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03/21/2007
Finding The Way Home
I’m starting this column on Sunday morning after watching the
TV program Sunday Morning, which had an interesting segment
on pigeons and pigeon fanciers. I was amazed at the number of
different types of exotic pigeons. The segment also dealt with
homing pigeons and pigeon racing, in which pigeons are released
perhaps hundreds of miles from their home base. The question,
of course, is how do these birds find their way back home? And
there’s the more general question as to how not only homing
pigeons but also many migratory birds find their way thousands
of miles back to a certain neighborhood or even a certain tree in
that neighborhood.
Theories have abounded, with explanations ranging from visual
cues to orientations with respect to the sun to interaction with
Earth’s magnetic field. The magnetic field as a guiding medium
has enjoyed major support from the experts and many have
suggested that birds incorporate bits of material that are affected
by a magnetic field. The idea is that, if you know the strength of
the field and your alignment in the field, then you know your
position on the earth. Frankly, you could give me all this
information and I wouldn’t have the foggiest idea where I was.
However, I don’t have a bird’s brain and, as noted in last week’s
and other previous columns, birds are actually pretty smart.
A March 14 Reuters dispatch posted on AOL News alerted me to
a significant study by some German researchers that points to the
homing pigeon’s beak as containing the key to its remarkable
homing capability. I then found that these same German workers
and others have published earlier works pointing to the beak.
For example, Cordula Mora led a study with colleagues at the
University of Auckland in New Zealand. The study, published in
a November 2004 issue of Nature, established that homing
pigeons do sense and respond to a magnetic field.
Mora and her coworkers trained pigeons in a situation where
they were placed in a tunnel with feeder platforms at either end
of the tunnel. The tunnel was placed in a coil of wire that
generated a magnetic field when the current was turned on. The
pigeons were trained to go to one end of the tunnel when the coil
was on and to the other end when the coil was off and the only
magnetic field was that of Earth. Their response to the switching
on and off of the coil showed they sensed a change in magnetic
field.
The researchers zeroed in on the birds’ beaks as their magnetic
field detector by anesthetizing the upper beak or by attaching
magnets to the upper beak. In either case, the birds no longer
responded consistently to the switching on and off of the coil. In
these birds there’s a so-called trigeminal nerve that has an
ophthalmic branch and an olfactory branch. (In humans, the
trigeminal nerve is sometimes associated with severe facial pain.)
When the ophthalmic branch was cut the birds couldn’t sense
whether the coil was on or off. They performed normally,
however, when the olfactory branch was severed, showing that
odor wasn’t likely to play a role in their homing ability.
The New Zealanders cited the view that the beak contains small
iron-containing particles that interact with a magnetic field and
noted that rainbow trout also respond to a magnetic field. Mora
is quoted in a November 24, 2004 article by Michael Hopkin on
BioEd Online as saying that nobody had seen the iron-containing
particles because of their small size and the interfering presence
of iron in many biological materials. However, I found a
reference to a study by Gerta Fleissner and 6 colleagues at the
University of Frankfurt published in the April 14, 2003 issue of
the Journal of Comparative Neurology.
Fleissner and her team looked at “afferent” trigeminal nerve
terminals in the upper beak of the homing pigeon. If you’re like
me, you may not know “afferent”, as opposed to “efferent”. My
dictionary states that afferent refers to nerves transmitting signals
to the brain, while efferent refers to nerves transmitting signals
from the brain. I learn lots of new words writing these columns!
At any rate, what the workers found in the nerve terminals were
clusters of nanocrystals of magnetite, a form of iron oxide. They
suggested that changes in the magnetic field could induce small
deformations in the clusters in the cell membranes, inducing
changes in the membrane potentials resulting in signals to the
brain. Later in 2004, Fleissner and colleagues published a paper
in a December issue of BioMetals that reported the size of the
nanocrystals to be in the 1-5 nanometer range and the clusters to
be only 1-3 microns in size. (A micron is a thousandth of a
millimeter and a nanometer is a millionth of a millimeter.)
Now, in a paper either published or about to be published in the
journal Naturwissenschaften, Fleissner and colleagues have
looked more closely at the clusters of tiny crystals of magnetite.
What they’ve found is that the crystals and clusters are arranged
in a 3-dimensional manner that allows them to sense the three-
dimensional aspects of Earth’s magnetic field. It seems this 3-D
arrangement really does give the pigeons the equivalent of a mini
global positioning system. The researchers also have found iron-
containing crystals in the beaks of other birds, including robins.
Speaking of robins, while the nationally televised St. Patrick’s
Day parade was in progress in New York last Saturday, I looked
out our kitchen window to see a robin walking on top of the 4-5
inches of hard packed snow and ice on our driveway. It certainly
wasn’t finding any sustenance there and was scared away when a
squirrel appeared on the scene. I was surprised when the squirrel
walked up the driveway and suddenly dove into the snow-
covered ground bordering the driveway and came up with a nut
that it carried away. Apropos of last week’s column, it certainly
appeared that the squirrel remembered burying that nut in that
location earlier. Or was it just luck?
The next day I saw a squirrel (the same one?) on the driveway
again dive into the snow. This time the squirrel dug and dug and
dug until half his body was in the hole – no nut. It was as though
it knew that it had buried something there but finally concluded a
thief had stolen it. That squirrel then moved on about ten feet to
another spot, dove into the snow and quickly came up with a nut!
Incidentally, for those of you who watched the parade, I was
amazed at how quickly and thoroughly New Yorkers managed to
clear the streets for that parade. Would that the same workers
have been available to clear my driveway! Our snowplowers
arrived hours after the expected time only to say that their
snowplow had “exploded”! After an hour’s hand shoveling of
the “wintry mix” of ice and snow on a 1-2 inch base of ice, only
enough of the snow piled up by the city plows was removed to
allow our cars to exit, if we could make it there from the
turnaround in back of our house. I’m thinking we, and the
robins, should spend two months on Marco Island next year.
I had planned to write this week about the plight of the vultures
in India, based on an article in the February Smithsonian
magazine. However, Brian Trumbore beat me to it in his January
25 Bar Chat (see archives or type “vulture” in our site’s search
engine), where he covers the situation thoroughly. Vultures play
an important role in the disposal of dead animals and a horrific
plunge in their population in India has been nailed down as due
to a drug fed to cattle and other animals. Another example of the
fragile nature of our environment.
Allen F. Bortrum
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