Migration

By Jon Friedman

(The topic of avian migration is too broad and extensive to examine in a single newsletter article. This article focuses on basic and historical information related to our understanding of migration. Future articles will focus on other important aspects that enable us to more fully understand this natural phenomenon.)

Introduction
Scientists and researchers from across a broad spectrum of the natural sciences (including ornithology, meteorology, climate change, paleontology, geology, etc.) have been piecing together the puzzle of avian migration for many years now. With each passing year and the technological advancements available to those who study migration, a fuller and clearer picture continues to come into sharper focus. And, while many facts have emerged that are well-founded and form the basis of our knowledge of the phenomenon of migration, there is still much to be discovered and learn from.

 We now know that the annual process of migration dates back far in the earth's history. Migration patterns were established while the earth's continents were still forming and in flux. These same patterns and routes have been used as long as there have been birds on the planet and have continued relatively unchanged into modern times. Geologists and paleontologists can demonstrate with great accuracy when and where birds have lived in all seasons around the globe. Migration occurs from the Arctic to the Antarctic, north and south in the hemispheres, and even east and west across the continents. Migrations also occur from lower to higher elevations. It occurs on land and sea as well as the more familiar flyways. Scientists believe that a majority of the earth's avian population, i.e. thousands of species, migrate in some fashion or other. Some travel seasonally for relatively short distances, such as birds that spend their winters at lower elevations and move up to mountaintops for the summers. Others, like the Arctic Tern, travel more than 25,000 miles seasonally between the northern and southern poles

Historical Record
What causes birds to undertake these arduous and dangerous migrations? There is no one simple answer. Many theories have been developed over time. Most of those theories have been debunked by modern science. For example, during biblical times, it was believed that when birds left an area or region en masse, they lived underwater in the seas and oceans. Another theory for their regular appearance and disappearance was that when they left a familiar place, they moved into muddy areas near water sources where they stayed hidden in hollow trees and caves or buried themselves in deep mud and hibernated till the weather changed and foods became abundant again. The concept that birds migrated for purposes of hibernation persisted as a result of early observations of poorwills and other members of the goatsucker family of birds that have the ability to become torpid and thereby avoid dealing with the realities of surviving harsh northern winters.

Bird migrations were probably among the first natural phenomena to attract the attention and intrigue the imagination of man. Regularly recorded observations date back about 3,000 years, to the times of Homer, Herodotus, Aristotle, and others. In the Bible, there are several references to the periodic movements of birds, as in the Book of Job (39:26), where it mentions observations of hawks seasonally flying south. In Jeremiah (8:7), it is written, "the stork in the heavens knoweth her appointed time; and the turtle dove, and the crane, and the swallow, observe the time of their coming." And the flight of masses of quail that saved the Israelites from starvation in their wanderings in the wilderness of Sinai is now recognized as a vast movement of migratory quail between their breeding grounds and their winter home in Africa.

 The fact that bird migration was gradually discovered over many centuries through a variety of observations, without the benefits of modern science and technology, led to many conclusions that we now recognize as error-filled and somewhat amusing. However, some observations proved helpful for early mankind. More than 5,000 years ago, the seasonal movements of large numbers of birds over the Mediterranean island of Cypress was understood as a signal that it was the right time of year to plant crops.

Aristotle's Observations

Aristotle image from Wikipedia

One of the earliest naturalists and philosophers from ancient Greece was Aristotle who was the first in written history to discuss the subject of bird migration. He carefully observed and noted that cranes traveled from the steppes of Scythia to the marshes at the headwaters of the Nile and that pelicans, geese, swans, rails, doves, and many other birds likewise passed to warmer regions to spend the winter. Aristotle was also the originator of the theory of transmutation, basing it on the fact that frequently one species will arrive from the north just as another species departs for more southerly latitudes. From this he reasoned that although it was commonly believed that such birds were of two different species, there really was only one, and that this one assumed the different plumages to correspond with the summer and winter seasons. He wrote, for example, that the summer Redstarts, which are now known to leave Greece for sub-Saharan Africa, were transformed into Robins, which breed farther north and winter in Greece. Likewise, he thought that summer Garden Warblers became winter Blackcaps. Both assumptions are incorrect interpretations of accurate observations, but they are understandable given that the two pairs of species are similar in shape and size.

In the earliest years of the Christian era, the elder Pliny, a celebrated Roman naturalist, in his Historia Naturalis, enlarged what Aristotle had previously said about migration and added comments of his own concerning the movements of the European blackbird, the starling, and the thrushes.

Perhaps the most remarkable and outlandish theory regarding bird migration was published in 1703 by Clarke, "a person of Learning and Piety," under the title "An Essay Toward the Probable Solution of this Question: Whence they Know and Observe the Appointed Time of their Coming." His "probable solution" was that migratory birds simply flew to the moon and spent their winters there before returning to Earth as the warmer seasons approached. Another long-held theory was that larger birds with greater wingspans would actually carry smaller-winged, less able flyers. This may have influenced Europeans to believe that storks delivered babies to their homes. Similar beliefs have persisted among the legends and folklore of some tribes of Native Americans and other indigenous cultures around the globe.

Modern Era
Various early hibernation and more far-fetched theories persisted for over 2000 years, but it wasn't until 1944, with the introduction of bird banding, that a more concrete and factual basis for understanding migration began to emerge. Modern science and technological advances have enabled researchers to factually arrive at more well-developed, understood, and accepted reasons for annual bird migration. We now know that the advance and retreat of the ice ages in prehistoric times forced birds to move where food and shelter would protect them from extreme climatic conditions. We are also keenly aware that in our own time, climate change is once again having a big impact on when, where, and how birds continue to migrate, guaranteeing their survival and breeding abilities in order to insure future generations of bird life.

Sandhill Cranes photo by Doris Evans

Contributing Causes
No matter which or how many theories have been developed over historic time, it is now accepted that migration is influenced by several factors, not just any single one. While it was believed that migration was an ingrained habit, we know that the urge and need to migrate is passed from one generation of birds to the next. It's in their genes. Another principle and uncontested factor for the necessity of avian migration, which applies to all migrating birds, is that they need favorable conditions to breed in spring and summer and to have more abundant sources of food and shelter in the colder seasons. 

Photoperiodism
Another factor that carries considerable weight with scientists and researchers is the theory of photoperiodism. This modern theory is based on studies of living behavior and suggests that there is good reason to believe that migration is an annually induced movement. This theory holds as its major premise that the quantity of daylight and the changing length of daylight hours throughout the seasons are the stimulating causes for migration. When conditions are correct, the birds' hormones are aroused by the increasing and decreasing number of hours of daylight. So, regardless of the abundance or dearth of foods, the birds inherently know when to migrate in order to survive and take advantage of what they need for the upcoming season.

 Along with this increase/decrease in the length of daylight hours, we now know that the development and readiness of their sexual organs is tied to increased hours of sunlight. When the days exhibit the greatest number of hours of sunlight, the birds' sexual organs are at their largest and most active and the need to breed and reproduce is greatest. This understanding was originally tied to the life cycle of plants. Increased light has positive influence on the growth, flowering, and fruiting of plants. With birds, experiments were done in laboratory conditions utilizing artificial light sources. It was discovered that birds have more energy and movement when they have more hours of sunlight. Shorter hours of daylight, as during cold winters, materially restrict the ability of the birds to obtain sufficient food at a time when the cold requires an increased supply to maintain body heat.

Genetics
One of the true wonders of annual avian migration is that it is instinctive: generation after generation, birds are able to make their dangerous and often complicated patterns of movement to ensure the continuation of entire species. Aside from genetic studies that point this out, simple observation also brings us to the same conclusion. Many, if not most, juveniles must have the ability to correctly know how, when, and where to migrate without the benefit of being taught this essential skill by their parents. Juveniles get separated from their parents for a variety of reasons, yet will successfully arrive at the correct destinations. Whether migrating solo or within a flock of a single species or a mixed flock, the genetics they inherit will guide them to their destinations from the first attempt and continue throughout their lives.

 Juveniles will spend time prior to their first migrations exploring the local features of their breeding and wintering territories, as well as adjacent territories. Becoming familiar with the exact information they need to know about these territories will enable them to quickly find food, shelter, and mates. Genetics serve as a sort of built-in software program that allows the birds to successfully accomplish annual migrations, what we humans may refer to as a "super-human task." Genetics is also responsible for the birds inheriting the annual rhythm that triggers the necessary physiological changes that cause them to migrate at the appropriate time. Equally important, the rhythm stops the cycle when they reach their wintering or breeding territories.

Red-tailed hawk photo by Doris Evans

The genetic basis of migration also causes birds to be conservative in their choice of wintering grounds. It's no accident that birds "choose" winter territories that enable them to flourish. The exact locations were "encoded" in their ancestral genetic material and no mutation has happened to provide a population with a viable alternative. Even during the warmest interglacial of the last cycle of ice ages, which ended 10,000 years ago, the wintering areas do not shift far enough north to cause any interruption to the migration cycle. Gradual change is always possible; sudden and radical change is not.

 No single reason for avian migration is a full explanation by itself, but when combined with other known causes, the picture becomes fuller and more realistic.

Future Topics in the Migration Series of Articles
Continuation of this series of articles in future newsletters will focus on several other important aspects of avian migration. Subjects to be covered include migration patterns and routes, timing of departures and arrivals, flight techniques, flight power and speed, flight variations in elevation during migrations, preparations necessary for migration, orientation and navigation, dangers migrating birds encounter, feeding and sleeping during migration, weather and climate, threats and conservation, and understanding the science and techniques of how migration is studied.

 Understanding as much migration knowledge as we can expands our appreciation of all that migration involves and furthers our fascination and comprehension of the birds at the same time. 

THE WONDER AND MYSTERY OF MIGRATION
Part Two

By Jon Friedman

Introduction
Migration may be the most awe-inspiring and wonderful natural phenomenon that mankind has observed since prehistoric times. Only the violent power of earthquakes, volcanoes, and the dramatic onset and acceleration of global climate change eclipse the natural wonder of migrations. To experience a migration fallout or to observe large masses of birds all flying in one direction gives rise to speculation, wonder, and awe of the natural world. Unlike violent natural phenomena, avian migration has inspired artists and scientists to ponder the regularity of bird movement, what it means to the natural world, and how it affects the earth's human population. There's much romance in considering how birds must outwit many obstacles in order to accomplish a life or death journey that is truly superhuman.

Over the centuries mankind has struggled to understand annual bird migration. Many theories have come forward only to be debunked later. We no longer believe that birds go to the moon, are transformed into other species, or spend winters buried in the mud at the bottom of lakes and ponds as was thought for many years. Over the centuries people have tried to understand and explain birds' annual migrations. Year after year, people have wondered how birds manage to fly tremendous distances and arrive and depart from the same locations at the same time of year without getting lost or becoming so exhausted that they die. The same problems and obstacles that have plagued bird migrations throughout history continue today. Today birds face additional hardships with man-made problems such as habitat loss; collisions with buildings, cell towers, wind turbines, aircraft, and other obstacles along their migratory paths. And scientists now believe that global climate change is accelerating at such a rapid pace that entire species will not be able to cope and mass extinctions will become the norm.

Science is struggling to understand both the particular and general problems that face avian migrations in our modern world. And while many parts of the migration mystery have been unraveled, there is still much to be discovered.

Bird Migration by Wikipedia

Patterns of Migration - North/South
Bird movements can be attributed to a variety of causes and involve many directions. Some birds travel along north/south pathways. Some travel east/west. Still others may migrate from lower elevations to higher elevations within a given range. Some species reside where they are found and don't migrate at all. With other species, only the juveniles migrate and the adults don't. Sometimes males migrate ahead of females. About half of the world's species migrate, yet there are differences for each species and each journey. Partial migration means that some percentage of a given species does migrate while the rest don't.

Most people think of long-distance north/south travel when they think of migration. An easy example is Barn Swallows, which live in most of North America during spring and summer breeding season, feeding on the large numbers of flying insects that are available then. Over most of their breeding range, the winter weather is so severe that they wouldn't be able to survive. So they simply migrate to southern regions where insects can be found in winter.

However, migration is a complex subject and there are always exceptions and alternatives. If the climate is livable year-round and doesn't affect the availability of food, then birds have no reason to migrate. But when any given species is widespread in its range and extends into more northern latitudes, individuals from the northern parts of the range will have to fly further to reach the southern portion of their range. In contrast, those from the southern parts of their range - where winter conditions are less severe - may not need to migrate south. Killdeer in North American exemplify this pattern.

In many species where this pattern occurs, another migratory phenomenon can be happening: partial migration, in which some birds of the overall population in a given area migrate and others do not.

Altitudinal Migration
Blue Grouse, found in the Rocky Mountains, are good examples of altitudinal migrators. Their migration is limited to around 1000 yards. They breed at lower altitudes in deciduous woodland clearings, feeding on berries and insects. In winter they move up the mountains to coniferous forests. Here they feed on pine needles which, though abundant, are not nutritious enough for the birds to be able to breed and raise their young, too.

East/West Migration

Our familiar Phainopepla (looks like a black cardinal) provides a good example of east/west migration. Widespread south of the U.S. border, Phainopepla are resident birds, i.e., they live year-round in their range and do not migrate. In the northernmost part of their range, they are found in the southern parts of the four states north of the Mexican border. At this most northern part of their range, some are resident and some migrate - but east and west as opposed to north and south. Birds that are found in their breeding range of extreme S.E. Arizona, extreme S.W. New Mexico, and the Big Bend portion of the Rio Grande River in Texas, migrate west to Southern Arizona and Southern California. In their more sub-tropical range in Mexico and points south, these Silky Flycatchers can feed on insects throughout the year. In the northern part of their range they will consume berries, particularly mistletoe berries, when insects aren't abundant or available.

While many species migrate east/west, more often than not these birds are taking advantage of the better winter climate provided by the sea at the edge of a continent. In North America most White-winged Scoters breeding in Western Canada and Alaska winter on the eastern seaboard. Shorebird migration is also dominated by the need to stay close to water, with migration routes hugging coastlines and major stopovers on estuaries. Birds breeding in the European Alps may migrate east/west, but they also move vertically to escape the harsh winter weather on the upper slopes. Scandinavian and Russian breeding birds often migrate westward to winter in France, Spain, or Britain.

Day/Night Migration
There are some species whose migrations are entirely confined to daytime hours because they rely on the rising warm air from thermals. Thermal soaring uses gliding techniques rather than wing-flapping and is therefore one of the most cost-effective methods of migrating. By using the warm rising air of the thermals to gain over 1600 feet in elevation, they convert the height advantage into distance by gliding, wings outstretched, to the base of another thermal and repeating the process. Birds that use thermal soaring for their migrations - birds of prey, cranes, and storks, for example - usually have long, broad wings with separated primary feathers (the major flight feathers are often referred to as fingers) for delicate directional control. Their preferred migration time is determined by the thermals and is usually around mid-morning, when they tend to be strongest.

Birds rarely fly only at night, although nighttime migrants are common. Terrestrial birds are unable to land on the seas and oceans, and since many nocturnal migrants make ocean crossings, they are forced to continue to fly by day. Birds that are considered nocturnal migrants, such as thrushes and starlings, may continue over open water to arrive in the early afternoon in the south or throughout the day into the evening in the north. They will be vocal, calling throughout, as a means of keeping the flock together in the dark.

Other Flight Techniques
Birds not specially adapted to soaring move forward in one of three ways: continuous flapping, bouts of flapping interspersed with periods of gliding with wings outstretched, and flapping and ballistic flight, with wings closed, known as bounding flight. Continuous flapping is primarily practiced by birds with a relatively high weight compared with their wing area. They are often birds that use their wings to assist them in swimming under water. Water is denser than air so only birds with relatively small, strong wings can use them for swimming. The best strategy for birds that weigh over 5 ounces is to flap and glide, alternating periods of continuous flapping flight, in which the bird gains both height and speed, with periods of gliding. Gliding uses less than one-twentieth of the energy needed for flapping. The length of the glide depends on the direction and speed of the wind. Small- to medium-size birds, such as warblers, finches, and thrushes, use bounding flight. Bounding flight can be seen locally by watching woodpeckers fly. The drag created as the air passes over an outstretched small, broad wing cancels out any lift the bird would gain from gliding. Smaller birds, therefore, fold their wings and drop between periods of flapping. Surprisingly, the bird's bodies, with wings closed, generate significant lift and reduce drag.

Flight techniques are varied and depend to a large extent on the weight of the bird and its wing size and shape. Birds can also exploit the air to their full advantage. Takeoff, for example, is energy-efficient, so the bird faces into the wind to become airborne. Once airborne it turns and increases speed using tailwind. Flight is more efficient at higher speed. Soaring birds clearly make use of the lift provided by thermals, but other birds also use the free ride to reduce their energy consumption. Most birds can use continuous wing beats for at least a short period, to escape predators or in other emergencies - think of all the wing-flapping when a hawk swoops down into your feeding station.

The characteristic formation for many flapping and gliding large birds is a V. The V formation is an efficient way for birds to keep together on long flights. It allows each bird to see the one in front, without being impeded by the air it disturbs, particularly when the bird in front flaps and huge swirls of air are shed off the tips of its wings. The leading bird in a V is usually an adult, unless the flock is on a routine flight, to a nighttime roost for example, when the young may take a turn and gain valuable experience.

Walking Migrants

Not all birds fly to achieve migration. On the African plains, Ostriches undertake long movements timed with the regular patterns of rainy and dry seasons. Emus are among the most mobile of flightless birds. Banded Emus have been reportedly found over 300 miles from their banding sites. Once the rainy season begins, they usually move to more arid areas for breeding, then return to places with a reliable water supply when the breeding season is over. Their numbers are declining due to agricultural development and long, Emu-proof fences to restrict their movements. Emperor Penguins in the Antarctic have been known to trek many miles over the ice to their remote breeding grounds, as documented in the popular movie March of the Penguins.

Swimming Migrants
Other species of penguins and auks regularly migrate long distances by swimming. Many auks fledge before they can fly and, accompanied by a parent, set off for new waters. Migrating male Razorbills and their chicks swim, the chick while it grows and the adult still flightless from its molt.

Elevation during Migration Flight
Migrating birds have been recorded at many heights above sea level. Some birds, like Bar-headed Geese, are among the highest fliers and have been documented flying over Mount Everest, which is nearly 30,000 feet above sea level. But the upward limit of bird flight is reached when the reduced amount of oxygen in the air, and the lower density of that higher air, prevents them from functioning normally. Height of flight varies greatly among migrating birds. Clouds are another problem to navigate. Birds prefer not to fly in clouds and heavy clouds greatly impair their performance. This is because birds in flight produce so much water when they burn the fat in their muscles as fuel that getting rid of it in cool, damp air is difficult.

A flock of 30 Whooper Swans migrating from Iceland to Western Europe was logged by an airplane pilot at 27,000 feet. Mallards reach a height of 21,000 feet during migration. White Storks have been recorded at 15,750 feet during their migration. Whistling Swans traveled across North America at 8,850 feet. Most swifts, among the most aerial of birds, migrate at heights of around 6,600 feet above sea level. Snow Geese have been noted at 4,900 feet and Black-bellied Plovers at no more than 2,600 feet.

Sandhill Crane by Doris Evans

Many small birds keep relatively close to the ground. Many finches fly below 3,000 feet, while Black-and-white Warblers and other wood warblers reach a maximum altitude of around 1,600 feet. Tiny hummingbirds may fly only dozens of feet above ground level and perhaps up to 200 to 300 feet above ground level. Hummingbirds also migrate as individuals, never in a species or mixed flock.

Head and Tail Winds
For migrating birds, the environmental factor that has the greatest effect is the wind. Many small birds have a flight speed that gives them a maximum range for each unit of energy consumed of some 25mph, so that even a relatively light direct head wind of 12 mph reduces their range by 50% and a gale wind of 30 mph would blow them backward.

Birds have no problem in deciding the strategy for a moderately light head wind: stay low, hug the ground, and make use of whatever shelter it affords. In a tail wind, the best option is to get up high and fly with it.

Perils and Dangers
There are perils in nature along with man-made dangers that can cause serious problems for all migrating birds throughout the world. Among the natural perils birds encounter are extreme winds and terrific storms; the need to cross large bodies of water non-stop; energy depletion and exhaustion; mountain ranges; fog; predation by birds, mammals, and reptiles; habitat fragmentation and complete habitat loss as a result of wildfires; effects of global climate change; collisions; drought and loss of food sources; and more.

Man-made dangers include, but are not limited to, habitat fragmentation and complete habitat loss due to wildfires and deforestation, as well as agricultural, suburban, urban, and industrial development; collisions with buildings, cell towers, windmills, lighthouses, tall bridges, windows, and aircraft at takeoffs and landings as well as at higher altitudes, along with monuments such as the Statue of Liberty and the Washington Monument; legal and illegal hunting; domestic cats; spraying of pesticides and other toxic chemicals; and more.