Featherweights and Heavyweights: Curious Extremes in Avian Evolution

by David Greer

Anna’s hummingbird

There’s a bird that weighs no more than an average paper clip and is one of the fiercest fliers on the planet. There once was a bird that weighed around half a ton, the same as an average cow, and laid an egg as large as 150 chicken eggs. The elephant bird is long gone but the bee hummingbird remains fighting fit. The only dinosaurs to survive the last mass extinction sixty-six million years ago, birds have evolved since then to fit into every available ecological niche, and today are the most widely distributed form of life on the planet other than microscopic organisms.

Birds are fascinating for any number of reasons, not least because of the mind-boggling variations in size that evolved through the tens of millions of years before humans stumbled onto the planetary stage. For the most part, the large flightless birds had been driven to extinction as humans spread across the planet, and close to two hundred other bird species are believed to have made their final exit during the last five hundred years as humans have largely converted the natural world to serve their own purposes during the latest geological epoch, the Anthropocene. The Featherweights

The morning after the blizzard, the temperature has plunged well below freezing after a three-dog night, rare in these parts. The flowers that reliably bloom in late December on the Pacific coast of British Columbia—purple rosemary, pink heather, scarlet salvia, yellow Oregon grape—are either covered in snow or instantly wilted beyond recognition. It’s a soundless morning save for the whisper of drifting snow, the soft chatter of a pair of Pacific wrens, and an insistent dronelike whirring.

The Anna’s hummingbird hovers a few feet from my head, gazing intently at the red plastic feeder I’m hanging to replace another one half full of frozen sugared water. At these temperatures the feeders need to be switched every three or four hours before their contents turn to ice. Hummingbirds have evolved a trick to help survive the cold snap, entering a state of controlled hypothermia when darkness falls in which body temperature drops by more than half and the bird’s metabolism slows by as much as 95 percent. Even so, the mortality of small birds during winter storms is high, and hummingbirds are no exception.

Nevertheless, the avian equivalent of a fighter pilot cum acrobat still needs to feed to support its energy output. For a bird the size of a ping-pong ball and weighing less than a nickel, that still means consuming the human equivalent of more than 150,000 calories a day.

The hummingbird is the only bird that can fly both forwards and backwards, not to mention up and down and sideways while hovering. Its acrobatic skills come into play most dramatically in war and sex. A hummingbird’s defense of its territory is fierce and fast and can be so intense that hummingbird combatants have been known to have expired together, impaled on one another’s rapier-like bills. But the hummingbird’s most gob-smacking acrobatic feat is a courtship behavior rather than a territorial defense.

When Anna’s hummingbirds first showed up in these parts, I remember being puzzled by a sharp sound that seemed identical to a marmot’s alarm whistle, but in a neighborhood where marmots do not exist. If anyone had told me at the time that it was merely the sound of a hummingbird’s tail feathers, I might have been skeptical, but tail feathers they were. To make the sound, the male flies towards the sun, folds its wings, and drops straight down at lightning speed, all for the purpose of displaying its iridescent crown (see photo) to the object of his desire. The hummingbird may reach a top speed of sixty miles an hour during the dive and, at its fastest point, cover 385 times its own body length every second. For its size, it’s the fastest aerial maneuver performed by any bird. In contrast, the attack dive of the peregrine falcon, while faster in absolute terms, only covers 200 body lengths per second.

The dive, of course, has no purpose other than to impress an Anna’s female, a tough little bird herself, one whose beady gaze (at least at my feeder) suggests she doesn’t suffer fools gladly. However, she’s apparently a sucker for a sunlit magenta iridescent crown, and the male Anna’s plays that card for all it’s worth, often repeating the dive multiple times. In subdued light the male crown appears dull and black, but when the sun catches it, the effect is stunning.

Iridescent plumage is not unique to the male Anna’s hummingbird, though it’s a particularly dramatic example. It’s found in birds as varied as tree swallows and pigeons and starlings, and different species have evolved just about every color in the rainbow to paint their glittering displays. Hummingbirds that employ iridescence as a courtship device are known as brilliants, topazes, emeralds and gems.

So visually striking is iridescent plumage in birds that it became a sticking point in the controversy that raged around the theory of evolution in the late nineteenth century. Flummoxed by the evidence of natural selection produced by Charles Darwin and Alfred Russel Wallace, some creationists thought they had hit upon an airtight proof of divine creation in the form of the “excessive beauty” of birds such as hummingbirds and peacocks, which had no evident purpose other than to please the human eye.

The Duke of Argyll in particular took aim at the theory of evolution in his 1868 book The Reign of Law. As David Quammen describes Argyll’s argument in The Song of the Dodo, “the reign of law in the realm of nature entailed an omnipotent, intrusive Creator with the mentality of a parlour magician. Yes, Argyll conceded, evolutionary change among species might occur. But natural selection wasn’t driving it. God himself was, the tricky devil. God decreed and guided those marvelous modifications. God produced excessive beauty by manipulating the mechanisms of inheritance and evolutionary change. Why did He do it? Good question. Argyll’s answer was this: as evidence of His greatness. God produced excessive beauty for the purpose of edifying humanity. Why else, why on earth, could the peacock’s tail exist?”

A cynic might say it was beyond the ken of a Victorian peer to fathom that a male of any species might need to resort to decorative raiment to win a lady’s heart. In any event, it was in due course accepted that there was more to excessive beauty than pleasing the Anglican eye. Evolution had evidently covered all the bases, including the natural selection of attributes best suited to ensure the perpetuation of the species, and in due course Darwin’s theory convincingly trumped the duke’s.

Hummingbirds are thought to have evolved about forty-two million years earlier than the eighth Duke of Argyll. They arose in Eurasia after splitting from their sister group, the swifts and treeswifts. What hummingbirds and swifts have in common today is their habit of rarely landing, spending much more of their time aloft compared to other birds, though swifts take it to an extreme, to the point where the common swift holds the record for unbroken flight, up to ten months aloft with all the necessities of life accounted for in the air: eating, sleep, courtship, and mating (in haste), followed by two months of raising young on terra firma.

Naturally, it took a while for bird life to evolve to a point where one of its members no longer needed to touch the ground for months at a time. The first vertebrates to take to the air were the pterosaurs, winged reptiles without feathers that inhabited the plant in one form or another between 228 and 66 million years ago. One might logically presume that birds are descended from pterosaurs, but they aren’t. Nature finds many different ways to resolve an evolutionary challenge such as flight, and the pterosaur adventure ended in an evolutionary blind alley in the asteroid-induced fifth mass extinction in the history of the planet. Only the little bird dinosaurs made it through.

As Steve Brusatte puts it in The Rise and Fall of the Dinosaurs, birds are not just related to dinosaurs; each one is “a real, honest-to-goodness, living, breathing, moving dinosaur”. It just took a little time for a hummingbird to evolve from something that looked more like Tyrannosaurus rex (though T. rex wasn’t a direct ancestor, just a distant cousin). It was long after Darwin and Wallace that convincing evidence emerged of a clear link between dinosaurs and birds. Though the link had long been suspected, the most definitive proof came to light in the 1990s with the unearthing in China of fossils of feathered dinosaurs.

A remarkable thing, feathers. Noting that, as “nature’s ultimate Swiss army knife”, feathers were useful for display, insulation, egg protection, and flight, Brusatte suggests that they initially evolved as a source of warmth and that wings evolved for display (including a show of iridescent plumage). In Brusatte’s telling, flight evolved more or less by accident, after dinosaurs with different airfoils (wings) and feather arrangements found themselves generating lift while leaping from the ground or between branches of trees.

Dinosaurs first evolved feathers many millions of years before the last mass extinction 66 million years ago, but it may have been another anatomical feature that enabled only the bird dinosaurs to survive that extinction: a toothless beak well suited for retrieving the seeds and nuts that would have survived the asteroid collision. In the evolution of bird species following the mass extinction, the beak began to assume bizarre shapes and unusual proportions in gigantic flightless birds that began to appear in far corners of the Earth.

To date, more than 350 species of hummingbird have been identified, all of them in the Americas. The smallest, though no less fierce because of its size, is Cuba’s bee hummingbird, which tips the scales at slightly under two grams, roughly the weight of an average paperclip. Hummingbirds are tough little beasts and the Anna’s particularly so, aggressive not only its claim for local territory but also in the dramatic expansion of its range. Until recent decades the range of the Anna’s hummingbird was limited to southern California. It was first sighted north of the US-Canada border in the 1960s, and only in the 1990s did it become common in southwestern British Columbia. Since then it has been spotted as far north as Alaska, where its hypothermia trick must be very useful indeed. Not only has the Anna’s adapted to winter climates but, unlike this region’s native hummingbird, the rufous, it does not migrate south to central America but stays put, no matter how low the temperature gets.

The Heavyweights

Birds are the most widely distributed form of life on the planet other than microscopic organisms. They live on land and water, in the tropics and at the poles, in jungle and desert and downtown. The number of bird species is estimated at roughly 10,000 but constantly changes with identification of new species and extinction of others, the latter as a result of habitat destruction, invasive species, pollution, human population growth or overhunting—collectively known by conservation scientists as “HIPPO”.

E. O. Wilson, in his 2016 book Half-Earth: Our Planet’s Fight for Life, estimated that, as a result of human activity, the overall current rate of extinction of species is between a hundred and a thousand times higher than it was before the arrival of humanity about two hundred thousand years ago, and is increasing. Birdlife International’s most recent State of the World’s Birds report from September 2022 provides a more detailed and alarming overview, estimating that 49 percent of bird species are declining globally, one in eight are threatened with extinction, and at least 187 species are confirmed or believed to have gone extinct since 1500. Small surprise that the usual suspects in those 187 extinctions are us humans.

In recent centuries, avian extinctions have largely been the result of human degradation or pollution of natural areas for the better and higher purpose of human convenience. But long before that came the less sophisticated and more purposeful objective of killing birds for food. The bigger and less mobile the bird, the easier to kill and eat.

The largest birds to evolve over millions of years after the last great extinction 66 million years ago inhabited islands scattered across remote reaches of the oceans and along the fringes of continents. As David Quammen explains in The Song of the Dodo, isolated islands allow evolution to take life forms in directions not afforded by a mainland existence, one notable example being size. Humans evolved about two hundred thousand years ago and by around seventy thousand years ago began to venture beyond the African continent and into southeast Asia. By twenty thousand years later—another geological blink of the eye—humans had reached Australia. This was bad news for big birds, in this case the species later labelled Genyornis newtonii, an Australian flightless bird more than six feet tall and weighing over four hundred pounds. Fossil records suggest that human invaders wasted little time hunting down Genyornis, whose extinction happened so quickly that it was far more likely a result of that hunting than of climate change. Genyornis newtonii was probably one of the first gigantic flightless bird species to succumb to what E. O. Wilson called the “basic rule of extinction biology: the first to fall are the slow, the dumb, and the tasty.”

The tallest bird species ever to have evolved, at over eleven feet tall, appears to have been the giant moa (Dinornis robustus), which inhabited New Zealand’s South Island. Reaching a weight of over five hundred pounds, the bird was a formidable presence but, unlike some other gigantic flightless birds, it did have to worry about one significant non-human predator. Haast’s eagle (Hieraaetus moorei), the largest eagle ever to have existed, is thought to have evolved to to prey on the giant moa. Nevertheless, humans appear to have had their way with it. In a paper in Science hypothesizing that the extinction of the giant moa was the result of human intervention rather than climate change, the evolutionary biologist Morten Allentoft argued that “we like to think of indigenous people as living in harmony with nature . . . but this is rarely the case. Humans everywhere will take what they need to survive. That’s how it works.”

Its height notwithstanding, the giant moa was a relative pipsqueak compared to Madagascar’s elephant bird (Aepyornis maximus), which at 1,400 pounds weighed more than twice as much as a giant moa (and more than three times as much as today’s largest bird, the ostrich), and is considered to have been the largest bird that ever lived. More than one species of elephant bird survived the arrival of humans on Madagascar for several thousand years until the last of its kind succumbed about a thousand years ago. It also laid the largest eggs of any vertebrate (including dinosaurs), over a foot in length and weighing more than three pounds. That’s a lot of scrambled egg. It’s hardly surprising that egg collection by humans is assumed to have been a key contributor to the elephant bird’s extinction.

One of the roughly forty elephant bird eggs in existence around the world resides in the 60,000-strong bird egg collection of Chicago’s Field Museum. Fittingly, given that the collection includes eggs from several extinct species, it has contributed to the conservation of existing species in surprising ways. After Rachel Carson brought attention to the risk of DDT to bird populations in her 1962 book Silent Spring, the museum’s egg collection provided the evidence needed to show that the shells of bird eggs had been thicker before use of DDT became common.

One of the stranger birds ever to inhabit North America was the aptly named “Terror Bird” (Titanis walleri). It found its way north from South America to Texas about five million years ago (likely by navigating between the islands that later merged to form the isthmus of Panama) and then dispersed along the Gulf Coast to Florida. A carnivorous predator seven feet tall, the flightless Terror Bird looked superficially like a miniature Tyrannosaurus rex and may have used its massive beak to club prey into submission. It became extinct about 1.8 million years ago. A metal representation of its skeleton, together with fossilized bones discovered in Florida’s Santa Fe River in the 1960s, is on display at the fossil hall at the Florida Museum in Gainesville.

Smaller but more famous flightless birds include the dodo (Raphus cucullatus), which was rapidly driven to extinction in the seventeenth century after Dutch sailors landed on the island of Mauritius and ate it out of existence. The dodo did achieve a level of immortality thanks to the Alice in Wonderland character created by Charles Dodgson, aka Lewis Carroll, who is thought to have visited Oxford University’s Museum of Natural History with Alice Liddell and seen the remains of a dodo and perhaps Jan Savery’s painting of the bird. The dodo skull at the museum represents the only surviving soft tissue remains of a dodo still in existence.

The last great auk (Pinguinis impennis) literally met its demise at human hands when it was strangled on an island off the coast of Iceland in 1844 while incubating eggs. This occurred during an age of increasing popularity of natural history museums, which competed to acquire specimens of exotic creatures brought back from the far corners of the world by collectors. Even Alfred Russel Wallace, whose years of dedicated research in the Amazon basin and the Malay archipelago led him to the concept of natural selection more or less at the same time as Charles Darwin, planned to finance his research in part by selling bird specimens to museums back home.

For pre-European peoples, large flightless birds were simply food. Today, however, bird populations in decline are likely to be collateral damage from human carelessness in the pursuit of economic development. The same applies to the extinction of non-bird species as well and to the gradual shredding of biological diversity that is the sad reality of the times that have come to be called the Anthropocene epoch, in which the entire global environment has been altered by humanity. A worldview in which nature is valued primarily for its importance to human welfare is unfortunately a major stumbling block to the protection of biological diversity.

Biodiversity Conservation: A Tough Sell Then and Now

The 1992 Earth Summit reached two primary agreements: the Convention on Biological Diversity and the Climate Change Convention. In the decade following, the Convention on Biological Diversity received greater attention, in large part because the environmental movement in those days largely focused on destruction of and protection of biodiversity, and public awareness of human-caused climate change was far more limited.

Since 1992, successive governments have set increasingly ambitious targets for the protection of natural areas—first 12 percent in the 1990s, then 17 percent at the 2010 Conference of the Parties (COP) to the Convention on Biological Diversity held in Aichi, Japan, and most recently 30 percent at the 2022 Conference of the Parties held in Montreal. Not one of the targets established at the Aichi COP was subsequently met. Hopefully the targets set at the 2022 COP will be. Even if they are, they come nowhere near approaching the 50 percent protection advocated by E. O. Wilson, in Half-Earth, “to save the living part of the environment and achieve the stabilization needed for our own survival”.

Three decades after the Earth Summit, however, the public focus has changed, and in a big way. Today’s Rachel Carson is Greta Thunberg, and it’s fair to say that fears of ever-increasing climate change trump concern about threats to biological diversity in most people’s minds. This preference is unsurprising. The obstacles to ordinary persons’ actions to help mitigate the impacts of climate change and to protect ecosystems are fairly easy to understand. We’re genetically programmed, as a result of the brevity of our species’ evolution on this planet compared to the evolution of most other species, to fight or flee from threats we perceive to be immediate and serious.

Climate change seems somehow worse than biodiversity loss because it affects us so obviously and directly, whereas diminished biodiversity impacts other species first. Do we really care that much about the warty jumping slug or, for that matter, the ivory-billed woodpecker (most likely extinct, but reported observations keep surfacing from time to time)? We’re far more likely to worry about wildfires/floods in central California or blizzards in Texas or tornadoes in Tennessee. People might actually get hurt there.

The fragility of biodiversity is also a harder problem to imagine than unpleasant weather. David Quammen pictures an ecosystem as a finely woven Persian carpet. Remove an isolated square here and there, no problem. But the rug is not simply a patchwork of squares, it’s an interconnected framework of threads extending the length and the breadth of the carpet. The more squares are removed, the more the framework of threads is broken and allowed to fray, until a carpet that looks relatively sound begins to fall apart all over the place. By then it’s too late to save. Ecosystems are a lot like that, and every species, whether cute and cuddly or an underground fungus, is an essential thread in the carpet.

Loss of biodiversity is also a tricky problem because it’s harder to see how it impacts our lives. A species here, a species there, just drops in the bucket, really. On the other hand, it’s useful to keep in mind that the natural world and the myriad species it comprises are the natural capital for all human enterprises. Capital is finite, and we live in an age where our withdrawals of capital seem to increasingly exceed our contributions. “Sustainability”, which once upon a time meant sustaining indefinitely the natural capital bequeathed to us by the planet, has become a term so misused and distorted by the apologists of profit that it has ceased to have much meaning at all.

The bottom line is that nature bats last and the human presence on this planet will be as fleeting as a flash of lightning in the evening sky. Life may take millions of years to recuperate from another mass extinction, just as it did after the previous five, but life will go on. Possibly the outcome of another mass extinction may be something a little like humans but with the self-destructive kinks ironed out. Who knows?

In his book Half-Earth, E. O. Wilson was optimistic that humanity still has time to reverse the threats to biological diversity by reducing its ecological footprint through a combination of gradual population reduction and technology that reduces the amount of material and energy needed to provide goods and services. He cautioned, however, that unsustainable consumption must be urgently reduced and that “only a major shift in moral reasoning, with greater commitment given to the rest of life, can meet this greatest challenge of the century”.

Humanity does not have a good record for walking the talk where the long-term duration of species is concerned. In a nutshell, the time has come to put up or shut up when it comes to doing what’s needed to sustain biological diversity —including bird diversity. Perhaps Colin Tudge put it best in his book Consider the Birds: Who They Are and What They Do:

We have the kind of economy we have, and we do the kind of science we do, for all kinds of reasons, including those of history. But the main reason in the end is that we just don’t care enough. We do not get angry enough when we see animals, or indeed people, treated badly, or when forest is swept aside. Those who do protest are commonly derided as ‘greenies’. They are perceived to be in the way of economic progress, and so to be ‘unrealistic’. Yet they are the defenders of reality: the real realities of landscapes and of living creatures. It’s the present economy, that recognizes no limits to financial growth, that is unrealistic.

So it’s attitude that matters most. We need to give a damn. Hope for the future—for ourselves and all other creatures—lies not with new technologies or even economics but in the possibility that human beings, collectively, might undergo a change of heart, of mindset, of the things we have come to take for granted and the way we look at the world.