Beaks, Bones and Bird Songs: How the Struggle for Survival Has Shaped Birds and Their Behavior - Roger Lederer (2016)
TRAVEL HITHER AND YON
Migration and Navigation in an Endless Sky
Bird migration is the one truly unifying natural phenomenon in the world, stitching the continents together in a way that even the great weather systems, which roar out from the poles but fizzle at the equator, fail to do. It is an enormously complex subject, perhaps the most compelling drama in all of natural history.
—SCOTT WEIDENSAUL, Living on the Wind
Migration is perhaps the most death-defying behavior that birds participate in and it almost seems ironic that the behavior evolved to ensure survival. Flying for days, sometimes nonstop, while facing wind and weather, unfamiliar territory, and predators en route is a big gamble, but apparently a worthwhile one because about 40 percent of the world’s avian species participate in annual migrations. Most birds fly at about 15–50 mph during migration and travel 100–500 miles in a day. Larger birds tend to fly faster than smaller ones; ducks and geese average 40–50 mph while flycatchers average more like 17 mph. So the voyage is generally not quick. The Arctic Tern migrates from the Arctic to the Antarctic and back every year. Bar-tailed Godwits fly nonstop from Alaska to New Zealand. And Bar-headed Geese fly over the Himalayas, even above Mount Everest.
A mutation in the gene that controls hemoglobin allows Bar-headed Geese to carry a greater concentration of oxygen than other geese.
Although birds are the quintessential wanderers, many animals migrate to sustain their lives. The massive annual migration of wildebeest, along with enormous herds of zebras, gazelles, elands, and impalas, is closely intertwined with the rainfall patterns of the Serengeti. Great White Sharks feed off Alaska’s coast in the summer and return to the coast of California during the colder times of the year. Monarch butterflies migrate from the Rocky Mountains to Pacific Grove, California, taking four generations to complete the trip. In the Shawnee National Forest of southern Illinois, snakes, turtles, and frogs move from the dry limestone cliffs where they spend the winter to breed in the nearby LaRue Swamp in the spring.
Establishing the evolution of a behavior is challenging but we can make some guesses, based on the distribution of fossils and present-day behaviors, as to why migration has proven more advantageous for long-term survival than being sedentary (non-migratory). Here is one scenario: a sedentary population of birds grows and eventually fills a habitat, competing ever more strongly for resources such as food and nest sites. To insure their own survival, some of the younger birds move to another area and establish their own population. If the new area is farther north, it might become unsuitable in the winter, so these northern birds return south to the sedentary population, along with their young. This movement lessens competition for resources for the entire population during the breeding season although competition during the winter season may be more intense. The following year, the population of migratory individuals moves northward again, perhaps occupying an even larger area. Over the years the sedentary population, being invaded in the winter by an ever-larger migratory population, eventually succumbs to competition from the migrants, resulting in an entirely migratory population. We once assumed that today’s migratory bird populations all evolved from sedentary populations, implying that migration is an evolutionary advancement to enhance survival. But it appears that patterns of avian migration developed almost randomly and that the migratory habit is elastic. A study published in 2012 of the wood-warbler family of North and South America reveals that the ancestral wood-warbler was migratory and that the loss of the migratory habit—becoming sedentary again—was as frequent as becoming migratory, another case of regressive evolution like flightlessness.
Although we know significantly more now than we did in 1703—when a pamphlet written “By a Person of Learning and Piety” stated that migratory birds flew to the moon for the winter—the whys and hows of migratory behavior are still being discovered. Why do some migrants fly nonstop for days and others seemingly take their time? Is the path of migration instinctive or learned? Do birds follow established routes? Why do some but not all birds migrate? Why do some species migrate but similar species do not? Why do some individuals of a species migrate and others not, and why is it that one individual may be migratory one year and sedentary the next? Both genetic and environmental components are at play here, but their roles have yet to be sorted out. Until scientists found ways to track birds, these were all mysteries, and all the answers are not in yet.
HOW WE LEARNED ABOUT MIGRATION: BANDING BIRDS
The study of migration began by marking birds with tags or bands. At least as far back as 1000 BC the Chinese kept birds for falconry, each one marked to indicate ownership. Early Romans sent simple messages across the countryside by tying threads to the feet of crows. Perhaps the earliest record of a metal band on a bird’s leg was in 1595 when a Peregrine Falcon belonging to Henry IV of France disappeared in pursuit of a bustard (a large, swift-running bird of the Old World plains) and was later found on the island of Malta, south of Italy, 1350 miles away. A duck shot in Sussex in the United Kingdom in 1709 had a silver band around its neck inscribed with the arms of the King of Denmark. J. J. Audubon tied silver cords around the legs of young Eastern Phoebes and later identified them when they were adults. But the first attempt to gather any real information about bird movement might have been the marking of a Great Gray Heron captured in Germany in the early 18th century. It had metal bands on its legs, one of which indicated it was banded in Turkey a few years earlier. In 1899 Denmark, C. C. Mortenson was apparently the first person to systematically mark birds with numbered bands to collect data on their travels.
In 1920 the Federal Fish and Wildlife Services of Canada and the United States began to oversee bird-banding activities and the collection of data in the two countries. Since 1960, approximately 64 million birds have been banded, and either through field observation or recovery of bands, there have been approximately 681,000 encounters with banded birds. From these encounters we can determine information about migratory routes, longevity, social structure, population sizes, and disease. The Bird Banding Laboratory of the U.S. Geological Survey collects comprehensive data, requiring painstaking attention to detail on the part of licensed banders, resulting in extensive and accurate information on bird movements. In addition to the date of banding and recapture, information like age, sex, stage of molting, whether blood or feather samples were taken, if auxiliary leg or wing or neck markers will be used, and signs of disease are all noted.
I was a licensed bird-bander for several years and banded a few hundred songbirds. Although I did recapture a few birds in the same location a year or two later, I never got a recovery from outside my banding site. This is typical: band returns from songbirds are around 1 percent because the birds have relatively short life spans and small bands are hard to find after a bird has died. However, about 15–20 percent of waterfowl bands are recovered each year because the birds are hunted and if a duck dies in the wild, the aluminum band is big enough and may last long enough to be found someday.
Yellow-throated Fulvetta of Southeast Asia showing metal leg band.
Banding, or ringing as it is called in Europe, has supplied important information on the movements of birds but little about what the birds did between banding and later band recovery. Research with new techniques and equipment—including radar, chemical isotopes, radio frequency identification tags, very high frequency radios, videography, and more recently GPS loggers and satellite tracking—has provided considerably more information. The British Antarctic Survey uses fingertip-sized data loggers attached to the legs of seabirds to record light intensities at different latitudes and longitudes to provide position information. Even smaller data loggers, as light as 0.05 ounces, are being made for use on songbirds. And sophisticated technology using isotopic ratios adds even more information. A study of Black-throated Blue Warblers measured naturally occurring isotopes of carbon and hydrogen in their feathers. The ratios of the isotopes reflected the diets of the birds and determined that the northern breeding populations winter farther west in the Caribbean than the southern breeding populations. Most of these techniques are expensive and require considerable training on the part of users, so bird banding will continue to be a primary source of data on bird movements for a while.
Banding is allowed under the provisions of the Migratory Bird Treaty Act but one needs to have a legitimate reason for banding birds; it is not looked upon as a casual hobby. Bird banding can have a negative impact on the survival of birds as bands can cause injury from friction against the leg and add weight to the bird, and the process of capturing the birds causes stress. Neck bands on Trumpeter Swans have iced up, causing severe mobility problems, and aluminum bands put on the flippers of King Penguins appeared to result in a 16 percent higher mortality rate. Banders argue that these problems are exaggerated and that the information gleaned from banding is worth the risk, but I suspect banding will slowly be supplanted by new techniques as they become more accessible and less expensive.
ALL IN THE TIMING: HOW DO BIRDS KNOW WHEN TO MIGRATE?
Are you a sports fan? Here’s something to consider: statisticians studied 40 years of football scores and determined that West Coast teams have an advantage over East Coast teams when the game is played after 8 P.M. on the East Coast of the United States. Similar results were found with baseball scores. What’s happening? It’s the players’ internal clock. West Coast teams are more alert in the evening on the East Coast than are East Coast teams. Did you get up at about the same time this morning as you did yesterday? Do you feel tired in the mid-afternoon? Do you go to bed at about the same time every evening? Probably. You are showing circadian (Latin for around the day) rhythms that are reflected in your brain activity, body temperature, hormone levels, blood pressure, and other metabolic functions. This is your body’s internal clock. These patterns of daily activity are tied to the photoperiod (length of daylight). You interrupt your circadian cycle every time you experience jet lag by flying several time zones away from home, but in a few days you adjust to the new daylight regime. (The quickest way to adjust is to adapt to the local time regime and expose yourself to sunlight, as the sun will reset your cycle.)
Now think about why you awoke and got out of bed this morning. The proximate factor is your daily rhythm: you usually get up at that time, perhaps assisted by an alarm clock or your significant other bringing you breakfast in bed. As unlikely as the latter might be, these are the immediate reasons you rousted yourself out of the sheets. But why get up at all? Because you have things to do—your boss expects you at work, you have a fishing trip planned, you have to get the kids to school, you are hungry. These are ultimate factors—the actual reason(s) you get up and out of bed.
The proximate factors of migration are those environmental cues that set birds off on their journey, primarily day length; and the ultimate factors—the reasons they leave on this annual journey—are such things as food and a milder climate. The timing of migration is pretty predictable. Birds arrive on their breeding grounds at about the same time every year, the key word here being “about.” Some species show little variation in average arrival dates and other species demonstrate considerable variability. Insectivorous birds appear to be more predictable than other kinds of foragers. Tradition has it that swallows return to California’s Mission San Juan Capistrano on March 19th, but the actual times vary, often by several days. (Unfortunately, remodeling and increasing development in the area has caused the Cliff Swallow population to decline to virtually zero, but efforts are being made to entice the birds back.) Since 1957 the town of Hinckley, Ohio, has been holding a “Return of the Buzzards” festival to celebrate the arrival of Turkey Vultures, predicted to be March 15th each year. Vagaries of the weather affect the travel of migrants so the dates of arrival are not exact, but they are pretty close. Photoperiod is the main proximate factor.
Ad for buzzard festival in Hinckley, Ohio.
Photoperiod or Day Length
The length of daylight each day changes gradually all over the earth except at the equator and poles. At the equator, the day length is constant at 12 hours, 7 minutes. At the North Pole there are 163 days of total darkness and 187 days of “midnight sun.” Tromso, Norway, experiences “polar nights” in which the sun does not rise for 60 days in the winter. Day length in London in mid-August is 14 hours, 51 minutes versus 7 hours, 51 minutes in mid-December. In Mexico City the corresponding figures are 12 hours, 9 minutes and 10 hours, 59 minutes. These changes in photoperiods can have significant effects on organisms.
We have known for a long time the importance of circadian rhythms in humans and other animals. Irregular changes in light and dark periods, such as some workers experience with frequent shift changes, can interrupt the growth of blood vessels and lead to diseases or conditions such as heart attack, stroke, and the delayed healing of wounds. The importance of circannual cycles has also been demonstrated, such as differences in the rate of cell division in the bone marrow and intestine, the volume of red blood cells, blood pressure, and cholesterol levels over a year’s time. (Don’t confuse these annual cycles with biorhythms, the wacky idea that our lives go though certain cycles based upon the day of our birth.)
Unlike weather, day length is predictable. The times of dawn and dusk change the same way at the same time in the same place. Birds are cued by the lengthening photoperiod to prepare for and depart on their migratory journey. This cue is called the Zeitgeber (German for time-giver). With increasing photoperiod, birds begin to exhibit physiological and behavioral changes. In the equatorial area with little change in the photoperiod, the cue may be the intensity of the light in the wet and dry seasons.
As the time to leave approaches, birds change their behavior. They begin to eat more to put on weight, particularly fat, for energy. For many years researchers observed captive birds as migration time drew near and noticed that the birds began to show restlessness, known as Zugunruhe (German for movement anxiety). The activity and orientation of caged birds during this time were measured by various electronic and mechanical means. In 1966 the Emlen team of father-and-son researchers invented the Emlen funnel, an elegant but simple mechanism to measure Zugunruhe. This funnel-cage was lined with paper on the sides and sloped down to an ink pad on the bottom. Whenever a bird jumped up the side of the funnel, its inked feet marked the paper. When the paper was later removed, the relative density of ink markings on various areas of the funnel indicated the bird’s directional preferences. The photoperiod, orientation of daylight or moonlight, and the night sky could be manipulated (via a planetarium) to see what the effects would be on the captive birds’ orientation.
As the spring photoperiod lengthens, birds begin their northward exodus. Although weather is unpredictable, the date(s) of departure is, on average, the best date(s) to go. If the birds were to leave too early because of a bout of mild weather, they could encounter bad weather during the passage. If they delayed their departure significantly during a low-pressure spell, they might find the resources depleted when they arrive. Insects emerge and become active, and plants germinate, emerge, and bloom as the weather warms and the birds need to be there to take advantage of the food supply. Extenuating factors such as soil moisture and snow cover as well as temperature affect the appearance of plants and insects, but they emerge at about the same time each year. Thus the timing of when birds migrate each spring is tied to the photoperiod and their food supply to the increasing temperature. As with most genetic characteristics, there is variation among the individuals of a species, including their date of departure. In cold years the earliest arrivals may be at a disadvantage; in warm weather tardy individuals may find a depleted food supply. In years of weather extremes, either early- or late-arriving individuals of any one species may have a higher (or lower) survival rate.
The Emlen funnel.
Density of ink markings indicate the bird’s directional preferences.
The entire population of many bird species moves from the wintering to breeding grounds in about three to six weeks. The White Wagtail arrives in Finland from North Africa over a period of about three weeks, similar to that of the Barn Swallow that winters in the southern half of Africa and arrives in Europe a little later. The Common Swift, wintering in the southern third of Africa, arrives later still, but with only a two-week difference between the arrival of the first and last swift. All three species depend upon insects for food, but the White Wagtail is a bit more catholic in its tastes and will eat some vegetable matter; the Barn Swallow feeds mainly in mid-air but will pick insects off of vegetation, rocks, and the water’s surface; the Common Swift is restricted to aerial foraging, a dietary difference that probably explains the difference in arrival times to coincide with the peak of insect emergence.
UNDERSTANDING MIGRATORY BEHAVIOR
Migratory behavior varies widely, depending on the geography and the bird species. There are long- and short-distance migrants, species with both migratory and sedentary populations, summer-winter and wet-dry seasonal migrants, and altitudinal migrants. All the individuals of a population might migrate at about the same time or spread out the timing depending on age and/or sex. Red-winged Blackbird males, for example, winter in all-male flocks and travel north early to set up breeding territories to attract females upon their later arrival. I could go on, but you get the idea—migration is very labile and changes as necessary to assure the survival of a particular population.
In the midst of writing this chapter during the month of October, I traveled to Bolivia and spent a week on one of the Amazon’s upper tributaries. While fishing and birdwatching, I was fascinated by the variety of both kinds of animals, each taxon totaling around 1400 species. I didn’t see all those birds or catch that many different fish, but I did see an amazing number of both. Interspersed with the native Bolivian birds were migrants from North America—the Peregrine Falcon, Osprey, Lesser and Greater Sandpipers, Solitary Sandpiper, and Barn and Bank Swallows. It has always fascinated me that these interlopers from the north seem to adroitly slip into the tropical Amazonian ecosystem unnoticed and unbothered by the residents. This certainly says something about the productivity of the tropical forest.
Migratory behavior varies considerably among species and between populations within a species. Song Sparrows and Blue Jays in North America are sedentary while orioles and tanagers migrate, and in the United Kingdom the Chough and kingfisher are sedentary while swallows and wagtails move southward. American Robins in North America are non-migratory in the southern United States but breeding populations of Alaska and Canada move southward in the fall. Chiffchaff generally breed in northern Europe and Asia and winter in southern Asia and Africa, but in recent years some have been overwintering in the United Kingdom. Black-headed Grosbeaks breed across western North America and winter in Mexico, meeting up with sedentary populations of their species.
The Southern Hemisphere has a smaller land area than the Northern Hemisphere, and migration patterns are not as strongly developed. No birds breeding in the Southern Hemisphere winter on a different continent and some Southern Hemisphere migrants never cross the equator. Rainfall also has a greater influence on migratory behavior in the Southern Hemisphere because insect populations increase with more rain. The Pied or Jacobin Cuckoo arrives in India from Africa at the beginning of the monsoon season, and the bird and the weather phenomenon have become inextricable in Indian folklore as the harbinger of the monsoon. Nightjars, birds named for their crepuscular feeding habits and the “jarring” sound of their voices, are good examples. The Standard-wing Nightjar breeds from Senegal east to Ethiopia and migrates northward to avoid the wet season. The Pennant-winged Nightjar breeds south of the African equator and winters somewhat north of it; the bird’s migratory journeys are very protracted and may or may not be related to the wet season. In Australia, only 7 percent of migratory species have totally separate breeding and wintering areas and, on average, they move only about 9 degrees of latitude, compared to Northern Hemisphere migrants, which average 22 degrees of latitudinal movement.
Altitudinal migrants, such as the Red Grouse, Spotted Owl, and Steller’s Jay, move from a higher elevation in the summer to a lower one for the winter. The White-throated Dipper of Europe and the American Dipper migrate slightly southward or downslope from a higher elevation, depending on the severity of the winter. But the American Dipper’s situation is more complicated and gives us some idea of how partial migration might have evolved. Studies indicate that although a large population of dippers might be resident in the lower areas of a watershed—a main river or stream, for example—when breeding season arrives some birds move to higher elevations because of competition for food and nest sites. Even though nesting success is comparable, riverside breeding birds at the lower elevations often have second broods. So the partial migration upslope benefits the entire population. In the tropics up to 20 percent of the bird species participate in altitudinal migrations. One study indicates that the elevation at which tropical birds in Costa Rica breed is inversely proportional to predation on their nests, so higher nests have less predation. But higher elevations may also be subject to the vagaries of the weather as evidence also shows that an unusually rainy season will cause birds to migrate downslope from higher elevations.
John Gould’s (English ornithologist and painter) depiction of the White-throated Dipper.
How Long Does Migration Take?
Birds begin and end their various journeys at different locations, so the time it takes to travel varies, and their speed depends on environmental conditions. Weather can slow or accelerate the trip considerably. In early February, Canada Geese leave their wintering grounds in the southern United States and arrive in northern Canada at the end of April. They more or less follow the 35°F isotherm (a line of equal temperature at a given time or date on a weather map) northward. This makes sense because if they were to fly in front of the isotherm, they would confront freezing conditions and a dearth of food. The Baltimore Oriole spends summers in eastern North America and winters in the Caribbean, Central America, and northern South America. At about 20 mph, migrating mainly at night, they might cover 150 miles a day, taking two to three weeks for the entire trip.
Shorebirds on both sides of the Atlantic make incredibly long migrations from the Arctic tundra to the Southern Hemisphere. Bar-tailed Godwits nest in the Alaskan tundra and winter in New Zealand. In the spring they leave New Zealand and fly northwestward, stopping to refuel in Australia and the Korean and Russian peninsulas before arriving about four months later in Alaska. The return winter journey takes them directly from Alaska to New Zealand during the month of September—a nonstop journey of 6600 miles. In the summer of 2006 researchers attached satellite transmitters to 16 Bar-tailed Godwits. On the trip from Alaska to New Zealand, one female bird was recorded as traveling 7145 miles without stopping, flying at an average speed of 34.8 mph for nearly eight days.
Bar-tailed Godwit in winter plumage.
The Arctic Tern holds the record for the longest migratory route. Nothing is spectacular about the appearance of this medium-sized bird, but its lifestyle in the Arctic and Subarctic and wintering in the Antarctic have made it famous. They are circumpolar breeders in the Arctic from May to August where summer days are long. The birds typically lay two eggs and defend their nest vigorously. Although they feed mainly on fish and crustaceans from the ocean, they will consume insects during the nesting season. After the young are fledged, the colonies leave the breeding area to winter in the Antarctic region from November to March when the photoperiod is long there. From about April to May the terns migrate northward again, traveling an average of about 300 miles per day, although individual birds have been observed to fly as much as 400 miles per day. Some individually tracked birds flew up to 66,000 miles each year. As they follow the seasons and photoperiodic changes, Arctic Terns see more sunlight than any other animal.
Arctic Terns can live for more than 30 years, and it has been said that during their lifetimes some of these birds may travel the equivalent of three round trips to the moon! An impressive accomplishment, but how many of them actually live to be 30 years old? I found an old banding study that estimated the annual mortality rate of Arctic Tern adults at 18 percent. With that death rate, if we start with a population of 1000 birds in the Arctic and 18 percent of this group dies each year, 30 years later perhaps one bird will have survived long enough to have made 30 round trips; what an amazing life that bird led.
Arctic Tern migratory routes showing only southward migration.
An endless amount of information has been amassed on the locations of breeding and wintering grounds, the routes to and from, the speed of travel, and the dates of departure and return. But the big question has always been, “How do birds get the energy to cross large bodies of water and fly nonstop for days?”
Migrating birds face three challenges: the energy cost of flight, the need to refuel en route, and the endurance needed to complete the journey. The importance of fat in fueling bird, insect, and mammal migration cannot be overstated. Fat is an excellent energy source; compared to protein and carbohydrates, fats provide more than twice the energy per unit of weight, and because fat does not absorb water, supplies of fat weigh less than other fuels per calorie. But birds also need to accumulate protein to replace the loss of muscle mass during migration. The ratio of fat to protein accumulated before migration varies from 1:1 to 10:1, depending on the diet of the birds, their route, and distance of flight.
Fat accumulation is the main source of weight gain before migration. A rather extreme example of this is the Eskimo Curlew, an abundant shorebird in North America 200 years ago. J. J. Audubon and Elliot Coues, 19th-century naturalists, described enormous expanses of midwestern plains covered with curlews resting while en route north from South America to the tundra of western Canada and Alaska. When the birds were making their return trip, market hunters would kill two million curlews a year for food. The Eskimo Curlew was nicknamed the “doughbird” because it accumulated so much fat before its southward migration that when it was shot and plopped to the ground, the heavy layer of fat resembled dough. The hunters probably exterminated it, as it has not been reliably reported since 1963.
Birds store fat under their skin, as you probably have noticed when preparing a whole chicken. At first the fat accumulates in particular areas, such as in the depression formed by the furcula, and then in the skin below the breast muscles; additional fat is stored all around the skin. Measuring the amount of fat a bird has is important in understanding the migratory strategies of different species, so ornithologists have devised a crude method of assessing fat stores of captured birds by looking at the furcular depression through the translucent skin. All one needs to do is blow on the bird’s throat to separate the feathers and expose the skin. A score of one means no fat, the furcular depression full of fat is a three, and a score of five signifies a full load of blubber.
Garden Warblers of the Palearctic, weighing about 0.6 ounces, put on fat and increase their body weight 30–40 percent before initiating migration to their wintering grounds in Africa south of the Sahara. They need to fuel up as much as possible before crossing the Mediterranean Sea and the sands of the Sahara where they have little or no chance to feed; upon arrival they will have lost virtually all their body fat. On their spring return, the birds feed heavily before crossing the desert, some individuals increasing their weight by 50 percent and 10 percent of them doubling their weight. (Imagine if you weighed 130 pounds and you increased your weight by 50 percent—how much time would you have to spend at the gym to get back to your original weight?) In addition to fat, their flight muscles increase in size by 15 percent, giving them not only additional flying power, but protein that can be used if fat stores are depleted. Rufous Hummingbirds increase their body weight by 60 percent in preparation for flying from the Rocky Mountains to Mexico, but converting sugar to fat is expensive, costing 16 percent of the calories they ingest.
After birds leave the nesting area and head to their wintering grounds, most stop en route to refuel and rest. Food might be limited as in the case of shorebirds, which often have to wait until the tide goes out. Prior to migration, birds are efficient eating machines with their gut and liver increasing in size to digest food and store fat. As the storage capacity of fat reaches its limit, the heart, blood vessels, and flight muscles increase in size while the gut and liver atrophy as a means of weight reduction and some of their proteins used to build muscle. In addition, enzymes are activated that control the deposition and utilization of fat, and the number of red blood cells increases so that more oxygen can be carried.
Many birds like thrushes, rails, sparrows, flycatchers, orioles, waterfowl, warblers, blackbirds, and shorebirds migrate at night even though they are usually diurnal because, except for aerial insectivores, they have found it adaptive to fuel up during the day and fly at night when the winds have diminished. If smaller migrants flew during the day, stopping to feed at frequent intervals, their journey would take longer, and, if they spent the night resting, they would need to refuel again in the morning before resuming their trip. But some birds, like the tiny hummingbird, may need to enter a reduced state of metabolism, torpor, to make it through their journey.
One factor that affects gas mileage in an automobile is weight: cars that weigh more use more fuel. Another is shape. Birds fill up on fat but the efficient way the fat is distributed on the body minimizes drag. But weighing more means fewer “miles per gallon.” So birds can employ one of two survival strategies during migration. One is to spend less time en route by flying longer and farther nonstop to reduce the chances of encountering bad weather or predators—but this choice requires birds to store more fat and expend more energy carrying it. The other approach is to carry a minimal load of fat and stop frequently to refuel, a common pattern among shorebirds, songbirds, and waterfowl. The best tactic for many species seems to be some combination of these two approaches—short hops alternating with extended flights. Birds that feed on the wing like martins, nighthawks, swallows, and swifts, employ a “fly and forage” strategy so they are able to make long flights with minimum fat. Satellite tracking of Ospreys traveling from Sweden to West Africa revealed that the birds flew 60 percent of the time and were stationary for 40 percent. But there was a lot of variability; some birds never stopped. It had to be that they fed along the way, catching fish from lakes and rivers. Again, natural selection honed migratory behavior to be flexible in order to allow the greatest number of individuals to survive the trip.
Anna’s Hummingbirds may enter a state of torpor during some migratory nights to preserve energy.
Flying is energy intensive; the average hummingbird uses 5 to10 times the amount of energy in flight than at rest and its oxygen consumption per unit of body weight is about 10 times that of a human athlete in action. In the spring, Ruby-throated Hummingbirds fly north across the Gulf of Mexico, about 500 miles, nonstop, in about 20–24 hours. For years birdwatchers could not fathom how such small birds could perform such an amazing feat. Again, it’s fat stores increasing their weight by 40 percent. Many other bird species take this overwater route and one of the first places they land is Dauphin Island, Alabama, a prime birding spot. One spring I attended a meeting there and saw about 75 exhausted but gorgeous male Rose-breasted Grosbeaks, having just crossed the Gulf, in one tree. These were the days before bird festivals and the locals were bemused by us birdwatchers showing up in bars and restaurants in khaki outfits and befitted with binoculars. We were the butt of local jokes. Today the island is a bird sanctuary and the annual Alabama Coastal Bird Fest brings in tourists and a good deal of money; the locals now find birdwatchers less funny.
Migratory routes, distances, and strategies vary among and within species. Like all adaptations that evolution has crafted, the flexibility provided by variation in migratory behaviors allows birds to respond to changing conditions over time. As the climate changes, lakes dry, rivers alter course, and food sources move; a stop-and-rest route today might become a nonstop journey in a decade. Of course, in order to make any passage successful, birds have to find their way.
Blackbirds resting in a tree.
NAVIGATION: NO COMPASS, NO MAP, NO PROBLEM
“The winds and waves are always on the side of the ablest navigators,” says Edward Gibbon in The Decline and Fall of the Roman Empire. Most anyone with a bit of backwoods experience can employ a number of clues to orient themselves, even if the compass gets waterlogged and the map eaten by Bigfoot. Directional clues come in the form of the orientation and movement of the sun, moon, and stars across the sky; the flow of a stream; the slope of a mountain; the prevailing wind and clouds; the moss on the north side of trees; even smell. Birds, without any instruments at all, navigate just fine.
During World War I in the First Battle of the Marne in 1914, the French used 70 mobile pigeon lofts from which they dispatched messages rolled into small cylinders attached to the birds’ legs. Amazingly, even though the lofts were on the move, 95 percent of the messages arrived. Cher Ami, the most famous pigeon of all, having lost an eye and leg in combat, received the Croix de Guerre medal from the French for delivering a message that saved 200 American soldiers from an artillery bombardment. Cher Ami traveled by ship back to the United States with General Pershing, the American forces commander, and now resides in a glass case at the Smithsonian Institution. Homing, racing, or carrier pigeons are all the same species, Columba livia—the Rock Dove—and are known for their ability to navigate. This is why you will read about so many navigation studies of Rock Doves (although they are often just called pigeons).
Scientists generally agree that birds probably utilize a combination of navigational techniques, either independent or closely intertwined. Landmarks, the sun and stars, geomagnetism, olfactory clues, and low frequency sounds are all possible navigation tools. Besides discovering how different birds find their way, we might learn about our own senses. We all know people with a good sense of direction and others who get lost after turning a corner. What we don’t yet know is if this ability is innate or learned. Navigation is among the more difficult areas of ornithology to explore, but also one of the most exciting.
We find our way around our neighborhood and city every day by landmarks. Birds do the same. Learning where to locate food, water, protection from predators, and shelter from the rain, cold, and wind is essential for daily survival, so it seems logical that birds would use landmarks to orient themselves. Even when a bird wanders out of a familiar area, it probably takes note of streams, lakes, large rocks and trees, and other key parts of the environment. Birds released in a familiar area immediately head in the appropriate direction. Released in unfamiliar territory, they circle until they recognize some sort of landmark. The farther the birds are away from their destination, the longer it takes them to find a landmark. In the case of pigeons, 635 miles is a distance they can, with difficulty, navigate in a day or so. Any farther than that and they generally get lost. A 2005 Oxford University experiment fitted 50 pigeons with tiny tracking devices that showed some pigeons following specific highways, roundabouts, and exits to find their way to their loft. Each time they were released at a distant location, they followed the same routes home, even though a totally straight return route would have been faster.
Data from radio-tagged Canada Geese demonstrate that the birds use landmarks during their diurnal flights, and are especially focused as they approach their destination. The birds also compensate for wind drift by correcting their path as they pass familiar sights. In one case, two populations of Canada Geese migrated as one group through Minnesota; at a certain point the populations diverged. At about that point a large tree fell one year and the following year the migratory populations appeared disoriented for a time before they diverged on their own paths; the tree was apparently a significant landmark.
But the use of landmarks cannot explain navigation on the first migratory flight of immature birds that do not follow their parents or those birds that travel over the sea a considerable distance from the shore. Humans, too, have found their way across vast expanses of the earth by the use of landmarks, but when there were no definitive physical waypoints we had to find other ways to learn our location.
The Sun and Stars
For as long as people have traveled the earth, the sky has provided clues. The ancient Phoenicians of the Mediterranean were excellent sailors and navigators. Although they preferred to sail in sight of land, it was sometimes not possible, so they kept track of birds over the sea, which often indicated that land was not far off. Norsemen noted that a bird with a beak full of fish was headed to its nest and young, whereas a bird with an empty beak was headed away from land. If birds were not visible, sailors oriented by the North Star at night and during the day tracked the course of the sun across the sky. The position and movements of the sun, moon, stars, and constellations were always available under a clear sky.
Several experiments and observations have shown that birds use the sun’s location in the sky as a directional clue and support an idea called the sun compass. At Germany’s Max Planck Institute in the 1950s, Gustav Kramer discovered that caged European Starlings were able to orient in the direction they wanted to migrate—essentially by inputting information into their sun compass—if they could see the sun move during the day. How they account for the different positions of the sun as seasons change is unknown. The ability to use the sun compass is innate but does not show up in pigeons until about three months of age; it has also been found in fishes, salamanders, frogs, toads, turtles, voles, and bats. Knowing pigeons use landmarks, Knut Schmidt-Nielsen of Duke University and William Keeton of Cornell fitted birds with frosted lenses that prevented them from recognizing landmarks more than 20 feet away. When the birds were released the researchers took note of the direction they flew. On a clear day the birds oriented in the direction of the loft; on a cloudy day their orientations were more random. In another experiment a group of birds with frosted lenses was held in an artificial light regime that shifted the day by six hours. When these birds were released, they flew in a direction that was a six-hour sun shift from the loft direction.
The European Starling has often been used for experiments and observations to unravel the secrets of migration.
Although some invertebrates like amphipods and marine worms respond to moon clues, there is no “moon compass” for birds. The moon rises about an hour earlier each night as it goes through various phases, so it is not a good clue for orientation. Investigators have even found that the moon tends to confuse birds. Mallards using stars for navigation were less accurate in their orientation when the moon was half or more full.
The Big Dipper and Orion’s Belt are about the limit of my astronomical knowledge but for millennia travelers have used constellations for navigation. Birds probably do the same; they certainly use the starry night to find their way. An elegant experiment is one that is simple, concise, ingenious, and persuasive, and that’s what I would call German ornithologists Franz and Eleanore Sauer’s 1957 planetarium-based investigation. The Sauers put warblers in cages in a planetarium and each evening moved the night sky about 300 apparent miles in the direction that the birds were oriented during their Zugunruhe. The birds “migrated” their way around the Mediterranean Sea to Africa. This was the first real demonstration of the avian “star compass.” Later experiments were performed with birds whose annual cycle was manipulated so that their physiological condition primed them for either a northward or southward migration. Under the planetarium sky, the birds oriented in the direction of their physiologically primed condition. Shifting the birds’ internal clock with a different light regime had no effect on their star compass as it had on their sun compass. Further experimentation indicated that birds orient to a pattern of stars, perhaps a constellation or other group of stars, and follow that pattern as the stars move through the sky. A learning component is involved, as birds need to have some education and experience with the movement of the sun and the stars before they can effectively use them to navigate.
Before radar, night migration of birds was studied by aiming a telescope at a full moon and counting the birds that flew across its face. Sometimes the species could be determined by their silhouette and even the altitude of the birds could be calculated. On one night in 1952, 1400 birdwatchers and astronomers at 265 observation points in North America observed 35,400 birds crossing the moon. That’s about 133 birds per observation point, not a lot and only broadly indicative of migration patterns, but it did provide a sample of what was happening.
Birds crossing the moon.
Ornithologists have known for years that birds use some sort of magnetic sense to navigate. Franz Mesmer of 18th-century “mesmerism” fame claimed that animals produced a magnetic force that he called “animal magnetism,” a term that has morphed into “body energy fields” and other questionable notions. No evidence has shown that weak magnetic fields have any discernible effect on humans. But on birds? Read on.
Alexander Neckam, an English scholar of the late 12th and early 13th centuries, was apparently the first person to demonstrate the magnetized needle that aligns with the earth’s magnetic field. Humans have used compasses ever since and today we have evidence that turtles, lobsters, salmon, and birds use geomagnetic lines to navigate. Early geomagnetic studies on birds in the 1940s were ignored, but later investigations with European Robins by Wolfgang and Roswitha Wiltschko of Frankfurt, Germany, demonstrated that the birds could orient properly even in cages that did not allow them to see the sky. When they artificially reversed the magnetic field, the birds oriented in the opposite direction.
In the late 1960s, William Keeton devised a clever series of experiments, including gluing bar magnets to the back of pigeons before releasing them away from their home loft. (He attached non-magnetic brass bars to a control group of birds.) When the sun was shining, all the birds found their way back with no problem. Under overcast skies the birds with magnetic bars became disoriented but the control group did not. Keeton speculated that the magnets were interfering with how the birds orient to the geomagnetic lines of force. Further experiments by Charles Walcott of Cornell University involved putting Helmholtz coils, devices that produce magnetic fields, around the head of pigeons. When the coils were activated on a cloudy day, the direction the birds flew depended on the direction of the magnetic fields around their heads, while on a clear day the coils had no effect.
So birds use geomagnetic lines of force for orientation, but what is the sensory mechanism for detecting them? A number of animals have magnetite, a magnetic iron-containing mineral, in their heads or cranial nerves. Magnetite deposits have been found in the olfactory nerves of trout, as well as in sea turtles, newts, and several bird species. In birds, the magnetite crystals are located along the ophthalmic nerve running on the right side of the upper bill. Studies have shown that pigeons with opaque contact lenses over their right eye have difficulty navigating, but lenses over the left eye have little effect. There do not appear to be any specific sense organs to interpret the magnetic field, so magnetite crystals may sense only the strength of the magnetic field. The orientation of the magnetic field may be sensed by the birds’ innate magnetic compass. Cryptochromes, pigments in the ultraviolet-violet cone cells of bird retinas, allow birds to detect the earth’s magnetic field, although whether the pigments indicate the direction or strength of the field is unknown.
Do you remember your mother making your favorite lasagna or your grandmother baking pies? Like scenes and sounds, we often remember scents—both good and bad. Every time I smell a skunk I revisit the time when I was traveling in my uncle’s car when he made roadkill out of one of those animals.
We now know that birds can use odors in navigation, but this was not seriously discussed until about 1973 when an Italian researcher postulated the “olfactory hypothesis.” He said that pigeons create an olfactory map of the area around their loft and on any journey away from the loft the birds use aromas to help guide their return. Experiments in which pigeons were released in an unfamiliar area upwind of their loft disorientated them, whereas birds released downwind were more successful returning home. It is hard to explain how birds can create an olfactory map a long distance from the loft, especially with shifting winds and changing scents, but apparently pigeons can pick up clues from as few as three volatile compounds in the air, most likely the most abundant human-generated compounds in the area.
I saw very few seabirds in mid-ocean on my cruise line adventures. That’s because most of the ocean’s nutrients are concentrated near the shore where runoff from the continents and the upwelling of these nutrients by surface winds create a productive coastal zone, with blooms of phytoplankton (microscopic floating plants). Gabrielle Nevitt and colleagues at University of California, Davis, showed that when krill, small ocean crustaceans, eat phytoplankton the chemical dimethyl sulfide (DMS) is released. DMS is detected by seabirds, which come to feed on the krill. The concentration of DMS reflects the topography of the ocean, remains at stable concentration for weeks, and shows predictable seasonal patterns. Experiments with prions (small petrels that eat mainly plankton) indicate that they can detect even low concentrations of DMS. This strongly suggests that seabirds find their way over large expanses of ocean by following a chemical map superimposed over the sea.
Infrasound refers to sounds that humans can’t hear—sounds below 20 Hz, such as some ocean waves (with an average frequency of 16 Hz), distant storms, and earthquakes. Pigeons can hear frequencies as low as 0.05 Hz. Early experiments used conditioned responses—whenever a low frequency ultrasound was presented, it was followed by a mild electric shock. After training, the pigeons were again subjected to a series of sounds; each time an ultrasound was presented, their heart rate increased. This indicated that the pigeons expected an electric shock and proved that birds can hear ultrasound. But in the wild, birds need a “sound map,” a way to associate landmarks with the aural environment of an area.
Microseisms are small earth tremors caused by natural phenomena such as ocean waves. Since ocean waves are nonstop, they produce a continuous hum all over the earth. It’s not likely that birds follow the infrasonic hum because it is spread so widely; however, it is possible that topographic features like mountains, river valleys, and large buildings affect the hum so that a microseism map is produced. Perhaps birds follow the echoes landmarks make, as well as their visual manifestations.
A neighbor of mine races pigeons. He takes a group of birds to some remote location along with other pigeon racers, and releases the birds to see how fast they get home. Electronic bands on the pigeon’s legs register their arrival at the home loft while their owner reads the results on a distant computer. I asked how many birds get lost each race and he responded “about 40 percent.” Seemed like a lot to me. In 1997, a big race was held to celebrate the centenary of the Royal Pigeon Racing Association. More than 60,000 homing pigeons were released from Nantes in northwestern France and were expected to fly to their home lofts in southern England, a distance of about 400 miles. The birds were supposed to arrive in the late afternoon, but, in the end, only about 10 percent made it—a 90 percent loss of birds! Three other races took place near the Atlantic coast that year and in 1998, all with similar losses. One explanation postulates that the Concorde, a supersonic jet that passed the area of the races while they were in progress, produced a cone-shaped shockwave that may have interfered with the birds’ infrasonic hearing. There’s no real proof, but it certainly sounds plausible as similar evidence indicates that noise from ship traffic interferes with communication among whales.
FLYWAYS AND THE DANGERS OF PREDICTABLE ROUTES
In North America there are four major migratory routes or flyways: the Atlantic, Pacific, Mississippi, and Central. These are general routes along the East and West coasts, the Mississippi River, and east of the Rocky Mountains, but the boundaries are fuzzy. Birds that move in family groups such as ducks, geese, swans, and cranes generally follow flyways, so the four North American routes have been useful in establishing regulations for the harvest of waterfowl. The northern Sacramento Valley of California is on the Pacific flyway, wintering grounds for millions of waterfowl and other birds. Sometimes, if I am in the right spot, as I was one year when a flock of White-fronted Geese came in for a landing, the birds came so close I could feel the wind off their wings and hear their feathers slicing through the air.
North American waterfowl flyways.
The other major flyways of the world are the Black Sea–Mediterranean, East Africa–East Asia, Central Asia, and the Australasia–East Asia. All these flyways extend into the Southern Hemisphere, but birds stop at different locations along the route. These migratory paths are considered highways along which birds travel, but birds don’t restrict themselves to particular paths, so recognizing distinct belts of migration through North America and other continents is a bit unrealistic.
Songbirds wander east and west to a much greater degree than waterfowl. All we know for sure is that migrating songbirds travel between certain breeding areas in the North and certain wintering areas in the South and that certain species use a few heavily traveled corridors while other species follow more generalized routes. However, individual birds show strong fidelity to a particular route. Having traveled a certain path, a bird knows the way, the stopover points, and the hazards, and will follow the same route year after year.
These general migratory routes have evolved over millions of years, shaped by the topography of the land and water and stopover sites that provide adequate habitat, food sources, and protection from predators. Many birds wander widely but others such as the Red Knot and Purple Sandpiper restrict themselves to coastlines. Birds typically migrate south from various points across their wide northern breeding range. Ultimately the flight paths taken by individual birds and populations converge because of the constriction of the land mass and the suitability of the habitat. Eastern Kingbirds breed across a span of 2800 miles, from Newfoundland to British Columbia. On migration the width of the route they traverse narrows until the migratory path covers a width of only 900 miles from Florida to the mouth of the Rio Grande. Farther southward at the latitude of the Yucatan, it narrows even more to 400 miles.
Over the past couple of centuries, the increasing human population has had a considerable effect on birds as they follow their historical migration route. Although the United States government sets hunting limits and seasons for each species, the birds have considerably less protection once they enter Mexico and Central and South America. (There are exceptions: hunting was banned in Costa Rica in 2012, the first Latin American country to do so.) Attracted by lax hunting regulations, 7000 foreign hunters from the United States and Europe visit Argentina every year. In Bolivia I met a couple of American hunters who couldn’t tell me enough about the amazing numbers of birds they killed in two days. Across the Atlantic in the Adriatic Flyway there are major abuses of bird hunting; two hunting companies actually export birds to Italy for food. On Cyprus, an island in the Mediterranean that 100 million birds visit each year during migration, 10 million migrants are killed for food by trapping with mist nets and sticky glue on tree branches. Some rest stops, which birds have adapted for their safety and resource predictability, have either disappeared or become death traps for migrants.
At least several dozen species have significantly changed their migratory routes and timing. The Barnacle Goose now breeds 800 miles south of its historical breeding site and the Sand Martin’s wintering population around Africa’s Lake Victoria has grown because of an increase in sand flies brought about by a reduction in fish that prey on the insects. The Eurasian Blackcap is a common bird of Europe, Asia, and northern Africa. Before the 1950s, the bird rarely wintered in Britain, preferring southern Europe and parts of Africa. By the 1990s there were thousands of birds (almost 10 percent of the European population) that bred in Europe and migrated northwest to Britain for the winter. Peter Berthold and co-researchers in Germany bred British-wintering blackcaps in captivity and when they released the offspring, the birds headed northwest, unlike the typical European Blackcaps. This study is probably the first to demonstrate a genetic change in migratory behavior.
Barnacle Geese on migration.
Cultural evolution has influenced changes in migratory routes—some individuals of a species modify a route and the others follow. Geese, storks, and cranes, species that live for a long time, have older individuals that tend to lead the flock and may make changes in the route because of environmental alterations caused by the increasing human population. Migration patterns have been and continually are being sculpted by the environment as the survival of birds depends on their successful adaptations to changing conditions.
Years ago, on the western edge of the Great Basin desert in northeastern California, I was standing under a juniper tree with a colleague, no doubt discussing something of global significance. Above us, a throng of Tree Swallows was gathering in the branches in preparation for their southward trip. As I talked, I gesticulated with an upward-facing palm and a Tree Swallow fell in it, dead. Good grief, I thought, if one of these swallows drops dead on a nice summer day before migration is seriously underway, what are their chances for making the entire round-trip journey? The answer: slim. More than 79 percent of Tree Swallows hatched in the Northern Hemisphere die in their first year, many undoubtedly succumbing to the rigors of the round-trip passage. But those birds that endure the challenges and hazards of the trek survive to reproduce. It has to be worth it.