Navigation and Homing in Pigeons: A Brief Sketch
Email: gacdvm@telus.net
Navigation and homing in pigeons and migratory birds continue to be fascinating yet puzzling subjects. Although many strides have been made over a number of years in understanding some of the complexities associated with this subject, much remains a mystery to this day. Even so, modern research continues to peel away the layers – albeit slowly - surrounding this mystery. The following information discusses some of the currently available knowledge on the multifaceted subject of navigation in pigeons and other birds.
There seems to be general
support for
the idea that pigeons use a two-step method to home accurately: firstly, a map to determine their home
direction and secondly, a compass to guide them.
It is known
that presence of the sun (sun compass) allows pigeons to navigate
correctly. It seems that they are able
to compensate for the movement of the sun across the sky and hence, to
stay on
course. However, finding the home
direction doesn’t require a view of the sun.
In order to navigate in the absence of sun on cloudy days, the
birds
appear to use cues from the lines of magnetic force that run from pole
to pole
over the entire earth. The belief is
that pigeons, like other birds, have an inborn magnetic sense that they
use as
a directional compass.
The liquid
core of the earth produces a magnetic field that is a dipole (di = two)
field
in which the magnetic poles lie near the geographic north and south
poles. The field lines of the magnetic
field leave
the ground at the Antarctic pole, curve around the earth and then
re-enter the
earth at the Arctic pole. Magnetic north
and geographic north differ by a certain number of degrees. This variation or deviation may be
considerable near the magnetic poles but it soon becomes much less at
lower
latitudes. In most parts of the world
the field lines run roughly north-south.
The total intensity of the field decreases gradually from very
high
values at the poles to about half that strength near the magnetic
equator.
Superimposed on this
general pattern
are certain irregularities. For instance
different degrees of magnetization of rocks cause localized magnetic
anomalies;
as well, time variations of the geomagnetic field occur on a daily
basis. These regular daily variations plus
the
irregular fluctuations that occur because of magnetic storms from
space, etc.,
are important in consideration of a navigational map used by birds. Because of its structure, the magnetic field
is present at all times, and therefore, is a constant, reliable source
of
information. The magnetic field seems to
provide directional information for birds, and factors such as its
total
intensity and inclination may provide birds with information about
their
geographical position.
The use of magnetic
information in
the orientation of birds was discussed as early as the 1800s. As long ago as 1859, one German researcher
proposed what we now call a magnetic compass in birds. The magnetic
compass was
described first in the European robin, a small bird that migrates at
night, and
experimental work showed that these birds use the magnetic field in
direction
finding. The magnetic compass of birds
is what is known as an inclination compass – instead of indicating
north and
south it distinguishes between poleward and equatorward.
A number of other species
of birds,
including short and long-distance migrants from different countries
over the
world, have been found to use a magnetic compass; most members of this
group
are night migrants. Studies in pigeons
have suggested that their magnetic compass too, is an inclination
compass. This information indicates that
the magnetic
inclination compass is a very widespread mechanism among birds in
general
regardless of their relationship, their geographic distribution and
their
migratory habits. It also indicates that
migratory direction is genetically encoded in birds as a compass course
relative to the geomagnetic field. In
both north and south hemispheres, birds begin their Fall migration
heading
toward the equator – south for northern birds, north for southern birds.
During homing, the
magnetic compass
is involved not only in locating the course determined by navigational
methods. It seems that it may also be a
part of these navigational processes themselves, that is, when these
processes
are based on information obtained during the trip from the loft to the
release
point. One experiment showed that young,
inexperienced pigeons were disoriented after they were released when
they had
traveled to the release point in a planned distortion of the magnetic
field. However, a second group that had
experienced the same distorted magnetic field for the same length of
time, this
time while they waited at the release site, oriented correctly and
headed for
home. The authors of this study
concluded that it wasn’t the distorted magnetic field as such but being
transported
in a distorted field that was critical.
On this basis it has been assumed that pigeons use the magnetic
field as
a reference system for storing information about the course of the
outward trip
as well as the homeward course, and that the distortion of the magnetic
field during
the trip to the release point prevented the first group of birds from
storing
such information. Only very young
inexperienced birds apparently use this strategy; older more
experienced birds
don’t appear to be affected by magnetic distortions during the trip to
the
release point – which suggests a change in their navigational strategy
that may
be related to their greater experience.
The magnetic compass is
involved in
the learning curve that leads to the establishment of the sun compass. Young inexperienced pigeons use the magnetic
compass before they can use the sun compass.
There is evidence that the magnetic compass serves as a
directional
reference system to establish the sun compass.
Later on, the sun compass becomes the preferred system. When pigeons are used to flying in overcast
conditions, their orientation toward home under either sunny or
overcast skies
is similar, so it would appear that the accuracy of the magnetic
compass is
equal to that of the sun compass.
How accurate is the
magnetic
compass? The flight behavior of birds
suggests considerable accuracy. When
pigeons are released in cloudy conditions in which they have to depend
on their
magnetic compass, they may produce compass directions of high accuracy,
deviating from a direct line by less than 25o.
Research has also shown
that there is
a pronounced learning stage involving the navigational map during
training in
the first 2-3 months of life after weaning. There is marked development
in the
first to second month, followed by a critical period between the second
and
third months of age, during which development is impaired possibly as a
result
of a reorganization of the system, and then slight progress occurs in
subsequent months. A related study in
2005 concluded that lofts apparently play a central function in the
development
of the navigational system, but their exact role is unknown.
It is known that a wide
group of
bacteria and algae orient their movements along lines of the magnetic
field. The basis for this ability was
determined to be crystals of the magnetic, iron-based minerals
magnetite, also
known as lodestone (Fe3O4 -- ferric oxide), and
greigite
(Fe3S4 – ferric sulfide).
Later, magnetite was also found in honeybees, birds, salmon, sea
turtles
and a number of other animals known to orient with the magnetic field
of the
earth.
Most magnetite isolated
from animals
has been in the form of what are called single-domain crystals that are
particles of magnetite that are attracted by a magnet but do not
attract iron
particles to themselves. In fact, they
are extremely tiny permanently magnetized magnets that twist into
alignment
with the magnetic field. In some animals
such as pigeons, magnetite crystals are smaller than single-domain
size; these
smaller crystals are said to be super para-magnetic and have different
magnetic
properties. Unlike single-domain
crystals, they don’t have a permanent magnetic movement and so they
can’t
physically rotate into alignment with the magnetic field of the earth.
Even
though these crystals don’t move, their magnetic alignment tracks the
alignment
of the surrounding magnetic field.
In order for magnetite
crystals to
function as magnetic receptors, the assumption is that they must have
to make
contact with the nervous system. The
strongest evidence that this is so is derived from experiments with
trout and
pigeons. In pigeons and other birds,
crystals that are thought to be magnetite have been found in the upper
beak. Electron microscopic studies (that
magnify
many thousands of times) of this area in pigeons have shown that these
crystals
are located within the ends of nerves and are distributed along the
membrane of
nerve cells. In pigeons, these crystals
have been found to be super para-magnetic.
They are present in clusters, and in pigeons about 10-15 of
these
clusters are found inside the end of one nerve.
An interesting similarity between birds and fish is that the
location of
the crystals in the beak of pigeons and in the nasal area of trout,
appears to
receive its nerve supply from the ophthalmic (eye) branch of the
trigeminal
nerve, one of the 10 nerves that originate directly from the under side
of the
brain. The implication is that the
ophthalmic branch of the trigeminal nerve carries to the brain,
important
information about the magnetic field.
(As an aside, these findings indicate that pigeons and other
birds
readily obtain the extremely tiny amounts of magnetite they need from
their
regular diet, mineral mix, water, etc., so I have long wondered why
fanciers
would buy magnetite!)
As
another aside, it is interesting that Cary Oler, a scientist from my
area, is the son of one of my club mates, and an expert in geomagnetic
disturbances. He makes the point that
races from east or west are more difficult for pigeons to navigate,
since the
birds are crossing magnetic field lines.
Birds racing from north or south fare better because they are flying
along the field lines of the magnetic field.
Adding to the complexity
of the
problem was the interesting suggestion by some researchers that the
magnetic
compass of birds might require light for its function. This idea was
put to the
test by the German research team of Wolfgang and Roswitha Wiltschko who
found
that European robins that were exposed to the local geomagnetic field
oriented
correctly in the expected migratory direction under blue, turquoise and
green
light (short wavelength light) but were disoriented under yellow and
red light
(long wavelength light). Thus, the
orientational responses of birds to magnetic cues seem to depend on the
wavelength of light. (When ‘white’ light
from the sun is passed through a prism, it separates in this order into
its
seven colors -- red, orange, yellow, green, blue, indigo and violet --
each of
which has a different wavelength.
It is interesting that
many years
earlier, the first attempt by the same authors to establish the role of
light
in magnetic orientation involved experimental studies with young,
inexperienced
pigeons. As noted previously, to
determine the correct homeward direction, inexperienced young pigeons
rely on
compass information that they obtain during the trip from the loft to
the
release point. If these inexperienced
pigeons are transported in the normal geomagnetic field and in darkness
as
well, they become disoriented when they are released.
(However, experienced older pigeons aren’t
affected by similar treatment.) These
findings in young pigeons are consistent with their use of a magnetic
compass
mechanism that also depends on light for correct orientation to the
home
loft. Experiments with young
homing pigeons suggest a similar relationship between the
wavelength
of light and the ability to obtain directional information
from the
magnetic field. In summary,
the available data indicate that light from the blue-green
part
of the visual spectrum is required for magnetic reception in
birds.
To add to this discovery,
the same
researchers found that that the effect of light on the magnetic compass
in
European robins was processed through only one eye:
the right one. When a cap was
placed over the left eye,
these birds continued to orient correctly, but when the right eye was
covered,
the birds became disoriented. Similarly,
work with pigeons showed that birds with the left eye covered homed
consistently better than those with the right eye covered.
Because the left side of the brain controls
the right side of the body - and vice versa - these findings suggested
the
importance of the right eye and left side of the brain in processes
involving
flight control, navigation and homing in pigeons.
Adding further to the
navigational
mix is the controversial subject of odors and their role in the
navigation of
pigeons. In 1971 an Italian scientist
named Papi and his colleagues proposed the idea that odors played an
important
role in navigation and homing by pigeons.
In the years since then the issue has remained contentious and
has
resulted in many experiments, theories, proponents and of course,
dissent.
The basic premise behind
this idea is
that pigeons learn an odor ‘map’ by associating odors detected at the
home loft
with the direction from which they are carried on the wind, or by
sensing odors
during exercise around the loft and surrounding territory.
Some of the evidence in favor of this
idea: 1) birds whose sense of smell has
been blocked by anesthetic or surgery don’t orient or home from
unfamiliar
release sites; 2) birds that are shipped long distances don’t orient
unless
they can sample the natural air during the trip; 3) if the air supplied
to
pigeons is filtered through charcoal, their ability to orient is
eliminated. However, it’s not obvious
how different strengths of odors that might be useful over ranges of
hundreds
of miles could be maintained in the dynamic changing atmosphere, and
some
scientists use this point to reject the whole idea of odors and
navigation. Without knowing the identity
of air-borne substances that constitute the foundation for a map based
on
odors, there is no way to know much about their distribution in space
and over
time. Even so, some scientists offer the
conclusion that the accumulated evidence supports the idea that the
sense of
smell plays a role in the map component of homing.
Not everyone agrees and the debate carries
on.
The location of the loft
seems to
make an important difference in the orientation of pigeons. In one study in
The researchers explained
that the
difference between the two groups of birds was likely related to their
access
to the wind and the odors it carried.
The garden loft was sheltered from the wind; pigeons reared in
this loft
may have learned to rely on cues that were not related to odors in the
wind,
and as a result they oriented and homed correctly.
The other group in the four-storey loft where
there was free access to the wind and the odors it brought may have
relied
extensively on orientation/homing cues detected in these odors. Because of their reliance on wind-borne
odors, this group was unable to orient or home when their sense of
smell was
blocked.
On the basis of cumulative
research,
it seems evident that pigeons use many different cues, possibly
including
odors, to develop a map; how and why these multiple cues are selected
at any
given time is unknown.
Some
other interesting information - in recent
years, at least two similar research programs, one a 10-year study from
Oxford
University in the UK, and the other from a team of European
researchers, have
come to the same conclusion: that
pigeons often home along major highways and use landmarks to remember
where
they are. In the European study,
investigators
used the Global Positioning System (GPS) to record 216 flight paths of
pigeons
over distances up to 30 miles.
Information from this GPS study showed that experienced pigeons
released
from familiar sites over three years around Rome, were significantly
attracted
to highways and a railway track running toward home, and not to the
direct
bee-line toward home. Birds often broke
away from highways that veered away from home but many birds continued
along
these highways to a major junction even when the detour added
appreciably to
the flight. Significant road following
occurred in 40-50% of flight paths along one particular line of flight,
from
northwest to southeast. The authors
suggested that pigeons use a learned route-following strategy and the
use of
particular geographic locations – landmarks in other words -- for final
navigation
to the loft.
Because of the open prairie country in which I live, during training I have often been able to follow my own birds along a main highway and a nearby parallel-running railway track. Usually I see these birds flying at treetop level within 10 to 50 yards of these structures. Very commonly they will deviate slightly to pass over small towns, farms, etc. that are adjacent to the highway-railway. It’s very much a heart-stopping experience to see them racing into a headwind as they fly two to three feet directly above the highway, and often lifting up just barely in time to avoid traffic approaching from the front and rear!
The ability to detect ultraviolet
light and to hear
infrasound (frequency less than 20 Hz) has been demonstrated in pigeons. Note - Hz [hertz] is
the unit of frequency; one hertz has a periodic
interval of one second; kHz- one thousand periods per second. The range of human hearing is from 0 Hz
to
roughly 20 kHz. The possible negative
effects of infrasound on several races from
In summary, as researchers
have long
suspected, there are likely many factors involved in the complexity of
avian
navigation. Over all, the general role
of magnetic cues in the navigational map used by pigeons is still not
clear. Homing is hardly affected by
manipulations of the magnetic field, which suggests that magnetic cues
are not
essential for use of the map. It seems
that birds have several alternative cues from which they can select a
number of
possibilities to aid in navigation.
However, the conditions that determine the favored cues used at
any
given time remain a mystery. Research
into this complex, fascinating subject continues unabated!
*****