Michigan lies within important migration corridors for a wide variety of birds and insects.  In the article below, author Bruce Gill explains how and why animals make their often perilous movements.  Understanding migration is key to understanding and appreciating our state’s unique wildlife heritage.  Bruce Gill is retired from the Colorado Division of Wildlife where he worked as a wildlife researcher.

A herd of elk carefully picks its way through the snow that covers an age old trail across the Continental Divide.  Elk have crossed the Continental Divide every autumn and spring for centuries.  Like most long-lived mammals, they navigate primarily by memory, sight, and smell.  The older, experienced cows teach their offspring the travel routes and their offspring pass on this knowledge to future generations. 

Migrations, annual journeys back and forth from a winter to a summer home, are ancient rituals, widespread among several groups of animals.  Zooplankton among the smallest animals, migrate vertically several hundred feet each day in response to light, pressure, and the availability of nutrients.  Whales among the largest animals migrate 10,000-12,000 miles round trip each year from their calving areas along the coast of Baja California to their summer feeding grounds in the Arctic Sea. Among reptiles, the painted turtle summers at the pond’s surface and then migrates to the bottom and burrows into the muck when cold temperatures signal the long winter freeze, returning to the surface with the spring thaw.  Among fishes, juvenile salmon journey thousands of miles out to sea to mature and then unerringly navigate back to their birth sites to reproduce their own offspring.

The ability of animals to find their way home after traveling far afield is called homing.  Even land mammals that don’t migrate can find their way back home with remarkable accuracy.  Recently, a mature male lynx , originally captured in British Columbia and released into the San Juan Mountains of Colorado, was caught in Alberta apparently on his way home after traveling nearly1200 miles from his release site.  Migration probably developed as a refinement of this homing ability.

 The fact that animals migrate has been known for centuries.  Why and how they navigate is an unfolding mystery. During the last 4 decades experiments with migrating birds have begun to unravel some of these mysteries.  Four paramount questions have dominated the research:  why and how did bird migration evolve, how do birds know where to go, how do they know when they have reached their goal, and how will global climate change affect the migrants?

Migration in modern birds developed in response to pre-historic global climate change.  Fifty million years ago or so, the earth’s climate was relatively moist, warm, and stable.  So mild was the climate that tropical and subtropical vegetation spread as far north and south as the poles. During this period of plenty, competition for resources was probably relaxed somewhat.  Most bird populations were either sedentary or migrated only short distances.  But, current research indicates that even sedentary or short-range migrants possess the genetic machinery to evolve within 3 to 6 generations into long-range migrants whenever ecological conditions favor the change.

About 30 million years ago the climate began to cool steadily especially at northern latitudes, ushering in the last ice ages.  As the ice sheets advanced and receded, habitat availability contracted and advanced.  With each contraction of the ice sheets, populations of long-range migrants rapidly evolved to take advantage of new, favorable habitats.  With each glacial advance, habitat shrunk and populations retreated to the tropics and subtropics and bird populations became less migratory.  Since the retreat of the last great ice sheet, migration became an increasingly advantageous adaptation.  More bird species and more populations within species have become migratory and migration distances have become progressively longer.   

Migrating longer distances from summer to winter homes and back requires an accurate navigation system, raising the question, how do migrating birds know when and where to go?  At the base of the brain lies a gland called the hypothalamus that functions like a complex switchboard.  It controls chemical messages that switch critical bodily functions on and off like hunger, thirst, metabolism, growth, and sex.  Successful migration requires the coordinated development of breeding behavior, feather molting, diet selection, the accumulation of fat and muscle that will fuel the migratory journey.  Deep in the hypothalamus lies a collection of cells called the suprachiasmatic nucleus.  These cells function like a biological clock that monitors changes in day length.  Changing day length signals the biological clock that the time for migration is approaching.  The hypothalamus responds by stimulating the bird to change to a high carbohydrate diet, to store fat and protein, to molt in new flight feathers, and to prepare to turn on or off breeding behaviors.   When the body is completely ready for the migratory journey, the clock stimulates a restless surge that ultimately compels the bird to take flight.

Once the migratory journey has begun, information genetically stored in the biological clock tells the migrant how long and in what direction it must migrate before reaching its destination.  Because the timing and direction of migration are genetically pre-programmed, even naïve migrants such as recently fledged juveniles know how to get to their winter destinations without the aid of experienced adults. 

However, just knowing when to migrate and what direction to fly does not assure that a bird will be able to find the way home.  Most migrants fly at night through all kinds of weather, conditions that additionally require some kind of guidance or navigation system.  Birds have developed at least three distinct navigational compasses – a sun compass, a magnetic compass, and a star compass.

Like many an elk hunter, migrating birds use the position of the sun as a reference cue to guide them toward their destinations.  However, the position of the sun changes throughout the day and that change must be accounted for in navigation.  Unlike elk hunters, who consciously recalibrate their position relative to the moving sun, the bird does it subconsciously.  The biological clock tracks the sun as it arcs across the sky and continuously recalibrates the bird’s relative position. 

Nocturnal migrants prepare for each stage of their journey at dusk of each day.  Before departing, they check the position of the sun just at sunset when it is accompanied by a band of polarized light that lies perpendicular to its setting point.  The combination of the sunset point and the polarized light band provide the migrant with an initial navigation cue as it initiates its nocturnal journey.

The bird’s magnetic compass is located within special receptors on the retina at the back of its eye.  Pigments in those receptors, called cryptochromes, react to the earth’s magnetic field.  The receptors translate those reactions into a perceptible image, although no one knows exactly what that image looks like.  As the bird travels northward or southward, the inclination of the magnetic field increases towards the poles and decreases toward the equator.  Researchers believe that birds actually see those changes through their eyes and use them as visual cues to adjust their flight direction.

Birds apparently use the star compass much as sailors navigate from the position and regular movement of the stars.  Although a migratory bird comes equipped with a star compass at birth, it must learn how to use it.  At night during their development, young birds observe the regular rotation of the stars around the North Star.  They seem to learn the rotational pattern and subsequently use this fixed pattern as a navigational aid later in their adult life.  During migratory flight, night vision is enhanced by special nerve cells near the upper surface of the bird’s brain.  The cells seem not only to enhance night vision, but also to coordinate and interpret visual images of stellar patterns.

None of these compasses seems to be used independently.  Rather, they are all used together either as backup systems or as independent checks on navigational accuracy.  As the novice migrant prepares to start on its maiden voyage, its winter destination is pre-programmed within the biological clock.  At dusk on each leg of its migration, the bird checks its position with the sun compass and the pattern of polarized light.  As it sets off, it uses the star compass to maintain course.  If clouds obscure the stars, the magnetic compass is called into play.  If stars are visible, the magnetic compass is used to corroborate information from the star compass and to make corrections when winds blow it off course.

As it travels, the youngster notes the location and direction of prominent landmarks such as mountain chains, coastlines, rivers, and stores these in its memory in the form of a topographic reference map.  Birds also have well-developed olfactory systems and along its journey the young bird also notes changing smells of plants, earth, and water.  These too it stores in memory banks, creating an odor map.  Both the topographic and odor maps will be used as references in future migratory journeys to make the trips ever more accurate, efficient, and less costly energetically.

Over a period of several weeks, the migrant will make several stopovers to rest and refuel.  At last, the biological clock will signal that the bird has reached his general destination.  It will spend several days exploring habitats within the general area until it finds one that is suitable to spend the winter.  As winter slowly turns to spring, the young bird’s biological clock will begin to prepare it for the return journey north again when it will use genetic information, remarkable navigating devices, and previously constructed topographic and odor maps to lead the way back to the summer breeding grounds.

Migration is marvelously adapted to seasonal environments.  It coordinates physical development with the departure time so the bird is capable of the long journey.  It assures that a bird’s arrival on the breeding grounds is synchronous with the resurgence of spring food.  And it assures that the bird departs the breeding grounds before winter drastically reduces food availability.  It is a system that has been developed and fine-tuned over thousands of years during a period of slow environmental cooling.  Now, however, that system is under challenge.  For the first time since the retreat of the last ice sheet, the earth is experiencing a prolonged and rapid warming trend.  What this all means to the future of bird migration has touched off a fierce scientific debate and a frenetic scramble for facts.  The answer to the question of how global warming may affect migratory birds is far from clear.

Two schools of thought, pessimists and optimists, dominate the debates.  Pessimists predict that global warming presages the end of bird migration and perhaps the doom of many migrants, particularly long-range migrants.  They offer as evidence the apparent fate of long-range migrants in the face of global climate change.  The effects of global warming are most pronounced at higher latitudes, breeding grounds of most long-range migrants.  Timing of migration for long-distance migrants is critical because the birds must arrive on their northerly breeding grounds when food is most abundant to assure successful reproduction.  Although the timing of food abundance is affected primarily by temperature, the birds’ spring departure dates are driven primarily by day length.  Global warming seems to have uncoupled the synchrony between departure dates and optimal breeding conditions, causing steep declines in bird numbers.

Optimists point out that birds have successfully adapted to extreme climate change several times throughout their evolution.  They now come genetically pre-adapted to change.  It appears that all bird populations have individuals that are sedentary and others that are migratory.  Birds adapt to climate change by rapidly modifying the proportions of migrants and non-migrants.  This genetic adaptation should allow most bird populations to cope with climate change, although not with equal success.  In addition, although departure and arrival times and breeding times of birds seem to be hard-wired into their genes, those times are windows of opportunity rather than fixed dates.  This flexibility coupled with an ability to rapidly learn from experience, optimists argue, should provide migrants with the tools to cope with a rapidly changing climate.  Which viewpoint is correct?  Only time will tell.  Regardless, there is widespread agreement among natural scientists that the narrow window of opportunity to reverse the warming trend is closing rapidly.  

Some of Michigan’s Long-Distance Migrants

Species                                                Southernmost Wintering Grounds

Sandhill crane                                       Florida

Killdeer                                                Northern South America

Spotted sandpiper                                Northern Argentina

Common snipe                                     Northern South America

Woodcock                                           Louisiana, Mississippi, Alabama

Yellow-billed cuckoo                           Central Argentina

Chimney swift                                       Northern Chile

Ruby-throated hummingbird                 Panama, Cuba

Kirland’s warbler                                 Bahamas

Bruce Gill

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