Out there, in the wild, speciation often precipitates whenever a species colonizes a vacant, new niche. The shift in demands imposed by its new surroundings causes natural selection to enforce changes in its physique, its tolerances and behavior. When the ancestor of all Darwin’s finches first landed on the Galápagos, there were plenty of unused niches: a whole smorgasbord of different plants and other types of food, whose nutritional blessings were just up for grabs. There were benefits to be had for any finch that was a little bit better able to target a particular diet—a process that, as we saw at the head of this chapter, is still ongoing. Because beak shape also determines the voice of a songbird, specializing on different food also brought about reproductive isolation: birds with different beaks sing different songs and no longer respond to those of birds with other beak shapes.
Cities are the world’s new vacant niches, and the blackbird is one species that has embarked on the road toward speciating to maximize its profits from this horn of plenty, previously eschewed by its reclusive ancestors. Let us briefly explore all the different manners in which urban blackbirds have become different, as discovered by those many blackbird teams across Europe. (Think of them as the Blackbird Posse; there are too many teams, institutions, and persons to mention all of them individually. If you want to know the details, turn to the Notes.)
We begin with the most obvious but also the most confusing: their looks. At first, it seemed clear-cut. Researchers in the Netherlands and in France measured urban and forest blackbirds and found that the former had stubbier bills, were heavier, had longer intestines and shorter wings and legs. But when Karl Evans, a student of Kevin Gaston, looked at blackbirds from eleven cities all over Europe and North Africa, he found that this pattern did not hold everywhere. In some cities they had longer wings, in other cities shorter. For weight and leg length, the picture was similarly muddled. He did not check their intestines, but the only thing that was consistently different across all these cities was bill shape: everywhere, city blackbirds have shorter, stubbier bills than forest blackbirds, presumably thanks to the easy pickings at bird feeders and other places in the cities where food can be had without pecking, probing or pincering.
The Blackbird Posse has not yet investigated whether those different beaks have any effect on their voices, but for sure, urban blackbirds do have different songs. Male blackbirds possess a vast repertoire of melodious songs that they sing at dawn and dusk from high perches (branches and rocky ledges in the forest, TV aerials and rain gutters in the city). The score of each song, whether urban or otherwise, usually consists of a more or less elaborate motif, followed by a high-pitched twitter. And, just like most other songbirds in cities (see Chapter 16), the urban background noise forces the blackbird songsters to change their pitch and timing. As Hans Slabbekoorn’s student Erwin Ripmeester found out after recording almost 3,000 songs, urban blackbird concerts are performed at a higher pitch than forest ones, and their twitters tend to be longer. And a German team discovered that, as foretold by Paul McCartney, urban blackbirds are singing in the dead of night. In the city center of Leipzig, they start a full three hours before sunrise, well before the trams and cars start creating a racket—whereas forest blackbirds open their beaks only at dawn.
Yet that is not the only thing that urban blackbirds do earlier. They also start breeding earlier in the year than their sylvan relatives. One of the causes for this is that their biological clocks are advanced by more than a month. In young urban males, the production of luteinizing hormone (which triggers the springtime flood of testosterone into their blood) peaks in mid-March, whereas for forest blackbirds, the call of spring does not reach fever pitch until mid-May. This was discovered by Jesko Partecke of the Max Planck Institute for Ornithology in Seewiesen, near Munich.
What Partecke did was raid ten nests of urban blackbirds in a cemetery in metropolitan Munich and ten rural blackbird nests in a quiet forest outside the city. From these nests, he took baby birds, thirty from each location, and brought them to his lab. There, he did one of those “common garden” experiments. As you may recall from earlier chapters, common garden experiments are a tried and tested way of making sure that differences between organisms are indeed genetic. In the case of blackbirds, the trick is to hand-raise the kidnapped chicks under identical circumstances and see what differences you still find. This way, Partecke could rest assured that the hormonal differences he discovered, at least the ones for young males, were not due to urban light or the urban heat island, or some other external trigger, but were purely the result of a genetically determined jolt to the urban body clock.
A second reason that the city blackbirds breed earlier is that they don’t migrate. They spend the winter in the city, basking in their heat island and leisurely picking food off feeding tables, and can start breeding when they feel like it. The forest blackbirds on the other hand are, by and large, migratory: to escape the cold and the scarcity of food, they spend the winter in the south and only when they return to their home ground can they begin breeding. By that time, the blackbirds in the city are already smugly ensconced in their nests. And, as Partecke found out with those same hand-fed birds, the shift in migratory propensity is also genetic.
Since his birds were captive, he could not actually allow them to migrate away, so he did the next best thing: monitoring Zugunruhe. This German word literally means “migration restlessness” and it is ornithologists’ and bird-keepers’ jargon for the night-time jitteriness that befalls caged birds when their biological clock tells them to migrate, but the iron bars of their cage tell them otherwise. Partecke placed motion sensors in the individual cages where he held his blackbirds. This way, he could detect the signs of the onset of migration as indicated by the blackbirds’ Zugunruhe. Sure enough, in autumn and spring, the forest birds were restless: at night, they moved about in their cage, hopping on and off their perches all the time. The urban birds, on the other hand, were fast asleep at night no matter whether it was migration season or not. Not only that, but the forest birds also built up massive fat reserves to sustain them during their anticipated flight, whereas the urban birds stayed lean. Curiously though, it was the males who showed this difference; the females differed much less between city and forest.
Partecke also discovered more fascinating urban/forest differences with his common garden blackbirds. For example, it turns out that city birds are much more laid-back in nature. This emerged when Partecke gave them the mildly stressful experience of taking them out of their cage and putting them in a cloth bag for an hour; at five time points during that hour, he opened the bag and took a blood sample. Measurements of the stress hormone corticosterone in the blood showed that the forest birds were much more alarmed by this procedure than the urban birds, whose stress hormone levels were raised only half as much as in the forest blackbirds. Let’s remember that these birds had never seen a city or a forest; so urban blackbirds are really more even-tempered by nature. This may also explain why they allow humans to get three times closer to them before getting spooked than forest blackbirds.
One of the causes for this may lie in a gene called SERT, which stands for SERotonin Transporter. SERT is in charge of the removal of the mood-regulating hormone serotonin from the connecting points between nerve cells. That is why many antidepressants work by blocking SERT. As it turns out, forest blackbirds tend to have different versions of the SERT-gene than urban blackbirds.
So, that’s a big pile of differences we find between urban and rural blackbirds: the way they look, their behaviors and personalities, their biological clocks … What does it all mean? In the wrap-up sentences of their scientific papers, the members of the Blackbird Posse exhibit their customary academic carefulness, but I’ll just stick my neck out here and say it out loud: over the past centuries Turdus merula has spawned a new species, Turdus urbanicus, if you will. It’s not quite there yet, just like those two Galápagos finches aren’t quite there yet, but it’s a matter of time for the process
to be completed. It was only waiting for this moment to arise.
Not only does Turdus urbanicus boast a whole catalog of unique features, it’s also puddling about in its own private gene pool. Blackbirds normally nest less than two miles from where they are born, so that already keeps their gene pools separate. And even if a forest blackbird would accidentally wind up in the city, it might be so ill-adapted that it would find it hard to get by. We know this thanks to failed attempts to introduce forest blackbirds into the Polish cities of Białystok and Olsztyn—whereas trials in Lublin and Kiev, this time using urban blackbirds, succeeded. Another reason why the urban genes stay in the city and the rural genes in the forest is that the urban birds start breeding so much earlier, before the forest birds have returned from their wintering quarters.
You can also simply have a peek at those gene pools themselves. That is what Karl Evans did. With a kind of genetic fingerprinting technique he tested the DNA of urban and forest blackbirds in twelve different sites all across Europe and North Africa. In all those places, the urban and forest blackbirds were genetically different, but it was also clear that everywhere, the urban birds had descended from the local forest birds. So there is not yet enough movement of blackbirds between cities (occasionally, blackbirds disperse much farther than those two miles) to homogenize them all into one urban gene pool. This is one of the main reasons why the Blackbird Posse is still a little reluctant to fully embrace the notion of a single, newly evolved Turdus urbanicus.
Still, the work of the Blackbird Posse makes an overwhelming case for the appearance of an urban-adapted evolutionary novelty. And surely it is not unique. We have seen many examples in this book of species that show a whole suite of freshly evolved genetic adaptations to urban conditions. And thanks to the urban heat island, if nothing else, many of those plants and animals will be flowering or mating earlier in the year than their relatives outside of the city. This alone may be enough to start the splitting of the gene pools, and incipient urban speciation.
This also means that evolutionary biologists no longer need to travel to remote places like the Galápagos to discover that holy grail of evolutionary biology: catching speciation in the act. The process is going on right in the very cities where they live and work!
But the reverse is, curiously enough, also true: in the Galápagos it has become possible to do urban evolution research. For today’s Galápagos is no longer the deserted pristine place it was when Darwin first set foot there. Over 26,000 people live there, and the islands are visited by hundreds of thousands of tourists every year. The city of Puerto Ayora, on the island of Santa Cruz, where, if you recall, the Darwin’s finch Geospiza fortis is in the process of speciating itself into two, has a population of 19,000 people, to which an annual deluge of 200,000 tourists is added. It has an airport, a highway (straight as a die), hotels, football pitches, tour agencies (Natural Selection Tours), cafés (OMG! Galápagos) and lots of restaurants.
It is those restaurants that, over the past few decades, the Darwin finches have begun frequenting. Thanks to their tameness (a characteristic shared by many island animals), it is no skin off their famed Darwinian noses to land on tables and feast on the morsels left by diners. And that—oh, the irony!—is beginning to undo the inchoate speciation process. Since the 1970s, the division between large and small-beaked Darwin’s finches has begun to disappear in Puerto Ayora. Researchers such as Luis Fernando De León of the University of Massachusetts in Boston, who is studying those urban Darwin’s finches, think this breakdown is due to their fast-food habits.
De León and his colleagues studied the feeding habits of urban and rural Darwin’s finches and discovered that the ones in the city (where the two beak shapes have merged) eat mostly bread, potato chips, ice-cream cones, rice, and beans. They also drink water from the tap. The ones outside the city (where the finches still show two distinct beak sizes) feed, as they always did, on seeds of wild plants. Moreover, the urban finches also displayed the full checklist of urban bird personality changes, displaying curiosity when De Léon put out trays with unusual food items and being bolder at approaching humans making a “crinkle” sound with a bag of potato chips. The rural finches, on the other hand, showed no interest in humans or their food.
Thus are the ways of urban evolution. It giveth us a new blackbird species in Europe, and it taketh away a Darwin’s finch on the other side of the globe. These and the many other examples in this book should make it clear that urban evolution is seriously reshaping our ecosystems. What does that mean for the future? How can we monitor or even steer this process? What roles could citizen science play? And could we perhaps even harness the power of urban evolution in nature-inclusive architecture and design?
IV.
DARWIN CITY
This natural beauty-hunger is made manifest in the little windowsill gardens of the poor, though perhaps only a geranium slip in a broken cup, as well as in the carefully tended rose and lily gardens of the rich.
JOHN MUIR, The Yosemite (1912)
19
EVOLUTION IN A TELECOUPLED WORLD
One of my favorite cures for writer’s block (or, admittedly, sometimes, an act of procrastination) is my walk around the block. The centuries-old street plan of inner city Leiden, my hometown, is not actually divided so geometrically, so I take a more tortuous route. I descend from my writing den in the attic, and turn right upon exiting my front door and enter the Weddesteeg, the alley where Rembrandt was born. Taking slow and deliberate steps, trying to unfankle my mind, I cross the suspension bridge across the old, dead branch of the Rhine and turn left, and left again to cross the river once more, this time via the railway bridge. The embankment in between the pavement and the railroad is smothered in Japanese knotweed (Fallopia japonica), a relative of the rhubarb and, when it was first introduced into the Netherlands, highly prized for its spikes of white flowers, rich in nectar—large amounts were paid for the cuttings. Today, the knotweed’s reputation has been tarnished by its unbridled spread and the ability of its roots to force apart brickwork and pavement. It is widely, but unsuccessfully, combated as an invasive exotic—in Leiden and nearly everywhere else in Europe, North America, New Zealand, and Australia.
My walk takes me back to the Rhine’s left bank and then onto the Rapenburg, considered the Netherlands’ prettiest canal, flanked by the lavish residences of its erstwhile notables. One of these homes of note is Rapenburg 19, a sixteenth-century colossus with plaster frills around the center doors and windows, dwarfing the more modest, delicately gabled houses on either side. There is a connection between this building and those Japanese knotweed plants that grow a stone’s throw down the street. For this is the house where physician, botanist, ethnographer, and Japanologist Philipp Franz von Siebold settled after he had been kicked out of Japan in 1829.
Von Siebold is an interesting figure. Hugely famous in today’s Japan (where he’s fondly remembered as “Shiboruto-san” and the life story of his Japanese daughter Oine has been turned into a pop-culture manga), he was the only western explorer permitted into Japan during the more than two centuries of sakoku, Japan’s policy to sever most ties with the outside world. Working as a physician from the Dutch trading post on the artificial islet of Dejima, off Nagasaki, he amassed a huge collection of local fauna and especially flora. He also collected ethnographical objects and, a crucial mistake, maps. Those maps were discovered by the authorities, he was accused of espionage, placed under house arrest and eventually forced to return to the Netherlands.
But he did not leave without his entire biological collection of dead and live specimens, which essentially was his pension. Back in Leiden, he managed to retire comfortably by selling off valuable parts of his collection, writing books, running a Japan museum from his home, and starting a mail-order oriental plant business founded on the live specimens he had brought with him. These included one living sprout of Japanese knotweed. It is cuttings from this one plant that kick-started the entire invasive
world-domination of Japanese knotweed since. Those plants I passed on my walk along the railway bridge, a few hundred meters from Von Siebold’s home, are its direct descendants, but so are the vilified knotweeds all the way across the world in New Zealand.
Besides knotweed, Von Siebold was responsible for spreading nearly a hundred other Japanese plant species to gardens and parks all over Europe and beyond. Eventually, some of those began to run wild. The Wisteria, Japanese rose (Rosa rugosa), Hortensia (Hydrangaea macrophylla), and garden privet (Ligustrum ovalifolium), now such familiar sights in urban green spaces everywhere, ultimately derive from Von Siebold’s gardens. Even the Boston ivy (Parthenocissus tricuspidata), after which the “ivy league” is named, those prestigious ivy-clad American universities that house some of the urban biologists featured in this book, is originally a Japanese plant introduced to the rest of the world by Von Siebold.
Philipp Franz von Siebold set the scene. The nearly two centuries that have elapsed since he opened his Leiden-based Far-East flora shop have seen the birth of a million Von Siebolds. Global trade and commerce, garden centers, and pet shops cause the ever-increasing spread of animals and plants from their native homes to urbanized hubs around the globe. And then there is the unwitting transportation of seeds, fungi, microbes, and small animals in the clothes, luggage, shoes, and vehicles of the tourists, migrants, and other travelers who roam ever farther. Not to mention the teleportation of entire ecosystems by empty cargo ships who take in ballast water, soil, or rocks to improve their stability and then dump it near their port of call.
As I am writing this, I am on a two-month stint at Tōhoku University in the city of Sendai in Japan (home of the nutcracker crows). My writer’s-block-laxative walks take me around the university campus and part of the city center. Here—naturally, since they are native plants—I also pass banks of Japanese knotweed, festoons of Wisteria blossoms, bushes of garden privet, and buildings clad in Japanese ivy, just like I do on my jaunts in Leiden. But I also come across familiar European plants that have made the voyage in the opposite direction of Von Siebold’s transportations. In the poorly tended lawns near the Kawauchi subway station I spot shepherd’s purse (Bursa capsella-pastoris) with their funny-looking seed pods, and also white clover (Trifolium repens). In the roadside verges of Jozenji Street grow garden sorrel (Rumex acetosa) and the common broom (Cytisus scoparius). And not only plants, either. Above the city fly rock doves, while buff-tailed bumblebees (Bombus terrestris) buzz their way through the beds of white clover flowers. On wet nights, the European slug Lehmannia valentiana slithers across the walls of the Shintō shrines, as comfortably as if it were at home. In fact, the urban habitats of Leiden and Sendai are much more similar, share many more species, than was the case in Von Siebold’s days.
Darwin Comes to Town Page 19