These fascinating observations languished in relatively obscure Japanese scientific papers until 1997. That year, the BBC came to Sendai to film the crows for David Attenborough’s series The Life of Birds. Sir David’s voice-over made them an instant hit: “They station themselves at pedestrian crossings … Wait for the lights to stop the traffic. Then, collect your cracked nut in safety!”
So, finding ourselves in this city with its famous urban crows, my merry band devoted today to view them for ourselves. Minoru and Yawara tell us that the crows’ trick is well known in town. In fact, it is a favorite pastime to throw the crows nuts and watch them perform. So, with a bag of walnuts brought all the way from the Netherlands, we try our luck. But the crows are not cooperating. We have already spent the whole morning at traffic lights at intersections, stupidly waiting on canvas folding chairs at the mercy of the surprised stares of endless motorists, but so far, in vain. And we have now ended up at the reputed epicenter, the Kadan driving school. It is getting hot and we’re hungry and tired. With glazed-over eyes, we stare at the heaps of nuts we have laid out at various positions on the school’s test range. The school’s students carefully avoid them, and the crows fly over without even looking down. This is what urban fieldwork is like.
Perhaps, Minoru and Yawara finally admit, it is too early in the year. The nuts are not ripe yet, the young birds have just fledged, and groups of crows are marauding the city to feast on other things, like ripe mulberries that are in abundance everywhere. I sigh and stare a bit more. Then, I hear a cracking noise behind me. I turn around to see that Iva has begun eating our stock of walnuts. She looks at me defiantly. “What? They’re not going to come anyway!”
Carrion crows do not only occur in Japan. They also exist in western Europe, where you can similarly find plenty of cars, pedestrian crossings, and walnuts. And yet, carrion crows in Europe somehow never learned to exploit human automobile traffic in the Rube-Goldbergian way that they do in Japan. That is not to say that humans in Europe are safe from having their behavior manipulated by birds, as demonstrated to us for nearly a century by the famous (and annoying) milk bottle top-opening skills of tits—lively songbirds with a handsome pattern of yellow, black, and blue (the blue tit, Cyanistes caeruleus) and olive-green (the great tit, Parus major).
Tits—in fact, all birds—cannot digest milk. Unlike mammals, they lack the enzymes needed to break down the lactose. But the layer of cream that collects at the top of old-fashioned, unhomogenized milk contains very little lactose, and a hungry bird in winter could do worse than supplement its fat intake with a bit of rich cream snatched from the neck of a milk bottle. And that is exactly what tits had been doing for a while in the late nineteenth and early twentieth century in England and elsewhere in Europe, when milkmen were still in the habit of leaving open bottles of milk on people’s doorsteps in the morning. Before the resident mammal would have time to open the door and bring the bottles into safety, a tit would swoop in, land on the neck of a bottle and dip its beak in the cream inside, consuming up to an inch of the coveted food.
Unfortunately, the very first stages of the ensuing game of attrition between human and bird are lost in the mists of time. Presumably, it was a matter of racing to the front door as soon as the milkman’s cart was spotted, not to give the tits a chance to steal any cream. Tits, not be outdone, would be hanging out near people’s doorsteps at milk delivery time to still try to get there first. In any case, at some point in the early twentieth century, milk suppliers began closing their bottles with wax-board tops. This only gave momentary respite because in 1921, in Southampton, tits began prizing these off, or stripping away the cardboard layer by layer until the cap was thin enough to be pierced by the bird’s sharp beak. Changing the cardboard caps to aluminum ones did not help for long either: by 1930, tits in ten different towns all over England had learned how to open the metal-topped bottles. When faced with a metal cap, they would hammer a hole in and then pull away the foil in strips. They might also pull off the entire cap and fly away, holding it in one claw and, in a secluded spot, peck at the cream sticking on the inside of the cap. Under the birds’ favorite tree, the cleaned and discarded caps would gradually pile up to a respectable refuse heap. But sometimes their greed was their downfall. More than once, say Robert Hinde and James Fisher, two English ornithologists who studied the behavior, blue tits would be found “drowned head first in the bottle, presumably because they tried to drink too deep and lost their balance.”
Hinde and Fisher learned all this because, in a citizen science project avant la lettre, in 1947 they mailed hundreds of questionnaires to bird-watchers, naturalists, milkmen and milk-consuming homeowners, but also to doctors and other “people with a scientific training.” Using the replies they received, they were able to piece together a detailed history of the epidemic-like spread of milk bottle attack skills among tits, and the human counter-measures, all over the British Isles and, in a Europe-wide follow-up questionnaire, also on the Continent.
In an article in British Birds, they published snippets of some of the responses they got, and these betray the depth of the frustration felt by humans in this battle of wits with their mouse-sized opponents. People were exasperated at how quickly the tits were at their milk bottles, often within minutes of the milkman placing them there. As if the birds were waiting for it! (They probably were, since one milkman complained that some tits did not even wait for him to deliver the bottles to a home, but rather raided his cart while he was out placing bottles on somebody’s doorstep. And then as he ran back to his cart, other tits would befall on the bottles just delivered.) In one particularly wholesale onslaught, a gang of tits managed to open 57 out of 300 bottles left at a school before the schoolmaster had a chance to chase them off. In some areas, people would provide their milkmen with heavy metal lids, rocks, or cloths to leave on top of the bottles, but invariably the tits would learn how to remove those as well.
The maps that Hinde and Fisher published in their article show how the birds’ bottle-opening skills spread. Intriguingly, they did not spread out gradually from the source of the invention in Southampton. Rather, tits attacking bottles seemed to pop up independently in many towns and cities and then from there the behavior would catch on locally. Individual tits rarely move more than six to twelve miles in a year, and yet, new towns more than twelve miles away from the nearest affected town would suddenly also be afflicted by cream-hungry tits. So, it is more likely that the behavior was invented independently by multiple, particularly clever birds that then were imitated by others. For example, in the Welsh town of Llanelly, hundreds of miles away from the nearest knowledgeable tit, only one house in a neighborhood of 300 houses suffered from a single thieving tit in 1939. Seven years later, all the tits in this neighborhood were doing it. And in Amsterdam, Niko Tinbergen saw great tits opening milk bottles both before and after the Second World War, even though during the war, and in the lean years immediately after, milk was not delivered and no pre-war tits would have survived to 1947, when milkmen gradually began doing their rounds again.
Over the past few decades, the tits seem to have finally been defeated by their milk-guzzling human adversaries. First, skimmed and homogenized milk, which lack the cream on top, became more popular. For a while, the tits managed to circumvent these by learning the cap color denoting old-fashioned fatty milk. But since then, aluminum-capped glass milk bottles have slowly been replaced by other containers, and the traveling milkmen themselves have all but disappeared, in favor of the supermarket. Today, very few homeowners still know the infuriating feeling of finding their milk vandalized by neighborhood birds.
This ongoing battle between birds and bottles keeps inspiring urban biologists, because many mysteries remain. How did the bottle opening skills pass from bird to bird? Are city birds perhaps better or quicker at learning such new tricks or acquiring new tastes than rural birds? And, if so, why?
The answer to that first question—how a new trick learne
d by one clever bird is then passed on to others—was recently revealed by an Australian researcher, Lucy Aplin, working at Oxford University. Aplin’s research takes place in Wytham Woods, that same forest near Oxford where Bernard Kettlewell lived in a trailer and collected peppered moths in Chapter 8. But these days, researchers use much more snazzy equipment than Kettlewell’s muslin gauze sleeves. Aplin set up automated computerized “puzzle boxes” all over the forest. A puzzle box is the biologist’s devious way to assess an animal’s problem-solving skills. It usually is a contraption that requires a certain set of actions before it releases a reward in the form of a tasty morsel of food. In the case of Aplin’s tits, the puzzle box consisted of a plastic chest with a stick to perch on. Right by that perch was a door that a bird could slide either to the right or to the left by pushing it with its beak. Behind it, it would find a dish with delicious live mealworms.
And that was not all. Being next door to a dense cluster of restless biologists, the great tits of Wytham Woods are an intensively scrutinized group of birds. Each, for example, has been fitted with a minuscule transponder chip in a leg ring. By placing antennae at nest boxes and feeding tables, researchers can keep track of the personal history of each bird: how old it is, with whom it had built a nest, but also: who are its friends, with which other tits does it like to hang out? These individual identification codes were picked up by an antenna hidden in the perch on Aplin’s puzzle box each time a bird landed. Switches in the plastic door would register if the bird managed to open it, and—a crucial point, as we shall see—it detected the manner employed: by pushing it left or by pushing it right.
Wytham Woods, at least as far as its great tits are concerned, is divided into eight sections, each harboring about a hundred tits, which interact more with one another than with birds from the other sections. The Oxford tit-watchers call those sections “sub-populations.” In each of five sub-populations, Aplin caught two male birds that were given the honor of being the puzzle boxes’ early adopters: she taught these ten birds how to open the puzzle box by letting them watch captive birds that already knew the trick. Some she trained to open the door to the right, others to the left, and she made sure that the two birds from the same sub-population learned the same version of the puzzle (either both pushing left or both pushing right). The enlightened birds were then released back into their native sub-population to proselytize the puzzle-box gospel, while Aplin set up a battery of puzzle boxes, stocked with mealworms, all over the forest.
For four weeks, the circuits of switches, antennas, and digital hardware in the puzzle boxes whirred away, constantly recording the comings and goings of birds and the left- or rightward sliding of the door. When the feast was over, Aplin packed up her puzzle boxes, downloaded all the accumulated data and began analyzing. She discovered that the majority of the tits, in the five sub-populations where she had released the puzzle-box-savvy birds, had learned how to open the door. But in the sub-populations without any “trainer,” only very few birds figured out how to deal with the boxes—less than 10 percent in one case.
It was also clear that this knowledge passed through the sub-population via networks of friends: the best pals of the ones who had been trained were the first to pick up and then pass on the new knowledge. Since the set-up recorded the exact moment at which each individual bird learned the trick, Aplin could actually watch the meme spread throughout the birds’ social network until nearly everyone knew about it. And this is where the two alternative right-pushing and left-pushing door-opening solutions came in handy: in each sub-population, a tradition of door-opening took hold that depended on how the original trainers had been trained. If they had learned to slide the door to the right, that was how all tits in their sub-population eventually did it, and vice versa. Even a year later, Aplin found, this local box-opening custom was still there.
What the great tits of Britain show is that some animals can learn how to crack human code and then let their best friends in on the secret—at least until humans come up with a countermeasure. That’s how humans and city-dwelling animals are constantly at loggerheads, but for such information to be learned and passed on among animals, certain faculties are essential. First, the animals need to have a kind of problem-solving intelligence; the kind that helped blue and great tits understand that breaching an aluminum top on a bottle would give access to delightful cream underneath. Secondly, they need to have neophilia—an attraction to unknown objects; when the first glass milk bottles arrived, some tits were not freaked out by them, but instead began exploring them for possible nutritional benefits. And, finally, they need to be tolerant of angry milkmen, tea-cloth-wielding homeowners, and close proximity of people in general.
Clearly, the tits that successfully attacked milk bottles, or Lucy Aplin’s puzzle boxes, benefited from the fact that they were tolerant, problem-solving neophiles. But this is not always the case. Under more natural circumstances, it is often safer to be shy, conservative, and neophobic. In an environment that has been stable for a long time, humans and other large animals are better avoided since they can be dangerous—objects made by humans tend to have lethal moving parts, so better be safe than sorry.
But in cities, such traditional behavioral adagios might need to be reconsidered. Humans bring in their wake a superabundance of food, they create shelter and nesting sites, and generally offer new opportunities. Moreover, at least in cities, humans tend to be favorably disposed toward most small birds and mammals and not likely to harm them (though their pets might). Finally, humans are forever creating new stuff. Sometimes, like the McFlurry ice-cream cups hedgehogs get their spiny heads stuck into, these new objects are dangerous, but often (think milk bottles) the advantages outweigh the hazards. In other words, we might expect that city animals evolve to become better at exploiting their human neighbors. Not because some gene for opening bottle tops spreads in the population (surely no such gene exists), but because genetic tendencies to be tolerant and more inquisitive (and such genes do exist) will help an animal to quickly learn how to take advantage of humans and their ever changing ways. By enabling quicker learning, such genes will spread—and the species will evolve in the city to be a more street-smart version of its former rural, stuffy self.
There’s actually evidence that this is the case—that city animals are fearless problem-solvers with a penchant for anything new. Some of this evidence comes from the island-state of Barbados, where McGill University in Montréal, Canada, owns a field center. Located on the fringe of the city of Bridgetown, generations of McGill staff and students have done their field teaching and graduate research projects there. The field center has a perfectly fine canteen, but hey! it’s the sandy, sunny Caribbean coast, and the lavish Colony Club is right next door—so quite a few hours of pre- and after-fieldwork lounging are actually spent there. It was at the Colony Club’s immaculately decked-out tables that in the year 2000, a few McGill biologists first noticed how cheeky Barbados bullfinches (Loxigilla barbadensis) deftly opened the paper sachets of sugar that had been intended for human consumption. Pretty much like the British blue tits on milk bottles, a bullfinch would hold a sachet with one claw and use its heavy beak to pierce the paper wrapper and gobble up a few mouthfuls of sugar before flying off. Later, the bullfinches were seen mastering other restaurant table manners, like opening sugar bowls (by turning over the heavy ceramic lids with their beaks) or stealing coffee creamer. “When you sit at a terrace in Barbados, it’s almost guaranteed that you will share your table with bullfinches,” says graduate student Jean-Nicolas Audet.
For Audet and post-doc Simon Ducatez, studying these bullfinch behaviors provided the badly needed justification for spending extended periods at the restaurant tables of the Colony Club. Eventually they even managed to persuade their supervisor to let them conduct a part of their “field” work at the Colony Club. And also at the nearby Coral Reef Club. And the sumptuous Royal Pavilion. But Barbados is not all city and coastal resorts. Although dense
ly populated (on average nearly 700 people per square kilometer) and heavily urbanized, the northeast corner of the island is still rural. So Audet figured it might be interesting to see if the problem-solving skills of rural bullfinches matched those of the urban ones or not.
To study this, he devised two types of puzzle box. Both were made of transparent plastic and contained seeds as a reward, but one (the “drawer” box) could be opened either by tugging on a drawer or by pulling off a lid, while the other type (the “tunnel” box) required both actions: first tugging, then pulling. Audet caught twenty-six urban bullfinches and twenty-seven rural ones and, in the field center, tested if (and, if so, how quickly) they figured out the puzzle boxes. As it turned out, all birds managed to do the drawer box, but the urban birds were twice as quick at it than the rural ones. The more complicated tunnel box was solved by only thirteen of the urban bullfinches—but the rural ones did even worse: only seven of those figured it out, and it took them on average nearly three times as long as the city birds. Clearly, urban bullfinches will be better at coming up with new ways of accessing human-provided food. Whether the urban finches actually carry different genes for problem-solving skills from the rural ones is up for debate. Audet suspects the island might be too small for that and the birds too peripatetic. Then again, if the benefits are great enough, natural selection can still go against the stream, as it were, and slowly build up genetic differences.
Problem-solving was key trait number 1. But for an animal to even approach a problem to be solved, it needs to be less than circumspect about new and unfamiliar objects in its environment. Preferably, it needs to be neophilic: keen to approach and investigate anything that is out of the ordinary. In other words, it needs to be curious.
Darwin Comes to Town Page 15