Grounded! Why do some birds walk?
Flying versus walking. A no-contest, you might think, but many bird species have abandoned the wind beneath their wings for the earth beneath their feet, and nowhere are these pedestrians better represented than in New Zealand. What is the attraction of a life on the ground that sets the kiwi, and dozens of other birds, to striding rather:than soaring?
Ah, to be as free as a Bird! To be able to take off and soar away, leaving all cares behind. To rise up on wings of eagles; to glide across the oceans like an albatross.
For as long as humans have dreamed, they have dreamed of emulating birds: of sprouting wings and taking flight. Indeed, a bird in flight seems to us the very quintessence of unfettered liberty. Jonathan Livingston liberty.
Birds have used their ability well. With mastery of the air, they have colonised more parts of the globe than any other group of vertebrates. For 65 million years they have ruled the daytime skies, and they contested them with pterosaurs for millions of years before that.
Given their success as a group, it seems puzzling that many birds have lost the ability to fly—handed in their wings, so to speak. Many of their names are familiar to us, for New Zealand happens to be a world centre for flightlessness.
We are internationally known, of course, by our snuffling, shuffling national symbol, the kiwi. And crowds flock to the natural history sections of our museums to view our “giraffes on two legs”—the several species of extinct moa. Visitors to semiremote South Island destinations such as the caves on Takaka Hill or the seal colony at Cape Foulwind have to run the gauntlet of weka eager for food or for anything bright and shiny. Fly they cannot, but they can run fast enough to beat the careless tourist for his biscuit or hop into a car when the door is left ajar, and they can jump high enough to snatch car keys out of the hand of the unwary.
The list grows longer: ostriches have left the zoo to go down to the farm, where they have been joined lately by emus. Penguins are so well known that they strut products on TV. And hardly a week goes by that one or other of our most endangered birds—kakapo, takahe and, increasingly, kiwi themselves—doesn’t get a walk-on part to report some success or catastrophe in the fight to save our heritage. Step back in time and the catalogue of flightless birds from these islands increases yet again: three flightless wrens (the only flightless songbirds in the world), two flightless geese, a duck or two, seven rails and two adzebills. All as flightless as the dinosaurs that birds evolved from, and all now as extinct as the dodo.
If flight is the defining characteristic of a bird, why would these and many other species choose to give it up?
There is no single reason, but the answers all have much to do with how much it costs to fly. We all know that flight is expensive; your airline ticket makes that clear. And the costs are the same for birds. Just as an aeroplane needs an appropriate structure, fuel and maintenance, so birds need to grow the wings and muscles to get them off the ground, then maintain these structures and generate the energy to use them.
Birds pay that price to secure the advantages of more efficient mobility, for to fly a certain distance requires less energy than to walk or run the same course. This is simply a reflection of the relative costs of the different types of locomotion in energy terms. When one then considers that a blackbird can fly directly across a valley, whereas a rat must pick its way down one side and up the other, around obstacles and up and down every small undulation—probably covering at least three times the distance of the blackbird—flight becomes an even more attractive proposition. For aquatic birds, swimming uses even less energy than flight, for bodies are naturally supported in water without expending energy. Standing upright on land or hovering in air both require energy before you even start to move forward.
Speed, too, is relevant to energy efficiency, and birds fare well here also. In general, higher speeds result in less overall energy expenditure to cover a given distance. Running a kilometre uses less energy than walking it. At the end of the run you may feel more exhausted, but this is because your muscles have demanded energy more rapidly than your heart and lungs are accustomed to supplying it, and parts of your person have gone into a temporary energy overdraft. Flight, by its very nature, is generally a rapid form of locomotion. If a bird flies too slowly, then, like an aircraft, it will stall and plummet
High-speed locomotion and relatively lower energy demands allow birds to operate over much larger areas than comparably sized mammals or lizards. Birds are able to take advantage of highly dispersed food supplies that would be out of reach for earthbound animals. Improved mobility is useful even for short-range work. For a bird, the fruit or nectar in the next tree is not a long climb or dangerous jump away: it is just a short flight. It is far easier for a starling to move to a fresh field of grass grubs than it is for a hedgehog to find a new source of snails.
And birds can move with the seasons. Some migrate thousands of kilometres to take advantage of rich seasonal food supplies. Even in New Zealand, far from the great flyways of North America and the streams of hawks and storks crossing the Straits of Gibraltar and the Bosporus, the migrations of birds have caught the imagination from prehistoric times: godwits congregating in the Far North before leaving for Siberia, streams of sooty shearwaters passing Otago Peninsula and shining cuckoos announcing their arrival from the Solomon Islands in spring.
With migration comes colonisation. Even tiny islands far from any other land usually have a bird fauna, while almost the only mammals to have colonised distant islands are bats. Flightless mammals, large or small, make bad overseas travellers. Though theoretically they could float on rafts of vegetation across large distances of ocean, they rarely do. Flight has given birds an edge in populating the planet, and this is reflected in the number of species alive today. Birds outnumber mammals in number of species by nearly three to one, and the ratio would have been far greater had not many birds been exterminated in the recent past by foreign predators introduced to their domain by humans.
Which brings up a key point about the costs and benefits of flying. There are good reasons (which we will presently explore) for a species to abandon flight, but for most birds there is an even better reason not to: to avoid being eaten. Being able to get around to new places or exploit distant resources are good reasons for birds to keep flying, but the crucial advantage is in escaping from danger. To get away and survive is to live and breed. Flying allows birds to live in environments filled with sharp claws and hungry mouths. That imperative can only be set aside when birds find themselves in places without predators. Those places have classically been islands.
Most oceanic islands had no resident mammals and therefore no predators on the ground which hunted by smell, and few that could hunt in the dark. Under such circumstances, birds could “afford” to become flightless. On continents, the domain of mammals, the advantage of being able to fly away from a predator remained so important that there are almost no flightless birds from those parts of the globe. The few that do live on continents—ostriches and their relatives—are big, and their ancestors may have given up flight before there were any large predatory mammals to contend with.
On islands, where there were no claws or teeth to avoid, the way was open for some birds to save the energy normally spent on building and maintaining wings. Of course, that meant living on the ground, and it also usually meant occupying a new ecological niche (the biological “slot” into which a species fits in an ecosystem). Often that niche was one that would have been occupied by a mammal, had it been around: for example, the niche for an insect-eating nocturnal animal, or a leaf-browsing daytime herbivore.
The kiwi is described as an “honorary mammal” by virtue of its adaptations to a niche normally occupied by a mammal. Kiwi seem to do in New Zealand what anteaters and insectivores do elsewhere. Indeed, they are very like two-legged tenrecs (the fascinating Madagascan parallels to hedgehogs or giant shrews), or solenodons (their counterparts in the Caribbean), or Australian echidnas, sniffing through the leaf litter at night in search of worms and grubs—prey which they detect by sense of smell, a faculty essential to mammals but largely absent in birds. Even kiwi body temperatures are more like those of mammals than birds. Flighted birds normally have a high metabolic rate (because of their high levels of activity) and, related to this, maintain a body temperature of 38°43.8°C, compared with mammals at 36°-38°C. Kiwi temperatures range from 37° to 38.6°.
But kiwi evolved without the stoats and dogs that threaten their existence today. The 400 Northland brown kiwi killed by a single dog over a six-week period in 1987 might have had a chance if their wings had been more than rudiments hidden in the body feathers. The selective advantage of being able to fly even a very short distance to escape a predator is so strong that it takes the complete absence of ground predators for many generations before selection for other traits can begin the process leading to flightlessness.
Eat plants, grow large—and walk
Like all warm-blooded creatures, birds walk a metabolic tightrope, balancing the amount of energy they can get from their food with the amount they need in order to maintain body temperature, to grow, move and reproduce. A bird puts a lot of energy into building and maintaining the muscles, bones and feathers of its wings, and into fuel for its muscles when it flies. If a bird did not have to fly, the materials for construction and repair and the huge fuel-consumption bill—as much as 15-20 times the energy consumed when resting—could be channelled into growing rapidly and maturing earlier and into extra breeding effort. Take away ground predators and a flightless species would have a competitive edge.
But other factors determine whether a species can make the transition. Obviously, even where there are no predatory mammals, not all birds are flightless. New Zealand has been called the land of flightless birds, but most of our species can fly. Animals develop as a coherent entity, not as separate parts. If deleting or slowing the growth of one part of the animal affects the development of other parts, the embryo will die. To become flightless, a bird must be able to develop properly in every way except for its wings. Yet the wings and their supporting breast bones are so important that in most birds they develop early and rapidly.
A few groups which live on the ground and whose young leave the nest early develop their legs faster than their wings. It is in these groups that flightlessness is most easily achieved. The best examples are rails and waterfowl, and New Zealand was particularly rich in flightless rails and ducks. Ten of our 13 rails were flightless, but only two of the 10, the weka and the South Island takahe, survive.
One way for groups with normal patterns of development to become flightless is simply to become too large or heavy for the wings to function. For energetic and aerodynamic reasons, flying birds can exist in only a limited range of sizes. Some very large birds have gone beyond those limits because their ecology and diet demand large size. When circumstances permit, such species, including the largest swans and bustards (very large running birds of the steppes of Asia and Africa), can avoid predation by their sheer size and aggressiveness, or by running or swimming away. They retain their full-sized wings but find it hard to develop sufficient power to fly. In these species, the largest males fly very rarely, if at all.
In one species of Patagonian steamer duck, the young birds can fly, but as they grow, males gradually become too heavy for their wings to support them. Then they can escape only by ploughing across the water like miniature paddle-steamershence the name given by 19th-century sailors. They retain wings, but flight imposes too great a demand on the energy supply or the ability of the wing area to support the bird’s weight. In these ducks, gradual loss of flight occurs within the lifetime of an individual.
Another species of steamer duck has arrested the development of its wings to the point that it can never fly, even when young and light.
The evolution of large size is controlled to some extent by the absence or relaxation of predation pressure, but is mainly driven by diet, specifically a vegetarian diet. New Zealand, Hawaii and other oceanic islands are natural laboratories for observing the evolution of organisms isolated from the intense competition of life on continents. Not only did these islands lack mammalian predators, but herbivorous mammals as well. No large antelope or buffalo chomped on grasses, shrubs and tree leaves. No rodents chewed on fallen seeds and nuts. All that potential food was just sitting there unmolested. The opportunities for herbivores were wide open for any birds that could exploit them, and species from several groups did just that, nowhere more successfully than in New Zealand, where kakapo, rails, extinct geese and moa all feasted on plants.
The problem is, leaf material contains relatively little nourishment. Herbivores need to process a lot of plant matter to extract the energy and nutrients they need. Any means of lowering energy demands and improving digestion imparts great evolutionary advantages to the species able to do it. Discarding wings and breast muscles reduces energy requirements. Being big allows an animal to have a longer gut, which is more efficient at processing plant tissue because the material takes longer to pass through and so has more time to ferment and be digested.
Even so, much of what is eaten has no nutritional value and is voided.
Larger animals also use less energy per gram of body weight than do small animals, both for staying warm and moving about. Gram for gram, an elephant needs much less food than a mouse does, and an ostrich consumes relatively meagre rations compared with a hummingbird. The basic resting metabolic rate (effectively energy used while sleeping) for a 17 kg cassowary is 29 kcal/kg/day; for a 150-gram pigeon it is 235 kcal/kg/day, and for a tiny four-gram hummingbird it is a staggering 1410 kcal/kg/day! These rainbow-coloured midgets are in constant danger of burn-out, and must eat their own weight in food each day. The average human resting metabolic rate is 12.5 kcal/kg/day.
Given these figures, it might appear that life favours the large, but it’s not that simple. The daily food intake of one elephant would sustain a plague of mice, and this means that elephants will always be far less numerous than mice. When really hard times come, the elephant may starve, and so may most of the mice, but it is more than likely that a few mice will still be able to eke out an existence, poised to reinfest the Serengeti when conditions become favourable again. Fortunately for the safari industry, individual elephants don’t starve as quickly as mice do, and many survive long droughts.
For herbivorous birds then, there are advantages in being large: they save energy and they can recover more energy from their food. The price to be paid is a reduction in their ability to escape from predators and to move to new feeding grounds. Where there are no predators and a year-round supply of food within easy walking distance, these liabilities are easily ignored, and if the species can follow the evolutionary path to flightlessness, it will.
The dodo is perhaps the classic flightless vegetarian. An overgrown, docile pigeon, it did not long survive the arrival of hungry humans on Mauritius. Although the birds were not considered particularly good eating, sailors weren’t choosy, and the last birds were taken in 1662. We even have a written record: “When we held one by the leg he let out a cry; others came running forward to help the prisoner and were themselves caught.” So exited the dodo, and very nearly all traces of its existence. The last known dodo skin was consumed in a fire at Oxford early last century, only the head and a leg being saved from the flames.
Unlike New Zealand’s moa, there are woefully few dodo bones in collections. Surprisingly, a large proportion are in New Zealand, brought here by the first experts on New Zealand’s own flightless birds. These bones are helping today’s scientists unravel the further mysteries of extinct birds in the Pacific.
The Hawaiian counterparts of our moa were the moa-nalo, flightless ducks as large as geese but which evolved from smaller ducks which flew. Many ducks feed on land when they can, as anyone who has seen the flocks feeding on acorns in Hagley Park will know. But the Hagley Park ducks have to beware of dogs and children; the ancestors of the moanalo did not. The ducks evolved into new species on the different islands, each cropping the forest-floor plants in a different way and rubbing shoulders with a range of flightless geese, of which the endangered Hawaiian goose is the last survivor—it can fly.
Large vegetarian waterfowl also shared New Zealand with the moa. Both main islands had their own flightless geese. Close relatives of the living Australian Cape Barren goose, the two species of Cnemiornis seem to have been confined to small areas of grassland beside rivers and lakes. They are rare in fossil deposits but are known to have been eaten by the earliest New Zealanders.
It is worth noting that not all plant foods are low in energy. Flowers, seeds and fruit, for example, give high rewards, and those who eat them need not be large but must have mobility. Species that live on fallen fruit on the forest floor must move around to take advantage of different fruiting seasons. Seed-eaters must follow the seasonal supply of seeding grasses or depend on seeds shed at other seasons. The unpredictability of supply at any one point places a premium on mobility, and for birds that usually means flight. Only the cassowaries among the living flightless birds depend on fallen fruit. Their considerable size allows them to range far enough to eke out a living on the floor of the rainforest. Some of New Zealand’s forest moa probably depended on fruiting of the matai and miro trees.
Swimming to safety on virtual islands
Apart from islands, the other great refuges from predators are lakes and the sea. Here each bird can be its own island, well away from land-based enemies, and many of the most successful groups of flightless birds are aquatic.
Even flighted species, such as most ducks, geese and swans, use their water refuges for protection while they moult their flight feathers (usually all at once) and are unable to fly for some weeks each year. Species that moult a few feathers at a time must cope with the reduced efficiency of the imperfect wing.
That water for many ducks is important for protection rather than food is clear when waterfowl are isolated on predator-free islands. In prehuman (and pre-rat) New Zealand, several ducks moved out of the streams, ponds and lagoons and on to the land. Until the rats came, Finsch’s duck, the brown teal and the blue duck wandered in the forests and shrublands, well away from running or standing water. Fortunately for the survival of the brown teal, it also lived in watery habitats, but Finsch’s duck was wedded to the land. Fossils show that its wings began to shrink after the last Ice Age, as taller vegetation once more covered the land. Having lost its powers of flight, it had no refuges when the Pacific rat arrived, and died out about 500 years ago.
On outlying islands, other small ducks closely related to the brown teal responded to the absence of predators and an environment dominated by strong westerly winds (posing a threat to flying land birds of blowing them irrecoverably out to sea) by becoming completely flightless. The Auckland Island teal lives along the shoreline and in the dense grasslands, feeding on insects and crustaceans in kelp piles and litter. Its vulnerability to predators was starkly revealed when cats and pigs were introduced to the main Auckland Island in the 19th century. Very soon, the little ducks were confined to small islets offshore which the predators have never reached.
The same fate befell the Campbell Island teal. It was thought to be extinct until a few were discovered living on steep and exposed Dent Island off Campbell Island’s west coast. There, the ducks move about under the thick grassland and scrub and have never seen a pond or stream. Under the thick vegetation, they are safe from their only predator, the brown skua. On Campbell Island itself, the introduction of Norway rats in the 19th century doomed the ducks to early extinction.
Thousands of kilometres to the east of Campbell Island, the steamer ducks of Patagonia live on a continent, but with rocky stacks offshore for safe breeding, and they have been able to reduce their investment in flight as a result.
In the remote Galapagos Islands, a flightless cormorant lives now only on the western islands of the group where the cold waters around Narborough and Isabela Islands are rich enough in fish to make it unnecessary to go far to find food. The cormorant used to be found on other islands, but the cats and pigs introduced in the past 200 years found them easy prey. The Galapagos species is now the only flightless cormorant. One hundred and fifty years ago the spectacled cormorant lived among the rocks and islets of Bering Sea, again protected by its harsh environment, and near rich fishing grounds. People found them an easy source of bait for their own fishing, and they were gone so quickly that only six skins remain.
“Flight” of the watery kind
The oceans have also provided protection to some groups of birds that traded flight in the thin air for flight in water. The world’s 13 species of penguins are the remaining members of a group that seems to have evolved along the southern shores of the Pacific, before the Drake Passage opened and Antarctica became an island continent.
Fossil evidence shows that New Zealand was a centre for the evolution and radiation of penguins Some of the early forms were giants, fully two metres high, but all had the anatomical adaptations that make them amongst the most efficient swimmers of all. Their bones are dense, unlike those of birds that fly in air, their bodies are covered in a layer of small feathers that streamline their form, and the wings have become flippers.
The wing bones themselves are flat and broad, and there are many more and shorter feathers than on a normal bird wing. By flapping these wings in a medium that is 900 times denser than air, some penguins can dive to more than 300 metres; others migrate thousands of kilometres.
Although the perfection of their adaptation to the aquatic environment has made them both awkward and vulnerable on land, their breeding grounds are usually on islands or stretches of coastline where there are few or no mammalian predators to take their toll. Now, some species are in trouble. Yellow-eyed penguins once nested as far north as Cook Strait and Nelson, but hungry humans trapped the northern populations to extinction. Dogs, cats and people disturb and maim the survivors on the Otago Peninsula. The rare crested penguin, now almost confined to, and called after, Fiordland, was also much more widely distributed before people arrived.
Birds of the southern latitudes, penguins reach their northern limits at the Galapagos Islands, where the tiny endangered Galapagos penguin just crosses the Equator.
Further north, other groups of seabirds have abandoned the air for life beneath the ocean wave, notably auks. Auks are among the most abundant of northern seabirds. In colonies of many thousands, puffins, guillemots, razorbills and many others line the cliffs of the northern oceans in the brief summer. All these birds catch their prey by diving and swimming beneath the surface with half-folded wings. Their wings are compromises: short and narrow, they are still able to carry the birds in the air, but they are also compact enough to work under water.
At least one species in the North Atlantic, the great auk, went for optimisation of the underwater effort, and the wings became too short, stiff and paddle-like for flight. This bird is one of the most famous flightless species of them all, not least because we know when the last two were killed (in 1844 on an islet off the coast of Iceland).
Thousands of years ago, great auks inhabited the coasts of France and Spain. Until the Middle Ages there were still birds in Britain and Norway exploiting huge shoals of fish within swimming range of the cliffs and stacks where they bred. Other auks depended on food supplies further offshore, or moved between feeding grounds with the seasons.
By the 18th century, the great auk had been decimated by persistent raids on its breeding grounds. Human exploitation of the bird for use as food and fish bait as well as for its feathers doomed the only flightless bird native to Europe in recent times.
The great auk was the bird first called pingouin, and it was the ecological equivalent of the penguin in the North Atlantic. It even looked like a real penguin, and when the first west-European mariners visited the Cape of Good Hope and Patagonia they thought they were seeing the same animal. The name stuck, even after the great auk was harried to extinction.
On the other side of America, a few million years ago, another group of auks went wholeheartedly into flightlessness. All known species of mancalline auks, fossil birds from California, were flightless. In the warm seas, they took the niche now occupied by the smaller penguins in colder southern waters. In the North Pacific at the same time, the counterparts of the giant penguins were the huge plotopterids. Some of these birds were over two metres long. Distantly related to pelicans, in their day the plotopterids ranged from Japan to California. As with the giant penguins, the demise of the huge plotopterids may have coincided with the rise of the seals as major inshore predators of fish and squid.
Penguins and auks can at least run, hop and toboggan on land and snow. Grebes are so well adapted to water that their short legs cannot support them on land, and they flop on to nests of floating vegetation to brood their eggs. The young are carried on the parents’ backs, and they never move on dry land at all.
It is not surprising that at least some grebes have given up flying as well. For these, the islands of flightlessness have been lakes, in Guatemala, Peru and Madagascar. For the giant pied-billed grebe of Lake Managua, being isolated on one lake has already proved fatal. Its habitat was altered by the introduction of exotic predatory fish (for human sport), and its close relative (and probable ancestor) the pied-billed grebe has colonised the lake as well. A combination of predation on the young and hybridisation pushed the giant pied-billed grebe into oblivion in the 1980s.
The fate of two other flightless grebes, each in their own evolutionary prison, hangs in the balance.
Flightless grebes may be almost predictable, but ibises seem unlikely candidates for flightlessness. Long-legged, long-billed and swift on the wing, ibises live in wetlands in the warmer parts of the world. They can fly strongly: glossy ibises regularly cross the Tasman to New Zealand.
Several millennia ago, some ibises reached Jamaica. Others made the epic trip to Hawaii. In both places their descendants stayed and moved into the forests. In Hawaii, they became so much ground birds that they have been compared with kiwi. The convergence in habits and structure was amazing. They lived on the forest floor, eating snails and other small invertebrates. When people arrived, they burnt the forests, and the tasty flightless birds could not survive the dual impact of habitat destruction and the cooking pot.
Why are there no flightless eagles?
Vegetarians and insect-eaters are surrounded by their food, and their food is slow-moving or stationary. Predatory birds have to hunt, and their food usually tries to get away. So it is almost impossible for a bird to be a predator of rapid-moving prey and not to fly. Even in the sea, penguins and auks “fly” through the water.
There are so few flightless predatory birds because it is just too expensive for relatively small, warm-blooded animals to walk and run after living prey, and dead animals are spread too thinly to search for on foot. So there are no flightless eagles, hawks or vultures. But there were flightless owls.
Before humans arrived in Cuba, the island was home to a huge owl which had such a flat breast bone that it almost certainly did not have big enough muscles to allow flight. Hunting by night, the bird could catch enough large rodents and insectivorous mammals—even young ground sloths—to make a living. It shared the island with an immense eagle, almost as large as the moaeating Haast’s eagle in New Zealand.
The New Zealand adzebills were the exception that proves the “no flightless predator” rule. These strange distant relatives of the cranes took to eating the one supply of animal protein in New Zealand that was not too fast to catch: reptiles. With their strong, strangely-shaped beaks they could hold struggling tuatara or big lizards and even dig them out of the ground. Some may even have caught and eaten the petrels that bred extensively on the mainland until rats, pigs, cats and humans arrived.
Like all the most specialised predators, adzebills were few in number; there’s not much room at the top of the food chain.
Insect-eaters, seed-eaters and omnivores are better candidates for flightlessness than carnivores are. Their food is usually common and readily available, and escape without flying is easy for birds small enough to live in very dense vegetation or among rocks. Small, secretive and able to live in very dense vegetation, more rails have become flightless than any other bird group.
Another reason for the abundance of flightless rails is that, although rails are strong fliers, they fly buoyantly on their broad wings and are easily blown off course. This combination of attributes has probably led to their colonisation of most oceanic islands—even small specks of land far from any continent. But having flown there, it is the individuals that are not blown away that survive and breed. There is a paradox here: strong fliers most often reach places where it is easiest to become flightless.
Flightlessness and extinction
The apparent safety of predator-free islands has turned out to be an illusion for many flightless birds. People have colonised, or at least visited, nearly every speck of land on Earth. Visitation itself is not usually a problem, but people have a habit of taking camp followers with them. These include rats, cats, dogs and pigs. Alone or together, they have proved deadly to flightless birds.
Together with the destruction of habitat by burning and logging, and with hunting for food or sport, predatory mammals have been responsible for the extinction of most of the species of flightless birds alive in the past 10,000 years. New Zealand is a classic case. Of about 40 flightless species alive in New Zealand 1000 years ago, at least 29 have become extinct, 25 of those before Europeans arrived.
On islands everywhere, flightless birds have succumbed to the onslaught. It has been calculated that, in the South Pacific alone, about 4000 species of bird, perhaps more than 1000 of them flightless, have become extinct in the past 4000 years.
Their refuges were safe so long as their isolation remained. And although some birds can become flightless in remarkably few generations, they can never regain the power of flight quickly enough once the cat is out of the bag. Losses have accumulated at least 100,000 times more rapidly in the last few millennia than they do through the normal background rate of extinction.
Ultimately, the ease and speed with which flightless birds disappear when their isolation is ended show the true value of flight. There are always environmental and physiological pressures towards saving energy. For most species, those pressures are balanced by the realities of survival in environments filled with hungry mouths and where food is hard to come by. Species that relax their guard, in the water or on islands, and choose to walk or swim, must always be at the mercy of the next invader.
Paradoxically, flightlessness is, or was, not rare, because there are many islands, and many species have reached them and stayed and changed. On those islands the pressures of competing with mammals for aeons were stripped away.
For most species, the way of life adopted by their ancestors could not be altered, and they still fly. For others—those that were able—release from a world of mammals meant that the very organs of success on the continents—wings—were now superfluous. There were new realms to conquer and nothing to drive them into the air. The wings that were so efficient in giving them a competitive edge on the mainland could be dispensed with.
If man had not intruded, the number of flightless birds taking the place of stay-at-home mammals might well have equalled that of flying species. Such sheer abundance is a challenging thought for those who see birds as the epitome of freedom: the rulers of the sky.