Birds do it; bees do it, butterflies, bats and eels do it—all leave one habitat to migrate to another, often thousands of miles away. NOVA penetrates the mystery of where animals migrate, why and how they get there.
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00:00Tonight on NOVA, trumpeter swans, Atlantic eels, bats, monarch butterflies, they all
00:13have one thing in common, to survive they must follow a fierce urge to migrate.
00:19How animals find their way is the mystery of Animal Pathfinders.
00:29Major funding for NOVA is provided by this station and other public television stations
00:34nationwide.
00:37Additional funding was provided by the Johnson & Johnson family of companies, supplying health
00:41care products worldwide.
00:46And by Allied Signal, a technology leader in aerospace, electronics, automotive products,
00:51and engineered materials.
01:22To be an animal is to be on the move, and an animal migration is a special kind of movement.
01:31Some migrations are marathon transcontinental odysseys between homes that are hemispheres
01:36apart.
01:40Animals migrate once a day, once a season, or once a lifetime.
01:54How animals manage their feats of endurance and navigation is the mystery at the heart
01:59of these great adventures.
02:02This is the story of the Animal Pathfinders.
02:13It's April in southernmost Brazil, over 2,000 miles south of the equator.
02:20This desolate shore is one of the few places in the world to look for clues to the mysteries
02:25of one of the longest of all bird migrations.
02:29This team of Brazilian scientists, part of a worldwide network, has come here to catch
02:34and band birds going north for the summer.
02:38The birds they are looking for are red knots, small shore birds a little larger than robins,
02:44almost always on the move.
02:47Now they are on the way to the Canadian Arctic.
02:52After dark, some of the birds will careen into these gossamer thin nets, and the team
02:57will measure and bend them.
03:03Each bird they catch will add its own story to the history of its species.
03:08But this information is hard to come by.
03:13The next place these birds can be seen and studied in any numbers will be 4,000 miles
03:18to the north at Cape May, New Jersey.
03:25Each spring it is a mecca for migratory shore birds.
03:31This is the time of year when thousands of horseshoe crabs come ashore to mate and lay
03:35eggs.
03:38The eggs they lay are a high-energy delicacy for arriving migratory birds, and the coincidence
03:43of the two gatherings is perfectly timed.
03:51This red knot has come non-stop from South America, and there is nowhere else on the
03:56Atlantic seaboard where it can refuel as it can here.
04:05As the tide drops, this large female and her suitors are stranded, and rows of eggs are
04:10left exposed on the beach.
04:13These eggs have plenty of protein and carbohydrate to fuel the next leg of the red knot's journey.
04:20The red knot is a specialist.
04:22It feeds only where beaches are exposed by the tides.
04:28Oblivious to the inroads made by hungry birds, the horseshoe crabs make their way ashore
04:34as they have once a year for hundreds of millions of years when the moon is full and the tides
04:40are high.
04:44Red knots have no experience of winter in either hemisphere.
04:47They follow a perpetual summer, traveling great distances between seasonal feasts such
04:52as this one.
04:59This annual banquet attracts birds and bird watchers alike.
05:05Scientist Brian Harrington was alerted by the Brazilians when the red knots left for
05:08the north.
05:09He has brought a group from the Manamette Bird Observatory of Plymouth, Massachusetts
05:14for an annual banding operation.
05:23The researchers select a spot where the knots will be feeding on eggs.
05:27They lay out a net that will be shot over the birds with an explosive charge, trapping
05:31them harmlessly on the beach.
05:35After a wait, the red knots return and begin feeding.
05:40When the birds are in front of the net, it's fired.
05:55Banding and recapture records reveal some basic facts about speed, flyway routes and
06:01winter and summer quarters.
06:03But knowing these logistical details only deepens some of the other puzzles of the red
06:08knot migration.
06:10How the red knot knows where to go, how to stay on course and when to stop, no one fully
06:16understands.
06:18This small bird is one of the world's fastest and most fuel-efficient migrators.
06:23And how its metabolism really works is still something of a mystery.
06:27This bird was marked in southern Brazil, and it was marked by a Brazilian team this year,
06:45in April.
06:46And you can tell from the blue flag here, it's got a Brazilian bird band on it.
06:53And it was probably marked very late April or early May, two or three weeks ago, in southern
07:02most Brazil.
07:03This bird may have stopped on the north coast of South America.
07:07And from there, the trip may have been non-stop, taking only a few days.
07:13Migrations such as this one require the greatest level of exertion of any vertebrate in the
07:17world.
07:20A red knot may lose 50 percent of its body weight getting here, and may gain it back
07:24as quickly by gorging on the eggs of the horseshoe crab.
07:47The red knot's ability to fly great distances means its young can fatten up in one of the
07:52most productive seasonal environments on earth, summer in the Arctic.
08:01The Arctic spring is a brief season, bursting with new life under the 24-hour sunlight.
08:09Migratory birds make flights of thousands of miles to reach this short-lived but unparalleled
08:14seasonal food supply.
08:20Many kinds of birds come here to feed and grow, to build strength for the long flight
08:24south.
08:34Red knots face a 10,000-mile marathon from the top of the world.
08:39In July, they leave nesting grounds on Victoria Island in the Canadian Arctic.
08:45Groups are seen at James Bay, Ontario, and then on the coast of Massachusetts and New
08:49Jersey in August.
08:53Only about 30 percent of all red knots show up at these known staging areas.
09:01A few are seen in Venezuela in September, many on the coast of Argentina in October,
09:09and then Tierra del Fuego, where they meet the missing 70 percent.
09:14Where they have been since August, no one knows.
09:18In April, they move north again, turning into refuel at that very special stretch of beach
09:24in southernmost Brazil.
09:30In April of 1986, the red knots were an example of the exception that proves the rule.
09:37Many birds don't stay long where they can't eat well.
09:43Heavy rains and gale winds affected food supplies, and the red knots didn't come here in their
09:48usual numbers.
09:51Where there were thousands the year before, there was only a handful in 1986.
10:01What makes this place special is a huge lake just behind the dunes, usually a shallow sheet
10:07of water.
10:09But after so much rain, it's too deep for the knots to feed on the juvenile snails,
10:14as they usually do.
10:20Until recently, observers lost track of red knots as they flew north from Tierra del Fuego
10:25in the spring.
10:27Since this Brazilian staging area was discovered in 1984, researchers have been coming here
10:32to band the birds.
10:34This year, the storms have scattered the red knots, showing that migratory paths can be
10:38as unpredictable as the weather.
10:44For two weeks, the team tends its nets.
10:48They will only catch about 600 birds in all, and just 60 of those will be knots.
10:57But of those 60, one was a knot banded at Cape May, New Jersey in 1983.
11:03It was caught here on April 20, 1986, 40,000 miles and three years after its original capture.
11:14The team works all night banding birds.
11:19The plastic bands are fused with heat.
11:30They even paint the underside of their wings for recognition in flight.
11:36Red knots spend most of their lives on the move.
11:40Their particular form of globetrotting means they avoid winter.
11:45All animals need to survive the rigors of the environment, and migration is just one
11:50strategy.
11:57Seasonal change may bring bad weather, but far worse is the lack of food it causes.
12:03To solve this problem, animals have three basic choices.
12:08They can hole up, and beavers take this first choice, building their dams in the fall and
12:13then living quietly all winter without ever actually hibernating, as some animals do.
12:24In the autumn, they cut branches ashore and add them to their dam, or to an underwater
12:29stockpile which feeds them in the shelter of their lodge all winter long.
12:38Other animals, such as elk, take the second choice.
12:42They tough it out, exposed to the elements.
12:46As long as they can graze, they are well equipped to survive the cold.
12:51But harsh weather is an equalizer.
12:53It brings the severest test most animals will face.
12:57Starvation, predators, disease take a toll.
13:07Half a world away in Africa's Serengeti plain, harsh conditions brought by an annual
13:11dry season force the third choice on the wildebeest and other animals.
13:17They must journey hundreds of miles to find greener pastures.
13:25For terrestrial animals, such long migrations are unusual.
13:30Unable to leave the surface of the earth, most land animals have limits on their freedom
13:35to move.
13:41But for an animal that can swim or fly, there are fewer obstacles.
13:46These are the great migrators, traveling enormous distances between feeding and breeding grounds.
13:54These animals are also pathfinders, for they often have to go where they have never gone
13:58before.
14:03Migrations are among the most demanding of animal enterprises.
14:07They require navigating with uncanny accuracy and performing great feats of physical endurance.
14:14They do all this essentially out of our sight.
14:20But there are some simpler, less elusive organisms that must also move to live, and understanding
14:26them yields clues to the behavior of the great migrators.
14:36Just below the surface of this mudflat, there is a perpetual micro-migration taking place.
14:42For some bacteria, oxygen is a poison.
14:47The best way to avoid it is to stay deep in the mud.
14:52Bacteria like this exist all over the world.
14:56What they all have in common is a row of material called magnetite, iron particles that act
15:02like the magnetized needle of a compass when it responds to the magnetic field of the earth.
15:17The earth's magnetic field sweeps far out into space, and it intersects the surface
15:22at an angle, going down towards the giant magnet-like dynamo created by the churning
15:28molten iron at the earth's core.
15:40In the northern hemisphere, north is downward, because the lines of force descend into the
15:45earth.
15:52Like the needle of a compass, the magnetite in these bacteria guides them down, away from
15:57the poison, oxygen.
16:02A magnet can temporarily reverse the local field, so they swim in the wrong direction.
16:07Each time the field is reversed, the bacteria do an about-face.
16:21Other kinds of bacteria thrive on oxygen.
16:26There's more of it near the surface of this air bubble.
16:32When these randomly swimming bacteria discover a concentration of oxygen, they converge on it.
16:42The earth's magnetic field, and very small changes in the concentration of a chemical,
16:47are just two of the cues that guide organisms on their journeys.
16:53More complex animals can use these road signs too, but they also need increasingly sophisticated
16:58tools to direct their travels.
17:04Many of these migrations seem to use pinpoint navigation for extended round-trip travel.
17:11One of the most celebrated finds in animal behavior has been the discovery of how the
17:15honeybee navigates.
17:21Bees must search far and wide for food.
17:24They gather nectar and pollen from wildflowers that may be as far as half a mile from the
17:29hive.
17:31They may be a different kind from time to time, and their yield may vary.
17:40There are specialized forager bees whose job is to find flowers, and then tell the rest
17:45of the hive where they are.
17:49But random searching may take the forager far from the hive.
17:54The bee's problem is how to tell the others where it has been.
18:02The problem is compounded by the bee's vision.
18:07This blurred mosaic is roughly what the world looks like to a bee, so it must be able to
18:11give accurate directions without relying on landmarks.
18:17Bee language is as complex as their social organization.
18:22This forager performs a repetitive dance which tells the others which direction to go in,
18:27how far to go, and what they will find when they get there.
18:35If the other bees want to know more, they make a special noise.
18:41If the forager doesn't stop dancing instantly, they will sting it to death.
18:48These bees want to know what kind of nectar and pollen they should be looking for.
18:53The forager has brought some back which they sample with their antennae.
19:01The dance is a diagram of the trip, telling the others what heading to take relative to
19:06the sun.
19:08Each time the bee runs up and to the left, it conveys the same information.
19:14To find flowers, steer about 15 degrees to the left of the sun.
19:24Direction alone is not enough.
19:26The workers have to know how far to go.
19:30Bees can count, so the forager tells distance by the number of times it wags its abdomen.
19:35In this case, 1, 2, 3, 4, 5, 6.
19:44Experiments have shown that this translates to 300 meters.
19:49With a heading and a distance, the bees now have the right directions.
19:54They're looking for goldenrod pollen and nectar, 15 degrees left of the sun, 300 meters away.
20:03But they can't see very well, so what kind of navigational aids does the bee have that
20:08enable it to follow these directions so accurately?
20:18Since the sun moves an average of 1 degree every 4 minutes, 15 degrees left of the sun
20:24isn't accurate for very long.
20:27So the bee must compensate for the sun's movement over time.
20:34The bee does this with a clock accurate to within a few minutes over many hours.
20:40But no one has any idea just how this internal clock works.
20:45More is known about the solar compass in the eye of the bee that keeps it on course.
20:51A bee's compound eye is made of hundreds of tiny hexagonal lenses.
20:56Like a telescope, each lens sees a very narrow field of view.
21:02By sighting the sun through the correct lens for a particular time of day, the bee can
21:07hold a constant heading as it flies.
21:11And as the sun moves, the bee's internal clock tells it to sight through a series of adjacent
21:16lenses.
21:20Once it has a heading, the bee must measure the distance it was told to fly.
21:25For this, it has an odometer, which works by monitoring fuel consumption.
21:31The bee tells how far it has flown by measuring how much energy in the form of honey it's
21:36used up.
21:37A good thing, because it leaves the hive with only enough honey for the trip.
21:44Because of how a bee sees the world, vision alone is not enough to guide it on its way.
21:51Instead, it has an array of senses that act like sophisticated instruments.
22:00The bee learns the way from its mates and also from experience, and it travels many
22:05times a day.
22:07But there is another kind of migration, the great once-in-a-lifetime journey.
22:13Unlike honeybees, some animals must be born knowing how and when to go to far-off places
22:19they have never seen before.
22:27It's late summer in these New England hills, and one of the longest of all annual insect
22:31migrations is quietly taking shape.
22:36The key to this journey is the milkweed plant and a homing instinct carried by the brilliantly
22:40colored caterpillars of the monarch butterfly that are feeding on it.
22:46This is the fourth and last generation of the season.
22:51The caterpillars, the eggs, the pupae, even the adult butterflies of other species can
22:56survive the harsh northern winter, but the monarch cannot.
23:03After weeks of feeding and fast growth, the three-inch caterpillar begins the miraculous
23:08transformation that will allow it to escape the certain death that winter would bring.
23:17In a few hours, the caterpillar has shed its skin, and the hard, protective casing of a
23:22chrysalis has formed.
23:25Inside, an utterly different creature is taking shape, and with it, the instinct to make a
23:35great migration.
23:46The earthbound caterpillar has grown wings, and it has inherited the ability to find its
23:51way on an epic journey out of all proportion to its apparent fragility.
23:58Using a long, slender tongue, it will now stoke up on nectar.
24:05Monarchs usually breed and then die within a few weeks, but not this generation.
24:12The temperature and day length may keep them from breeding, and that gives them eight more
24:16months of life than their parents.
24:24The autumnal equinox seems to trigger the migration.
24:29Very little is known about how they navigate, but somehow they will find their way to the
24:33only known place they can survive the winter.
24:39One theory holds that monarchs may steer by the sun, correcting for its movement across
24:44the sky like honeybees.
24:51The monarchs that start in New England and eastern Canada have the longest trip.
25:01The monarchs west of the Rockies move to the coast of California, but the vast majority
25:05of North American monarchs, those east of the Rockies, move toward Mexico.
25:14When they cross the Tropic of Cancer, leaving cold temperatures far behind, they might be
25:19expected to disperse, but they don't.
25:23What they do instead is the greatest enigma of all.
25:27Having crossed half a continent, tens of millions of butterflies converge toward a pinpoint
25:32on the map of Mexico, a 30 by 50 mile area of high mountains and Spanish colonial towns.
25:47Except to the people who live here, the wintering colonies of the monarch butterfly were unknown
25:52until 1974.
25:59Above the hillside farms and the thin, cool air at 10,000 feet, there is a habitat that
26:04may be unique in Central America.
26:09Beyond the transvolcanic range, it is a rocky wilderness of ravines and fir forests.
26:18Since the butterfly colonies were discovered here in 1974, scientists like Lincoln Brower
26:23have been making annual visits of their own.
26:28Scientists had long, long wondered where in the world monarch butterflies migrate to out
26:32of the whole eastern part of the United States.
26:36And over a 30-year period, Professor Fred Urquhart of the University of Toronto developed
26:43a tagging program, recaptures over many years established a southwesterly migratory route
26:52for the monarch.
26:53But once the monarchs crossed into Mexico, nobody knew where they went.
26:59But in approximately in 1974, one of Professor Urquhart's associates in Mexico City was traveling
27:10through the mountains of Michoacan and came across a hint of the monarchs in the mountains.
27:18And then sleuthing the area, he finally discovered this overwintering site, tens of millions
27:25of butterflies in this tiny area of Mexico, which had been there since the beginning of time.
27:47These forests combine the seasonal stability of the tropics with the coolness and the moisture
27:52of the mountains.
27:54It's not warm enough to breed and lay eggs, so the monarchs are in reproductive cold storage
27:59until next spring.
28:07There are known to be about 15 separate colonies in the transvolcanic range.
28:12Precisely how the monarchs find them is still a mystery.
28:19From December until March, they eat nothing, living only on fat reserves.
28:25But they must drink water, and a stream is crucial to the location of each of the colonies.
28:33Brouwer, his graduate students and other scientists are trying to piece together the features
28:47that make this place so special to the monarch butterfly.
28:54Seven years ago we were here, and the butterflies were about, oh, a half a mile away from where
29:00we're sitting now. And last year we were here, too. We've been here every year. But last
29:06year the butterflies were in exactly the same place that they are now. And over the years
29:11they've been in the same places or very nearby. And there are many different mountain ranges
29:17in this part of Mexico, but they only come to a select few ranges. And one of the real
29:23mysteries is why do they do that?
29:27To understand the monarch's life story, it's necessary to appreciate its relationship with
29:32milkweed. The ancestors of both modern milkweed and monarch butterflies were once found no
29:38farther north than Mexico and the extreme southern United States.
29:4520,000 years ago there was a continental ice sheet. One theory suggests that when the ice
29:51sheet retreated over thousands of years, milkweed may have spread north, and the monarchs may
29:57have moved with the milkweed. But at the end of each summer they had to return to Mexico.
30:06Then as now they were truly tropical butterflies.
30:11The monarch never adapted to freezing conditions. Instead, it evolved the ability to travel
30:18great distances. Alfonso Alonso, a graduate student from the University of Mexico, is
30:27tagging butterflies. If this butterfly is recovered when it migrates north in the spring,
30:35it will add one more facet to the developing pattern of monarch movements. But first it
30:42must survive the winter, usually mild, but not always.
30:49Winter storms can be very devastating. In fact, one in 1981, 20% of the colony froze
30:57to death that year. We estimated that year that about 10 million butterflies froze to
31:04death. And then after that snowstorm, it cleared and it got extremely cold. And thousands of
31:16butterflies started dropping out of the clusters over a period of several days.
31:23We also know that the butterflies that are in the center of the clusters are better protected
31:31than those that are on the outside. And where there have been any trees cut at all, the
31:38storms get into the forest and do a great deal more damage. And so once again, the intact
31:45oyamel forest here is just absolutely of immense importance.
31:52The Mexican government and conservation groups are trying to protect the sites, but land
31:57use is a complex and controversial issue in Mexico. To make way for agriculture, fir forests
32:03are sometimes partially cleared. And without the fir forests, it is doubtful that monarchs
32:09could survive winter exposed to the elements in the mountains. This situation gives a special
32:15importance to the research conducted here.
32:22Jim, I was just walking along here looking at all these butterflies that have been eaten
32:29by birds, and I found one of Professor Urquhart's tags. Look at that. Wow, that's amazing. What
32:36number is it? 78,900. Wow. It says on it, send to Zoology University, Toronto. This
32:45butterfly could have been marked in Michigan or Canada. So this is the first one of these
32:52I've found in two years. In nine years working here, I found four. That's pretty amazing.
32:59I wonder how many he's tagged over the years. Thousands and thousands. Chances of getting
33:05one are remote. So this is worth its weight in gold. That's right.
33:13The spring flowers are as important to the monarchs as the fall flowers. After a four-month
33:21fast, the butterflies feast on nectar, storing fuel for the trip back into North America.
33:28Like this pair, the entire colony begins to mate before it disperses in late March.
33:47Around the spring equinox, the colony moves north. As much as a movement of individual
33:55butterflies, the monarch migration is a movement of genetic traits. The inherited characteristics
34:02that underlie the migration are strengthened each year. Virtually the only butterflies
34:08from east of the Rockies that breed successfully are those that survive the winter by coming
34:13here to rest and fast in the cool mountain air.
34:25A hundred miles away are the winter quarters of a colony of Mexican free-tailed bats. Unlike
34:37the butterflies who come south to fast, the bats come south to feast.
34:44What's special about this lake are the quantities of insects that breed here. The insects feed
34:50the fish that feed the fishermen. They are also food for the bats that live on the island
34:55in the middle of the lake. The bat colony is just under the statue and coexists with
35:03the people of a small fishing village just below it.
35:11Like the monarchs, the bats are seasonal visitors. They move north in the spring and south in
35:17the fall, returning faithfully to this cave as the monarchs do to their mountains.
35:33Around sunset, the colony emerges. First a few, then a flash flood of bats. This happens
35:41every night.
35:48Their seasonal treks north and south begin like regular nocturnal hunts. One night in
35:54the spring, the colony leaves the cave and doesn't return for six months. Bats navigate
36:00with a precision that is as much of a mystery as the way they pick the timing for their
36:04departures. Maybe it's the equinox, maybe a change in the food supply, or the weather.
36:11No one is certain.
36:14For whatever reason, from many locations in Mexico, the freetails make their way north
36:19in April. Many of the bats return to summer quarters at Bracken Cave near Austin, Texas,
36:26the largest bat colony in the world.
36:31Dr. Merlin Tuttle is a biologist who studies bat ecology and behavior. The breathing mask
36:40is a necessity. Without it, he could be overcome within minutes by ammonia fumes from bat droppings
36:45in the cave.
36:52Dr. Tuttle works to spread information about the benefits brought by bats, from insect
36:56control to plant pollination. Even a small colony of only a few thousand eats millions
37:04of insects nightly. This one is enormous, containing from 20 to 40 million bats.
37:14In a single night of hunting, they devour 250,000 pounds of insects. But they must hunt
37:20over a vast territory each night, and then find their way home in the dark from miles
37:25away. And once they do that, females must be able to pick out their own young from millions
37:36of others.
37:40Towards the end of the day, the bats become more active. First one goes, then the air
37:48is thick with bats. The exodus can begin long before sunset, and it lasts for hours.
38:06Except for nursing young, the entire colony leaves every night.
38:18They usually go in the same general direction at first, but if these bats are to find a
38:30quarter of a million pounds of food every night without eating themselves out of house
38:34and home, they have to cover a huge area. The colony spreads over thousands of square
38:42miles. A single bat may fly a hundred miles, twisting and turning, and yet it is able at
38:53the end of its hunt to fly straight home in the dark.
38:59Stragglers arriving after sunrise may find their way by memorized landmarks. Approaching
39:04the cave, sometimes from a height of two miles, they fold their wings and dive. Inside
39:12the cave, they use a sonar system, emitting high-pitched whistles. Echoes bounce off
39:19solid objects, warning of obstacles, indicating openings for safe flight.
39:27Females are known to return to roosts within the cave, perhaps by hearing the cries of
39:35their own young, perhaps by recognizing some subtle contour of the cave wall, but no one
39:41really knows. When the colony moves south in the fall, juvenile and adult bats migrate
39:49together, but other kinds of migration separate adults and their young. Such animals set off
39:55simultaneously in opposite directions, and yet their journeys are paired, essential to
40:01the survival of the species.
40:10Every spring and fall, a migration passes largely unseen through the estuaries of eastern
40:15North America. Sheltered and productive, these waterways are home to the American Atlantic
40:21eel, except when it breeds. Until the 1920s, no one even knew where the breeding grounds
40:28were until they were finally found thousands of miles away, east and north of the Caribbean
40:34islands, seaward of their fringing coral reefs, in the Sargasso Sea.
40:43Adult eels spawn here in late winter. When the eggs hatch, the non-swimming larvae slowly
40:49change from leaf-shaped drifters to tiny two-inch eels.
40:59Weak swimmers. At this stage, they must rely on ocean currents to carry them away from
41:04the unproductive Sargasso, where there is little to eat.
41:10They drift west and north, meeting the Gulf Stream off the Carolinas. It is thought they
41:15detect faint traces of river water far out at sea, attracting them toward the land.
41:21The first thousand-mile ride has been on ocean currents. Those that come ashore here at Penobscot
41:27Bay will now hitchhike on the great tides of the Gulf of Maine.
41:34Researchers from the University of Maine are studying the migration of the juvenile eels
41:37as they ride the flood tide inland during late May.
41:43They catch eels in nets towed behind a research vessel. At the same time, they record data
41:49about the water and the tide in order to try and solve one of the many mysteries of the
41:54eel migration.
41:59They know that when the tide goes out, the eels drop to the bottom and wait six hours
42:04for it to flow inland and carry them farther upstream.
42:11But how do the tiny three-inch eels read the tides? Maybe they sense the minute electric
42:16fields generated by flowing water. Or they may perceive a change in sound with the turn
42:22of the tide. Perhaps they detect a difference in the chemistry of the water, as they might
42:28have far out at sea.
42:32This tiny glass eel has covered the greatest part of its journey in a year and a half.
42:37The rest of the trip, though shorter, may take many years.
42:46When eels enter the estuary, some will stay and mature in this salty water.
42:54But many will continue inland, riding the tide as far as Bangor, 50 miles up the Penobscot.
43:02From here, they must swim upriver. The tide goes no further.
43:11Progress is extremely slow. It has been measured at 100 meters in three months. No one knows
43:18what makes them struggle upstream, but it is a powerful urge, and even great obstacles
43:23do not deter them.
43:26They get around these by clinging to wet moss, by slipping into cracks, avoiding the full
43:31force of the water.
43:46Several years later, and 100 miles inland, some of the eels reach the headwaters.
43:54They'll stay here for about 10 years, then return to the Sargasso Sea to spawn.
44:08Nearly all oceans have eel migrations, but none as long as the Atlantic eels.
44:13One theory says its great distance might be explained by continental drift.
44:19Two hundred million years ago, the continents were close together. Then they drifted apart.
44:25The line shows a hypothetical range for the ancestors of Atlantic eels, already migratory
44:31150 million years ago.
44:37As the Atlantic widened, the migration might have lengthened into the great journey it
44:41is today.
44:46For the eels that travel to Europe from the Sargasso, the trip is the longest of all.
44:51Then, as now, eel eggs could only hatch in salt water. But no one knows what is so special
44:58about the Sargasso, or even how returning adult eels find it.
45:09It's certain only that they all return here to spawn, and probably die, and that their
45:14young must then leave to mature in places thousands of miles away.
45:22To find its way, the eel relies on a powerful combination of senses. It has an extremely
45:28acute sense of smell that may help it catch prey, or guide it to fresh water from out
45:33at sea as a juvenile, and back to the Sargasso as an adult.
45:41Just behind the head, the lateral line runs down the body. It detects sound waves, water
45:49currents, and electrical impulses, and may be used in navigation.
45:56This adult eel will need a good guidance system. It is September, and it is about to retrace
46:02its epic journey.
46:07Eels only travel at night, and they wait for bad weather.
46:15The first heavy rains of the autumn raise water levels and increase current flow.
46:27The torrents of fresh water seem to be a signal to sexually mature eels that it's time to
46:31move.
46:35During the return journey, their gut degenerates, and they may not eat at all, for they have
46:40stored enough fat to fuel a 1,000 mile return swim to the Sargasso.
46:54There are many obstacles between the eels and their distant breeding grounds. They will
46:58have to face rapids, dams, and waterfalls, and some will get caught in eel weirs.
47:08For generations, fishermen have driven stakes into the riverbed here, and fastened wooden
47:13grills on them from bank to bank.
47:23The grillwork lets the water through, but it guides the eels into catch boxes set in
47:27the riverbed. When the eels are running heavily, this trap might catch six or seven hundred
47:33pounds of them a night. They are running in hundreds of rivers along the Atlantic coast
47:39of Europe and North America. By all going to the Sargasso to breed, a genetic mix is
47:45assured, and the migratory trait is reinforced.
47:52If the mature eels were to stay, they would perish. Their body chemistry is changing,
47:57and now they cannot survive long in fresh water.
48:03When adult eels leave this river, they will pass another generation of young eels swimming
48:07ashore.
48:11The instinct to migrate is a powerful, irresistible urge, but it is more than just a genetic trait.
48:18It is a strategy that works, for the only eels that breed successfully are those that
48:24complete the journey to the Sargasso Sea.
48:37Migrations are a long time in their development, and the instinct is strong, but not immutable.
48:45It is October in Minnesota. The call of the Canada goose is autumn music along the Mississippi
48:54flyway. But the voice of North America's largest waterfowl has been absent here for
49:02a hundred years. In the Midwest and throughout most of their original range, trumpeter swans
49:08were hunted out for their feathers by early settlers.
49:21Larry Gillette of Hennepin Parks near Minneapolis, Minnesota, has been working on reintroducing
49:26the trumpeter swan in the upper Midwest.
49:31We have been undertaking this project in an effort to restore the trumpeter swan as a
49:36nesting migratory species in Minnesota. And the real key to a successful project is migration.
49:45The fear had been raised that these birds had simply lost the instinct to migrate. So
49:50it was a great relief to us that in 1984, 29 of our 46 free-flying birds did migrate
49:56from Minnesota and travel down to Missouri, Kansas, and Oklahoma where they spent the
50:01winter. 18 of these birds returned north in the spring. This was the first documented
50:08migration of trumpeter swans in the lower 48 states in the past half-century. It showed
50:16in fact that these birds did still have that instinctive ability to enable them to migrate.
50:24The trumpeter swan was once widely distributed. But when early settlers hunted them out, there
50:32were only a few groups left, surviving by finding water that was open year-round. So
50:38they became non-migratory. In 1976, Hennepin Parks brought some of these to Minnesota.
50:46Cold is no problem for the swans, but finding food is. Underwater plants form their natural
50:55diet. So when ice forms, they must move or starve. In Hennepin Parks, a little water
51:04is kept open for swans that need to winter over. But they have to be fed on corn and
51:09grain because there is not enough open water for them to feed naturally. It is December
51:181985. A year ago, 29 swans went spontaneously south. They had never migrated before, so
51:26they had to learn the route by trial and error. This year, winter was early and cold. Most
51:34of the swans stayed at the preserve as there was little food to be had in the wild.
51:44But there were at least five that did go south, including some fledglings that had never gone
51:51before. Trumpeter swans usually migrate in family groups, so young swans learn the way
51:59from their close relatives. But these fledglings were unrelated and probably had to find their
52:06own way once they left the preserve. After leaving Minnesota, the swans flew south. Then
52:14they disappeared until some were seen briefly in Oklahoma and Kansas, where swans had always
52:33wintered. They were not seen again until they returned to Minnesota the following spring.
52:44This aggressive display warns other waterfowl to stay away from the nesting territory. Swans
52:53tend to mate for a long time, often for life. These rituals reinforce bonds and discourage
52:59competitors such as these Canada geese. This is what the swan migration is all about, surviving
53:07the winter in order to take advantage of northern wetlands. The instinct to build a nest is
53:17inherited, but some of the skills are learned. Apparently, the same is true of the ability
53:24to migrate. By late May, swans begin to migrate to the wild.
53:36In May, pairs of swans in ponds throughout the preserve have clutches of eggs. And by
53:45early June, the pair are parents. This effort to bring the trumpeter swan back to Minnesota
53:53is a fascinating study of the causes of migration. Fed by men during the winter, this group of
54:04swans does not have to migrate, but some of them do, and no one has yet understands exactly
54:11why. This experiment raises many questions, but it has yielded one important discovery.
54:23Even after a hundred non-migratory years, these swans can still read the seasons, fly
54:30in the right direction, and memorize landmarks. Their legacy has not failed.
54:38The urge to migrate dies hard. It has been a key to survival for hundreds of thousands
54:45of generations, and these swans are still the pathfinders their wild ancestors used
54:51to be.
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