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MY subject today is birds, the kind that come to the Philippines in
the winter and return in the spring to their nesting grounds in
Russia and China.
Last week, Candaba, the winter home of at least
49 species of migratory birds, inaugurated what could become an
annual celebration, the Ibon-Ebon Festival, a tiresome play on words
that amuse Tagalog speakers no end.
Jerry Pelayo, the town mayor, estimates that
some 17,000 wild fowl had come to visit the Candaba Swamp Wildlife
Reserve.
In December, three rarae aves were sighted and
photographed. A Great Cormorant (Phalacrocorax carbo) was seen by
members of the Wild Bird Club of the Philippines on Dec. 17. Three
days later, an employee of the Candaba Reserve spotted a
White-Shouldered Starling (Sturnus sinensis), a bird that had been
seen in the Philippines only six times since 1911. A really rare
bird, the Eurasian Spoonbill (Platalea leucorodia), was
photographed by a pair of birdwatchers.
But what fills Mayor Pelayo with justifiable
pride was the successful breeding in the reserve of endemic wild
ducks that had been hunted almost to extinction.
How did these birds find their way across
continents and oceans to the Philippines?
In the 1860s, the Russian biologist, Alexander
von Middendorff, noted that migrating birds traveled along fixed
paths later found to correspond to Earth’s magnetic field lines.
For some birds and fish, the Earth’s magnetic
field is enough to point to a direction. On most places on Earth’s
surface, the magnetic field points north. Its intensity grows
stronger nearer the pole, indicating latitude. Longitude is harder
to fix. It might be that birds watch the sun as it rises and sets.
But how do fish do it? According to John Bohannon (Science, Nov. 9,
2007), there’s a north-south magnetic ridge on the ocean floor
that fish probably memorize.
The question that remains unanswered is: do
these animals have an organ that can detect magnetic signals,
however faint?
The discovery in the 1970s of magnetite in
living cells gave a clue. Although magnetite has the obvious
function of locking up excess iron, Kenneth Lohmann of the
University of North Carolina at Chapel Hill was bold enough to
assert that “…it’s very hard to imagine that these crystals
aren’t there for magnetic detection.”
In 1984, Michael Walker, a biologist, and Joseph
Kirschvink, a physicist, both of the California Institute of
Technology in Pasadena, found magnetic crystals in tuna (Science,
May 18, 1984). They were convinced that they had found the
biological compass.
Pursuing this lead, Walker and his team reported
in Nature in 1997 and 2000 that they also found these magnetite-like
crystals in the nose of rainbow trout. What clinched the argument
for them were the strings of crystals in the cells that were
connected to the brain by a nerve sensitive to magnetism.
Not everyone was convinced. An alternative
mechanism for magnetoreception was hypothesized: the radical-pair
model. Magnetite detects the signals but at the same time birds and
fish keep track of them with a chemical reaction.
In 1998, a probable magnetoreceptor was found
in the eyes of animals as different from each other as fruit flies
and mice. This is cryptochrome, a protein that, when hit by light,
produces two possible intermediate states that differ in orientation
relative to the magnetic field.
Thorsten Ritz, a biophysicist, at the University
of California at Irvine says that because cryptochrome is in the
retina, the magnetic information is fed to the brain through the
optic nerves.
“Many pieces of the puzzle that never fit well
with the magnetite model have started to make a lot of sense,”
Ritz said.
Ritz reported in Nature in 2004 that when the
cryptochrome, but not the magnetite, was disrupted by changes in
frequencies of the electromagnetic fields, the birds were
disoriented.
Walker countered by saying that during
vertebrate evolution, the compass function of cryptochrome was
“gained and lost repeatedly” (Science, Nov. 9, 2007) whereas
magnetite remained constant. Besides, Walker thought that it was
absurd that evolution produced two organs for magnetoreception.
John Phillips, who had been working on
magnetoreception at Virginia Polytechnic Institute, pointed out that
the radical-pair model gave a role to both mechanisms. Cryptochrome
determined direction while magnetite mapped displacements when the
bird was on the wing.
Both camps are gearing up for what could be
final proofs. The radical-pair group, with cryptochrome-knockout
mice, are out to prove their model beyond doubt. Walker and
Kirschvink on the other hand have received US$1.4 million from the
Human Frontier Science Program to press on with their studies on
fish magnetite.
But whichever mechanism is found to be the right
explanation, the essential unity of science is key. Physics,
biology, chemistry and geology all contributed to our budding
knowledge of how animals find their way across trackless deserts and
featureless oceans.
But for me the significance of the Ibon-Ebon
Festival is to remind us that this smallish planet called Earth is
not for man alone. Birds have as much right also to call it home.
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