Loon Magic/Loon Magic - Wayzata Technology (8011) (1993).iso/mac/text/03close.b

Text File | 1993-07-26 | 9KB | 155 lines

** A CLOSE LOOK **

   Swimming and Diving

   While loon's feet are fully webbed with three toes, they are not
what scientists call perfect swimming feet, a distinction reserved for
birds like the cormorants and pelicans which have totipalmate feet
(four toes joined by webs). Not perfect maybe, but the loon's feet do
the job well. In the 1917 edition Birds of America, contributing
editor Edward Howe Forbush, compared the dive of the loon to a bolt
shot from a crossbow, and described their quickness as "truly
astonishing."

   Part of the loon's quickness is due to basic anatomy. The bird's
torpedo shape, with leg muscles blending into a streamlined body
mass, helps to give the loon amazing aquatic freedom. On a quiet,
glassy lake surface, loons swim effortlessly, as if friction were not a
physical property. Swimming with the ease of an otter, a loon bobs
slightly as the big feet push alternately. Moving and at rest, loons
ride low in the water with the water line just below the top of the
white breast feathers, maybe two inches from the dramatic necklace.

   Contrary to many reports of early naturalists, contemporary loon
researchers believe that loons, unlike underwater "flying" birds like
auks, use only their feet for propulsion when diving and use their
wings just for help in underwater turning maneuvers. This foot/wing
debate has been cooled with the advent of underwater photography:
several filmed sequences of diving loons show clearly the legs-only
style of underwater swimming. During dives, the feet kick
synchronously with powerful thrusts. According to Sigurd Olson, who
in the 1950s did pioneering loon research at the University of
Minnesota, this is not true for chicks. He often observed chicks trying
to use their wings in diving attempts. With their buoyant down
feathers, chicks need a lot of help to get down under.

   How deep do common loons really dive? This simple question still
sparks heated discussion among loon experts, professional and
amateur alike. Olson interviewed Lake Superior fishermen who
claimed they found loons in their gill nets which had been set at
depths of 240 feet. While such reports are too numerous to ignore,
they are also hard to swallow. Fishermen are not known for veracity.
At 200 feet the pressure is six times the normal sea level pressure,
far more than most air-breathing animals can tolerate. Having
unique half-bird/half-fish attributes, the loon is not like most
animals. If any top-side critter could function at 200 feet, the loon is
the one. Loons are not deep-dive champions. Another black and
white bird, the penguin, can dive to 800 feet and stay submerged for
about twenty minutes.

   How long loons stay underwater is a related question. Probably
every loon watcher has watched a loon dive and never come up.
While observing loons on a small bay of Burntside Lake near Ely,
Minnesota, I was amazed once when I "lost" my loon. While timing
feeding dives, I watched the loon submerge. About five minutes
later, I started to think I was timing a record dive. Ten minutes later,
I was worried about the loon. Fifteen minutes later, I realized that
the only record at stake was my gullibility. Loons are sneaky. They
can surface quickly and dive again with barely a trace of broken
water, and with their rapid underwater movements, they also cover
great distances in a short time. My loon, I'm sure, slipped around a
point with a single underwater swim.

   The scientific literature does have references to extended dives. In
1913, F.R. Jourdain reported several cases of immersion of ten
minutes and one case of a wounded loon which stayed submerged for
fifteen minutes, but these were extreme cases involving birds in a
state of panic. When pushed to the edge of survival, many animals
display extraordinary toughness, and loons are particularly
tenacious. In his 1924 book, Birds of the Lake Umbagog Region of
New Hampshire, William Brewster described the strength of a
wounded loon:" ...the heavy bullet passed directly through the middle
of the neck, about three inches above the body, partially shattering
some of the vertebrae and cutting the jugular vein, yet she the loon
dove twice ... going each time a long distance."

   Most dives are quite brief. P.K. Kinnear, a British researcher
watching common loons feeding on the ocean wintering range near
Tronda, off the Shetland Islands, estimated the median dive at ninety
seconds but did record several dives of up to five minutes. Carter,
another British researcher, estimated the median feeding dive to be
about twenty seconds. Of 258 dives, Carter recorded only three over
one minute with the longest ninety seconds. Sigurd Olson estimated
an undisturbed loon's average feeding dive at about forty seconds
and timed no dives over three minutes. The length of dives is
affected by several variables, but particularly the availability of
prey. Why dive to a hundred feet when most prey, small fish and
minnows, are usually near the surface?

   The questions of depth and length of dives are illuminated by
"Dewar's Rule", a rule-of-thumb estimate of time required for diving
birds to get under water. The rule, proposed by John M. Dewar in
1924, states diving birds need twenty seconds for the first six feet of
a dive and ten seconds for each additional six foot increment. The
journey to 200 feet would take a loon about six minutes. Of course
the loon has to get back to the surface which would require probably
an additional three or four minutes, placing a loon at the very edge
of any reported immersion times. Animals rarely subject themselves
to such danger, especially when so little is at stake. While loons
certainly dive to depths in excess of a hundred feet, their dives to
200 feet, supported only by the claims of commercial fishermen,
should be viewed with healthy skepticism.

   While the maximum depths and durations of dives are not well
documented, the physiological mechanisms the loon utilizes to make
extended dives are fairly well understood. Over a hundred years ago,
W.H. Slaney advanced the theory that diving birds could change their
specific gravity (the weight of an object divided by the weight of an
equal volume of water) by expelling air from their body cavities. His
contemporaries studying divers believed birds like the gallinule
remained underwater by grasping vegetation with their feet. Slaney
was right. To prepare for dives, loons expel air from their body and
compress their dense and waterproof plumage tightly, forcing out air
trapped between vanes of the feathers. With these air reduction
techniques, loons can sink into the water without noticeable effort.

   Just getting rid of unwanted air does not make a bird a great diver;
several other important physiological adaptations help the loon earn
its nickname of "the great northern diver." Fifty years ago noted
ornithologist Erwin Stresemann described the loon's skeleton as
completely lacking pneumaticity. While typical bird bones have an
interior sponge-like construction with lots of air spaces (a light-
weight skeletal construction ideal for organisms adapted for flight),
the loon is not typical; it is built for diving first and flying second.
Since any air within the body cavity, even in the bones, would
compromise diving abilities, the evolutionary path for the loon went
the way of water and favored the development of many solid bones
in the skeleton, which made diving much easier but flying a little
tougher.

   Loons also have ribs with overlapping spurs called, oddly enough,
uncinate processes which form a type of lattice preventing the rib
cage from collapsing from water pressure during deep dives.

   Another significant adaptation of loons is their ability to dive with
minimal oxygen. While underwater, loons derive needed oxygen
from the oxyhemoglobin and oxymyoglobin stored in the blood and
muscles. According to A.W. Schorger, the dark color of the flesh of
diving birds, including loons, is due to the presence of these
respiratory pigments. In addition, the loon's circulatory system helps
by reducing the heart rate.

   The loon's vital body organs have an unusual tolerance for low
levels of oxygen. In the textbook Avian Biology, edited by Donald
Farner and James King, this phenomenon is described as a system
which rations available oxygen preferentially to tissues particularly
susceptible to damage from oxygen deprivation. Farner and King
report a ninety-percent mechtachbolic reduction during
experimental submergence of some diving ducks. While such tests
have not been performed on loons, it is likely that similar
mechtachbolic reductions occur. Like most birds, loons normally have
a high metabolic rate which is reflected in their body temperature of
about 102íF.

*****