the eyes of migratory birds
are in fact able to not only detect light, but
wintering and breeding grounds. Such an
anatomical feature requires incredible levels
of sensitivity and detection of light at low
levels, and unfortunately, doesn’t come with
a simple explanation. How can the retina
detect the magnetic pull of the earth? Two
main theories have developed.
Light-dependent navigation
One such theory involves blue-light-sensitive
photoproteins known as cryptochromes.
Some scientists believe these molecules to be
key in the detection of the earth’s magnetic
magnetoreceptors at a bird’s disposal. The
cryptochromes, of which birds are known
send the information along ganglion nerve
cells to the brain, where it can be processed
into directional information that the bird can
respond to. These ganglion cells have been
found to be particularly active in migratory
birds such as garden warblers and European
robins when tested under moonlightstrength
white light. Although this ‘compass’
is dependent on the presence of light, it is
extremely sensitive and has been found to
function at light intensities equivalent to
that of a ‘partly clouded moonless night’.
Impressive, when you consider your
unfamiliar, lightless expanses.
In addition, an area in the front of the brain,
known as ‘Cluster-N’ has been shown to
be highly active when birds are orientating
through night-time migration. The lightdependent
information is transmitted from
the retina in the eye to Cluster-N, which
is activated under low light intensities.
Studies on European robins have shown
that deactivation of this cluster renders their
magnetic compass useless.
Magnetite-based navigation
Another theory on the ability of migratory
birds to navigate at night involves iron
deposits, found in the upper beak of several
bird species. The migration distance of these
birds range widely from year-round residents
such as pigeons, to long-distance transequatorial
migrants like garden warblers, and
it is therefore thought that iron deposits will
occur in most bird species.
The iron-rich mineral structures were
initially thought to act literally as magnets,
with the metal aligning with the magnetic
pull of the earth. This information was
thought to then be relayed to the brain
by the trigeminal nerve; a primary nerve
connecting the face and brain. Despite
this, experiments severing the trigeminal
nerve found that the birds’ sense of
recent experiments have severed the nerve
and subsequently relocated the birds up
to 1000km east of their original starting
point. This study showed that cutting the
their sense of location; birds would continue
as if they had never been moved, and end
up lost, rather than readjusting their course.
It seems therefore that the iron-deposits
‘compass sense’ - sense of direction, of a bird,
their sense of location.
Neither of these theories provides a complete
explanation of how birds navigate at night.
As with many aspects of biology, it is likely
that no one theory can explain all of the
variation in the adaptations that we see.
Essentially it seems probable that both
the light-dependent and magnetite-based
theories work together; depending on the
light available both mechanisms will work
to send directional information to the brain,
with the iron-based clusters only activated at
low light intensities.
The avian eye and
the magnet effect
Directional information provided by
the bird’s eye by chryptochromes. It is
thought that light absorption generates
radical pairs within chryptochromes
in the retina which are determined by
molecule orientation in relation to the
iris
lens
cornea
The earth’s
gives migrating
birds their sense
of direction, or
‘compass sense’.
N
retina
optic nerve
So next time you’re following your Sat-Nav
in the dark, or wandering home from the
pub late at night, spare a thought for these
miles a year whilst using nothing but their
fantastically clever, built-in GPS.
Georgia Cass, Alumnus of
the University of Exeter,
Cornwall Campus
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