Schieffelin’s greatest successes were achieved with Hotspur’s starlings. In 1890 and 1891, he had some eighty breeding pairs shipped from England and released them in New York’s Central Park. Instead of sitting around repeating royal names, the birds wasted no time and immediately proliferated into the vacant niche of winged inhabitant of American cities and villages. Researchers have calculated that, from their point of release, they multiplied and spread with a speed of some 50 miles per year, hopping from town to village to hamlet. In 1920, they occupied the entire US east coast. By the end of the Second World War, they had crossed the great plains. In the 1960s, they had established themselves on the west coast, pushing on into the interior of Alaska by 1978. Today, there are about as many starlings as there are people in North America.
Clearly, bolstered by its Shakespearean mandate, Sturnus vulgaris decided to be, and not not to be. But to establish itself in all those burgeoning American cities may have placed demands on the starling’s nimble body. And those demands, two Canadian researchers reckoned, might be different from what had shaped the bodies of the original English colonist-starlings. To check this, they consulted the bird collections of eight natural history museums in North America and took measurements on the shape of the wings of 312 starlings, stuffed in the 120 years since their departure from Central Park in 1890.
The scientists, Pierre-Paul Bitton and Brendan Graham from the University of Windsor in Canada, discovered something interesting. Over time, they found, the starlings’ wings had gradually become more rounded, because the secondary flight feathers (the ones at the bird’s “lower arm,” closest to the body) had become elongated by some 4 percent.
Now, the shape of a bird’s wing is not something that evolution can mess with with impunity. It is very closely wedded to a bird’s way of life. Long pointed wings are better for fast flying in a straight line, while short, rounded wings are good for making rapid turns or for quickly taking off. That’s why the dive-bombing peregrine falcon has the former, but an aerial acrobat like the lapwing has the latter. It is precisely this quick-response benefit of more rounded wings that may be one of the reasons that the settler starlings evolved. In those 120 years, the human population in western North America (the part of the continent that the starling expanded into) grew almost fifty-fold. What were tiny settlements when the starling arrived, blossomed into metropolises in a matter of decades. And with urbanization came new dangers for urban birds: cats and cars. It is quite likely that this is what caused the American starlings to evolve a wing shape that helped them get out of the way of a pouncing cat or a speeding motorcar hurtling toward them.
In the case of the starling’s rapid wing evolution, we can only speculate about what exactly caused it. But in the evolution of roadside American cliff swallows, we know for sure.
Blessed is the bird to whom a biologist devotes his or her entire life. In the case of the American cliff swallow (Petrochelidon pyrrhonota), it is even his and her. Since 1982, Mary Bomberger-Brown and Charles Brown have spent every spring studying colonies of these birds in Nebraska. Around the time they began their work, the swallows, which normally build their gourd-shaped clay nests on crumbly rocky overhangs and sandy cliffs, had just taken up the habit of colonizing firm, newly built concrete highway bridges and road-side culverts. “We built them a better cliff,” says Bomberger-Brown. Over the years, some colonies grew as large as a staggering 6,000 nests, all suspended from these artificial structures. And each year, the two biologists monitored the colonies, driving along the same roads for the same number of field days, using mist nests to catch the swallows, measure them and adorn their legs with tiny numbered rings. They also made it a habit to pick up any dead cliff swallow they found along the roadside, and to take its measurements—such as wing length.
As so often in scientific research, their meticulousness, stamina and utter immunity to boredom paid off in the end. In a two-page article in Current Biology in 2013, they assembled all the data that their thirty years of calipering swallow wings had yielded. In the 1980s, when the birds had just begun nesting on the roadside structures, all birds, dead or alive, had wings that were about the same length: around 10.8cm. But as time went on, they found that the living birds’ wings grew shorter, by about 2mm per decade. Not much, and perhaps not really worth noticing if their measurements on the roadkill had not shown the exact opposite pattern: by the 2010s, the wings of dead birds by the roadside were about half a centimeter longer than those of live birds still happily flapping along. Also, even though the pressure of traffic had remained the same or even increased, the numbers of dead birds declined by almost 90 percent.
The conclusion was inescapable: only cliff swallows with wings short enough to take off vertically from the tarmac to escape an oncoming car had managed to get away and spread their short-wing genes in the gene pool. The tardier long-winged ones ended up as ex-swallows on the hard shoulder, their long-wing genes excluded from the gene pool. And, as the surviving swallows became ever better adapted at evading approaching vehicles, the number of casualties plummeted.
On the other side of the Atlantic, in southern France, unforgiving tarmac also drives evolution. Not birds, in this case, but plants. Pierre-Olivier Cheptou, a botanist at the Montpellier’s branch of the National Center for Scientific Research (CNRS), has been studying weeds on the city’s pavements. Or rather, in the one-square-yard quadrats of soil around trees planted along those sidewalks. As Cheptou writes in one of his papers, “these patches are widely distributed (several thousand in the city), and regularly spaced, from 5 to 10 m[eters] from each other depending on the street.” Indeed, as I descend into Montpellier on spaceship Google Earth, the square snippets of soil become visible on my computer screen all over town. Like tiny bits of Versailles, the geometric pattern along Rue Auguste Broussonet, Avenue Henri Marès, and Chemin des Barques looks like a city-sized ecological experiment. And that is exactly how Cheptou and his colleagues used it.
Almost a hundred different species of wild plants grow in these patches of soil. One of them is the hawksbeard (Crepis sancta)—it looks a bit like a dandelion, but with the yellow flowers sitting on top of multiple branched stems, rather than a single one. Like dandelion, after blossoming, the flower changes into a head of fluffy seeds. Most of these seeds are small and light and have a delicate parachute-like umbrella attached to them. Others, however, lack the parachute and are much heavier. This seed-duality is the hawksbeard’s way of having your cake and eating it, seed-dispersal-wise. The heavy seeds fall straight to the floor, where they are sure to find a suitable bit of fertile soil at the parent plant’s root. The parachute-seeds, on the other hand, will be lifted into the air by a gust of wind or a child blowing them off their pedestal, and float on the wind until they drop somewhere far away from their parent. With a bit of luck they find a suitable, vacant bit of soil.
At least, that’s how it works with hawksbeards growing in the wild. In Centre Ville Montpellier, however, “a bit of luck” is hard to come by. Barring a few cracks in the pavement, those one-by-one-yard squares of soil strewn with dog poo, discarded candy wrappers, and Gauloises butts, are the sole spots suitable for seed germination. So, even though the plant owes its colonization of the city center to stray parachute seeds that were lucky enough to land on soil, once it had colonized the city’s streets, the heavier seeds that fall straight down were the hawksbeard’s only sure chance at reproduction.
For city hawksbeards, therefore, it would make evolutionary sense to produce more heavy seeds and fewer parachute seeds. And that is precisely what they do, Cheptou found. He sampled hawksbeard seeds from patches of soil along the sidewalks of seven city center streets. Then, he did the same for hawksbeards growing in four meadows and vineyards in the countryside around the city. He brought all of the seeds to his lab at CNRS and grew them in the same greenhouse, under the same conditions.
When the plants finished blossoming, he counted the numbers of heavy seeds and parachute-s
eeds in all of the flowerheads. Plants grown from seeds collected in the city, he found, produced almost one and a half times as many heavy seeds as countryside plants. And for the numbers of parachute seeds the reverse was true. In other words, in the city, the plants had evolved to sacrifice the parachute-seed production in favor of the heavier seeds. Based on the loss of progeny and the degree to which seed production is determined by genetics, Cheptou was able to calculate that it must have taken about twelve generations for this evolutionary change to take place. The plants have one generation per year, and since the streets where Cheptou plucked them had been repaved between ten and thirty-three years previously, this seems to be an example of extremely rapid, urban evolution.
Again, as rapid and inner-city urban as it may be, what evolution does here is nothing unprecedented. Basically, those Montpellier hawksbeards have become island plants. The one-square-yard patches of soil form an archipelago within a sea of impervious tarmac and concrete, and the hawksbeard’s investment in seed types evolves the same as it does in plants on real islands in the ocean. In fact, in the 1980s, Martin Cody and Jacob Overton of the University of California in Los Angeles found something very similar in the seeds of another dandelion-like weed, Hypochaeris radicata, or cat’s-ear, on twenty-nine tiny islands in the Barkley Sound of Canada. Unlike the hawksbeard, these plants only produce a single type of seed, suspended below a parachute. But when Cody and Overton measured the sizes of the seed and those of their parachutes, they found that the island plants produced much heavier seeds with tinier parachutes than plants of the same species growing on the Canadian mainland. The explanation is the same as for Montpellier’s hawksbeards: all the plants that produced light seeds saw their offspring fly off into the sea, and were punished by natural selection, which favored the evolution of plants producing ever heftier seeds suspended from ever punier parachutes!
Anolis lizards, too, have recently added their own urban twist to their already formidable conventional evolutionary portfolio. Dwarfed only by the 2,000 cichlid fish species of the great African lakes, the anoles of the Caribbean, Central and South America form one of the greatest evolutionary diversifications of the vertebrate animals. Literally a textbook case of “adaptive radiation,” they have fanned out into some 400 species, each with its own specialization and locale. They can be tiny, just a few centimeters long, or giant, up to almost one and a half feet. They can be pretty green or turquoise, gray or brown patterned, with a snout that may be snub-nosed like Anolis nitens, or long enough to warrant the nickname “pinocchio lizard” (Anolis proboscis). There are chameleon-like stocky ones that crush snails and moth pupae. There are sleek water-diving ones that catch crayfish. And then there are all manner of tree-dwellers: the tree trunk anoles have long legs to jump on the ground and chase prey. The twig dwellers have short legs to hold onto their twig. The canopy dwellers have large toe pads to stay stable on the slippery canopy leaves. (Besides geckos, anoles are the only lizards that can hang by a single toe.) There are also grass species with extremely long tails and lengthwise stripes—they look almost like a blade of grass when you see them. All of these types of anole have evolved repeatedly on different islands. “They’ve done it in quadruplicate across the four islands of the Greater Antilles!” exclaims Jonathan Losos of Harvard University, the world’s foremost Anolis-evolution expert.
Although it has taken 50 million years for this diversity to come about, this does not mean that anoles evolve slowly. In a famous experiment started in 1977, researchers caught Anolis sagrei on the small island of Staniel Cay in the Bahamas, measured all of them and then released handfuls of males and females on fourteen small, previously anole-free islets in the vicinity. After about ten years, they went back and caught and measured the colonists’ descendants. They discovered that, compared to the original Staniel Cay population, the settlers had evolved. Overall, their legs had got shorter and their toe pads bigger—the more so if the vegetation in their new home was more grassy. This is because to run fast on the big tree trunks of their original island, Staniel Cay, the lizards needed long legs and narrow toe pads. However, to get about on the thin twigs and blades of grass of their new homes, it’s important to cling on and the best way to do that on those narrow slippery stems is with short legs and sticky toes. (All this was duly confirmed in the lab where lizards were chased up inclined racetracks of various widths.)
The Staniel Cay experiments showed that Anolis lizards are capable of fast evolution. How fast? Evolutionary biologists have a unit for that: the darwin—one darwin being roughly an increase or decrease of 0.1 percent per 1,000 years. The Staniel Cay lizards evolved with a rate of between 90 and 1,200 darwins (that’s 1.2 kilodarwins, if you like that mental image). Not a world record, but quite impressive, as evolution goes.
This opened up perspectives for urban evolution, too. And Anolis does not eschew the city, as another Anolis researcher, Kristin Winchell, found out. She decided to work on the Puerto Rican Crested Anole (Anolis cristatellus), a small anole of the tree trunk type that occurs all over the island of Puerto Rico, in the countryside as well as in the city. Winchell picked residential neighborhoods in three of the island’s biggest cities and for comparison chose a forest on the edge of each of these cities. At each of these six sites, she caught fifty lizards with a silk noose, a kind of miniature lasso on the end of an extendable fishing rod, then snipped off a small piece of the tail, stuck the animals in a portable x-ray machine, scanned their feet on a flatbed scanner, and wrote a little number on their scales. After that, the perplexed lizards were returned to their original perch (and are probably still telling their grandchildren about that day they were abducted by aliens).
Winchell’s project (published in Evolution in 2016) showed a clear sign of urban evolution: the urban lizards always had longer limbs and more lamellae on the underside of their toe pads. And yet, the DNA from their tail tips showed that the urban lizards of each city were most closely related to the local forest lizards, not to those from the other cities. So, the differences had evolved three times independently. To check that the limb and toe pad differences were really genetic, Winchell also took a hundred urban and forest lizards to her lab in Boston, waited for them to lay eggs, and let all the offspring grow up under the exact same conditions. In these expat lizards, too, the ones from urban parents had longer legs and more toe-pad lamellae than those born from forest parents, which proved that the difference was truly in their DNA, and not simply induced by their growing up in a city environment.
All in all, this showed that the urban lizards had adapted to the urban kinds of perches they use: usually walls where you need to run quicker and farther to get out of harm’s way than on a tree trunk. Also, the urban perches were often very smooth (painted concrete walls and metal), which required heavier-duty toe pads to remain clung. In fact, a decade previously, other researchers had already discovered that urban lizards fall down more often (and usually onto a much harder surface) than forest lizards, leading to injury or even death. For them too, the old adage is true: most accidents happen in and around the home.
The Anolis lizards, hawksbeard plants, starlings, and swallows show us that urban evolution is fast, observable, and actually quite straightforward. In the following chapters, we’re going to see that urban evolution can also be complex, tortuous, and counterintuitive. But first we must pay some attention to the matter of fragmentation. How can evolution ever gain a foothold if the urban gene pools are cut into miniature shreds by all the barriers we create in our cities?
10
TOWN MOUSE, COUNTRY MOUSE
Long, thin needles stick up from the marble heads of Reine Mathilde, Marie Ire d’Écosse, and all the other statues around the Jardin de Luxembourg in central Paris. Though this gives a fleeting impression of them all being connected by wireless to the great mother ship of French aristocracy in the sky, the “antennae” are in fact meant to keep the park birds from landing and covering their regal heads in unsightly dr
oppings. Still, one ring-necked parakeet is undeterred and briefly perches on the crown of Reine Berthe, demonstratively deposits a dropping on her cheek, and then flaps off to join a loudly screeching group of parakeets zigzagging among the tall plane trees.
Parakeets in Paris … It could be the name of a hypnotic Matisse painting, but since the 1970s it has been a very realistic image for the French capital. In fact, the ring-necked parakeet (Psittacula krameri) is one of the birds that has been most successful in invading cities in Europe (on a smaller scale, also in Japan, North America, the Middle East, and Australia). Originally hailing from India and Africa, the bright green parakeet with its red beak and long tail (and, in males, a black-and-pink cravat and an azure tail) has been hugely popular in the caged-bird trade for much of the twentieth century. The numbers in trade have been so large (almost 400,000 imported into western Europe since the 1980s) that on the one hand the original populations in the tropics have declined, but, on the other hand, the species has established itself in cities all over Europe as booming populations founded by the inevitable escapees.
Darwin Comes to Town Page 9