Why We Run: A Natural History - Bernd Heinrich (2002)
Chapter 8. Ultramarathoners in the Sky
Fall and spring, untold billions of birds take to the skies for nonstop flights of thousands of miles that often take them over oceans and deserts. Their very survival depends on their athleticism, mental resolve, and navigational skills. My race on October 4 in Chicago would be puny relative to what they accomplish routinely. I’d race for only 100 kilometers, with the route mapped out in convenient loops and with all the food and drink I wanted provided along the way. Like other birders, I have sometimes trained my binoculars up at night, seeing the migrants’ silhouettes slip across the milky background of the full moon. I’ve heard the birds’ faint cheeps in the dark, and I’ve wondered how our brethren, of flesh and blood like ourselves, accomplish their amazing feats. Could they teach us something about endurance?
I want to discuss how they survive migration, but we first need to get a better fix on what they actually do. Answers are only recently coming to light, primarily as a result of the work of thousands of people all over the globe who have banded birds. An amazing picture is emerging that may seem scarcely more plausible than the old presumption that swallows must “obviously” overwinter in the mud rather than commute between continents.
I’m especially impressed by the small, beautiful, and exquisitely delicate wood warblers. Every June, the forests across northeastern North America resound with the sibilant lisping and chattering of the thirty-five species of warblers that have returned to set up territories, build nests, and rear their young. The journeying of one of these species, the blackpoll warbler, Dendroica striata, is now known better than most of the others and can serve as an example, although there are differences between species’ travel routes.
By mid-July, the blackpolls’ nesting in thick northern spruce or fir forests is complete, and both adults and young molt their feathers. The adults change from their bright nuptial garb to a drab and undistinguishable one. The molt is done in a month and the whole population, distributed from Maine to Alaska, begins to move. All the birds converge in the northeastern United States. Individual birds from Alaska and the west return to specific sites where they had been after previous transcontinental trips of thousands of miles.
After reaching their eastern seaboard staging areas, the birds, whose lean weight is 9 to 11 grams, now enter a phase of gluttony, technically known by the more polite term “hyperphagia.” Taking advantage of the berries ripening at that time, as well as outbreaks of aphids and other insects, they double their body weight in as little as ten days. Most of this weight gain is fat that is stored in thick masses under the skin on the abdomen and on the chest just below the neck. Fueled up, the birds next converge on their final staging areas on Cape Cod, Massachusetts. From there, these tiny wraiths of the coniferous forest embark en masse on an awesome nonstop transatlantic flight of about 2,200 miles, all the way to Venezuela.
The starting gun that launches them on this nonstop ultra of ultramarathons out onto and across the Atlantic Ocean is the passage of a cold front. Flying at a little over 20 miles per hour, they are at first wind-assisted, provided they pick a cold front that results in southeasterlies. Gradually, the departing birds merge into flocks of about five hundred to a thousand individuals. By the second day, these flocks reach the still air over the Sargasso Sea, and after the third consecutive day and night of continuous flight they are boosted by the trade winds, and the now much leaner birds begin to appear on the northern coast of South America.
Blackpoll warblers are perhaps not so exceptional among bird migrants, but like the other songbirds that fill our summer woodlands, they are a constant reminder of avian competence. Numerous species of Arctic sandpipers breed even farther north and overwinter even farther south, thus engaging in even more stupendous travels.
The white-rumped sandpiper, Calidris fuscicollis, is one. This shorebird, barely larger than a sparrow, breeds north of the Arctic Circle. In the fall, like the blackpoll warbler, it migrates east across the American continent to the northeastern shores. Slimmed down, it again fattens up before embarking on a nonstop journey of 2,900 miles lasting at least three days and nights. At the end of that flight, the flocks reach Suriname, on the north coast of South America. As in all long-distance travel, energy supplies are vital for success. The birds require rich feeding areas for fueling up to be able to embark on the third and final leg of their long journey, this one of 2,200 miles overland across the South American continent, the Amazon, and on to Argentina, at the southern tip of South America, to complete a total trip of more than 9,000 miles that spans the globe nearly pole to pole. The birds’ traveling involves a refined itinerary with major refueling stops at quite specific and essential wetlands and undisturbed coastline feeding areas. At the end of each of their epic fall and spring migrations, the birds again reach continuous daylight, after having just come from the midnight sun in either the Northern or Southern Hemisphere. In short, except for the days in transit, the world they experience most of the year is without nights; this enables continual feeding, frolicking, and flying.
Populations of the knot, another Arctic shorebird, have given us additional information, especially as regards migration energetics. Knots breed all around the North Pole, and they have widely divergent wintering areas. In the New World, the red knot, Calidris calidris rufa, makes a southward migration in the fall that covers approximately 7,800 miles. When not carrying fat reserves for migration, the birds normally weigh 120 grams, but when fully fat—just before takeoff—they weigh about 180–200 grams, and occasionally even reach 250 grams. Like the smaller songbird migrants, they regularly double their body weight prior to migrating.
When loaded to a body weight of 250 grams (130 grams fat), a knot has enough fuel for a theoretical maximum nonstop flight range of about 4,700 miles. Flying at a speed of 47 miles per hour, it has about 100 hours of flying time before it must stop to refuel. Knots make their migration in long nonstop flights between staging, or feeding, areas, where the distance covered in each flight is limited by the amount of fat they carry. Hence, the food supply at the feeding areas, where the birds forage for a week or two before resuming their migration to the next fueling stop, is critical.
Red knots leaving James Bay in northern Canada have three staging areas, or fueling stops, before reaching Tierra del Fuego, their ultimate destination, at the southern tip of South America. Knots from the western Arctic first fly to the eastern American seaboard, refuel for two weeks, then fly on to Suriname, in northern South America, refuel again, then fly across the Amazon basin to southern Brazil, before taking off on their last flight, to the tip of South America. Here they are in continuous daylight while their breeding grounds are in the continuous night of Arctic winter. Like many other sandpipers, plovers, and terns, they span the globe from pole to pole.
Tremendous amounts of fuel are required for the birds’ long flights, but weight is a burden as well. The sandpipers, like airliners, often fly higher than fifteen thousand feet, where the air is thinner and there is less aerodynamic drag. One downside of flying so high is that there is less oxygen available. To stay aloft in thinner air requires more speed to generate sufficient lift, which in turn requires greater energy expenditure and more oxygen to sustain the greater flight effort. This is a catch-22 situation, which the birds resolve by their digestive and respiratory physiology.
How have the birds’ remarkable capacities evolved from their terrestrial dinosaur ancestors? When the ancestors of birds first took to air, there must have been strong selection for weight reduction. Bones became lighter, in part by becoming hollow. Further weight reduction was probably also achieved by changes in diet. To extract the scant energy resources from otherwise plentiful foliage, herbivores require huge stomachs and long intestines. No airliner can fly using plant fiber as power source; it needs highly refined jet fuel, with a high ratio of energy to weight, that can be combusted easily and quickly. To support the high-energy habit of flight, the ancestors of birds must have become selective in their diet, choosing fruit and insect protein over foliage. A high-octane diet of insects and fruit could allow jettisoning of an enormous volume and weight of gut, as well as of teeth and heavy jawbones for anchoring the teeth. Becoming selective in diet eventually would have allowed them to become even more selective, because they could travel farther. I suspect that such a self-reinforcing cycle may have triggered the virtual explosion of bird evolution that allowed this group of animals to become one of the most diverse, numerous, and amazing life forms on earth.
Even now, changes in the diet of some birds and in such omnivorous animals as ourselves proximally affect gut mass. Simply eating more protein results in a physiological response of reductions in gut length and mass. Although change in diet from low-to high-energy foods would itself have increased the power/mass ratio to facilitate long-distance flight, the real breakthrough in ultraflight endurance in birds may have come later, as a secondary consequence of having the body cavity less crammed with gut.
A body cavity that has less digestive machinery crammed into it has more space for other components. That space could either be filled with other organs or left as air space to maintain lightness. As it happened in birds, it was the latter that occurred, and ironically that’s part of the great breakthrough that allowed them to fly fast even at high altitudes in an oxygen-poor environment.
Air-breathing fish, reptiles, and mammals are saddled with an inefficient in-out breathing system. We inhale into a baglike lung by raising our ribs and lowering our diaphragm to create negative pressure in the lung. We then push the air back out before we can take in another breath. It is practically impossible to deflate or empty our lungs totally since we can’t totally collapse our chest. There is always some residual air left inside—air that becomes partially depleted of oxygen. When we inspire, we mix the new, fresh air that is fully saturated with oxygen with this oxygen-depleted residual air. Not so with birds.
Somewhere, sometime, in protobirds there occurred the great innovation of connecting their extra body air spaces to the lungs. This connection then made possible the routing of air through the lungs. With the aid of inflatable air sacs in the body cavity, birds now route air through their relatively rigid lungs, and they do it during both the inhalation and the exhalation phases of the breathing cycle.
It might appear at first glance that if air goes through the lungs, rather than in and out, birds don’t exhale. However, they do exhale! The trick is that it takes two breaths for them to move a given mass of air in and back to the outside, as two separate slugs of air move through the respiratory system at the same time. The lungs are connected to a set of air sacs in front of and another in back of the lungs. Air goes through the lungs in both exhalation and inspiration because during the inspiration, when both sets of air sacs become inflated, the posterior air sacs become inflated with fresh air while used (stale) air (of a previous inhalation) passes from the lungs and into the anterior air sacs. During exhalation from the mouth, the air leaves the anterior air sacs to the outside, while the fresh air (from the previous breath into the posterior air sacs) goes into the lungs.
These breathing innovations set the stage for birds to make further stepwise modification in almost all aspects of their existing morphology, physiology, and behavior. It has made them the most impressive ultraendurance machines of flesh and blood the world has ever seen.
Birds’ unparalleled aerobic and respiratory capacity allows them to expend the necessarily high metabolic costs of flight even in the thin atmosphere above Mount Everest, where humans can barely crawl, as our O2 max is scarcely above resting levels. Bar-headed geese (Anser indicus) fly over the summit of Mount Everest (8,848 meters), making a journey of about 900 miles nonstop. How do they and many other high-altitude migrants get sufficient oxygen to their muscles where the fraction of oxygen in the air is only one-third that at sea level and where humans can take only a few steps before being exhausted?
Diagrammatic representation of bird breathing. Mammals inspire by expanding the chest cavity, which is accomplished by contracting the rib muscles and the diaphragm. Air pressure in the chest falls, and air rushes into the lungs. Birds have air sacs attached to the lungs, but their lungs are relatively rigid; the air sacs expand and contract to ventilate the lungs. Both anterior and posterior air sacs expand during inspiration, the anterior with already-used air from the lungs, the posterior with fresh air. During expiration, the air from the anterior air sacs leaves the body, while the air from the posterior air sacs (from the previous inspiration) enters the lungs. The two-cycle respiration results in one-way traffic of air through the lungs.
The birds’ lungs, with their impressive capacity to extract oxygen from the atmosphere, are only part of the solution. Next, the blood must extract the oxygen from the air going through the lungs and deliver it to the muscles. One of the major adaptations, as seen from differences between the goose and a domestic duck, for example, is that the goose’s hemoglobin has a very high capacity to bind oxygen in the red blood cells. Thus the blood delivers more oxygen to the tissues per unit of blood pumped by the heart.
Next, the oxygen must pass from the blood to the muscles where it is used. In all animals with the capacity to take up and use oxygen at high rates, the muscle tissues are dark in color because of high concentrations of myoglobin, the dark red protein that takes up (binds) and thus helps remove the oxygen from the blood and deliver it into the cells, where the mitochondria, the tiny energy-producing batteries, utilize it by use of appropriate enzymes. Migrating geese have much higher concentrations of these enzymes for fuel utilization in their mitochondria than the nonathletic, sedentary domestic duck. All these various features working together contribute to the animal’s O2 max, or aerobic capacity, which limits sustained power output. In humans, O2 max is an accurate predictor of middle-to-long-distance running performance, other things being equal.
But other things are seldom equal. Energy supplies must be used sparingly and carefully hoarded. In human runners, drafting, or running in the wind shadow of another, is a well-known strategy of saving energy. Birds do it routinely, especially the larger sociable species, such as geese, swans, and cranes, by flying one behind another to form large lines or Vs. In addition to drafting, they avoid head winds and wait for tailwinds before departing. As with runners, pacing—to achieve maximum distance with least overall effort—is also very important for achieving maximum range. Vance Tucker’s studies of flying budgerigars in a wind tunnel, in the late 1960s, proved the point.
Tucker successfully measured the rates of work output (from oxygen consumption) of bird flight at varying speeds by training his subjects to fly in place with a mask over their heads (to retrieve gas for measuring oxygen consumption) and against head winds of varying speed in a wind tunnel. Flight speed then equaled wind speed. Tucker’s apparatus was essentially what a treadmill ergometer (to measure rate of energy expenditure) is to a runner, and his results revealed, surprisingly, that birds’ power output in flight is not a simple function of flight speed. Flying at 20 kilometers per hour, his budgerigars were expending energy at near their maximum aerobic (that is, work) capacity, or a O2 max of about 35 milliliters of oxygen per gram of body weight per hour. (Human O2 is usually depicted in milliliters of oxygen per kilogram of body weight per minute, so by those units, for comparison, the birds’ O2 max is 583, not 35.) Their minimum energy expenditure during flight, of approximately 22 milliliters of oxygen per gram of body weight per hour, occurred at a much higher flight speed, of 32 kilometers per hour. To achieve flight speeds greater than 32 kilometers per hour, the birds’ metabolic costs of flight again increased, ultimately reaching their maximum of 35 milliliters of oxygen per gram of body weight per hour at a flight speed of 48 kilometers per hour. Simple calculations revealed that, given this relationship, the maximum flight range, per given amount of flight fuel, could be achieved at the flight speed of about 40 kilometers per hour, which corresponds to an energy expenditure slightly above the minimum. Thus, speeding up in flight would get them there faster, but with limited energy supplies, they might not get there at all.
There are now extensive data on flight speed of migrants derived from radar observation. These data have been compared with calculated flight speeds and energy costs on the basis of wingspans and body weights. These studies show that most species fly to achieve maximum range, rather than flying at minimum energy expenditure. Exceptions often prove the rule. For example, during the breeding season, swifts often sleep in flight—they spend the night flying back and forth at high altitude rather than perching. During such resting bouts, forward flight speed slows down and energy expenditure approaches minimum, because covering distance no longer matters as it does during migration. Apparently bird migrants exert themselves to achieve maximum distance, not with the least effort at any one time, nor at the greatest speed, but with a specific effort that yields the longest flight range at the least overall effort for their distance. Distance runners must also search for, and find, that specific level of effort that is precisely suited to them and the distance traveled. Timing is important as well.
Small birds migrate at night, it is thought, for two reasons. First, it gives them time to replenish their energy supplies in the daytime. Second, it solves the problem of dehydration. The large amounts of metabolic heat generated by the work of flying can be dissipated, as in hawk moths and most other insects, by passive convection (unassisted heat loss to air) alone, without resorting to additional cooling by evaporating water. By flying at night, the birds can continue in their strenuous effort without stopping to drink. Given what the birds do, it is clear that for a record human ultramarathon performance it would be advisable to run on a cool night in order to conserve water.
Many birds cross seas and hot deserts, where drinking and refueling are not possible. They must carry all their required water and energy supplies with them. Fat, when burned as fuel, does release water as a by-product, so fuel and water supplies are interrelated. If rate of water loss is not in excess of metabolic water production, as it is usually in insects and birds, but not humans, then burning fat for fuel can also serve the need for water.
As already mentioned, long-distance bird migrants fatten up like butterballs just before embarking on their long journeys. Do human ultramarathon racers have to do the same? It depends on the distance and the rules. If we were to race intercontinental distances and the rules were that we could not at any time eat, then yes, the winners would be those who fattened up. The thin runners, who could run fast at the beginning, would not stand a chance of finishing. However, we don’t race over such distances, and the organizers of our ultramarathon competitions ensure that there are refueling stations along the way. We can eat what, when, and almost as much as we want. That being the case, human ultramarathoners are better off lean. As long as we can refuel and rehydrate on the way, any extra weight we carry is a burden that slows us down. Most elite runners, men and women, carry at least several percentage points of body weight fat (1 to 6 percent), and if we could use it all, it would be sufficient to carry us hundreds of miles.
The common supposition that women could be better ultramarathon runners than men because, on average, they have more body fat is false. On average, women are slower runners than men at all distances, and this sexual difference shows up especially at the longest distances. As I’ll indicate later, there are biological reasons that relate this to genetic trade-offs. Some animal traits are sex specific. When women do run as fast and far as men (as many can), they likely do so at a reproductive cost. They must lose so much body fat that ovulation ceases. Animals are consummately pro-choice. Their bodies commit to the massive task of reproduction only when the resources to pull it off are available.
I’m unable to run hundreds of miles without stopping to refuel, despite my body fat, and I’m humbled by what is routine to any songbird or sandpiper. I’m awed by their ability to fly unbelievably long distances to and from specific pinpoints on the globe. Some might argue that if I were a bird, I would not be able to enjoy my fantastic annual journeys, following the sun from perpetual daylight on the high Arctic tundra to the pampas in Argentina and back again, but I think they are wrong. Birds are not likely driven by the logic of what they have to do. Instead, they are motivated by powerful urges. They behave in ways that feel right and pleasurable to them. Feelings of pleasure are a product of evolution that makes healthy organisms do what helps them survive and produce offspring, in the same way that fear makes them shy away from danger. What makes the blackpoll warbler strike out south in the fall after a cold front is probably not fundamentally different from what motivates me to jog down a country road on a warm, sunny day. We both respond to ancient urges that have adaptive roots. We have much in common, but our differences make direct comparisons of our endurance physiology difficult. With antelopes we’re much closer.