The following is an excerpt from Darwin Comes To Town: Darwin Comes to Town: How the Urban Jungle Drives Evolution by Menno Schilthuizen.

In Henry IV Part 1, Hotspur is planning to drive King Henry crazy by letting a starling endlessly repeat the name of Hotspur’s brother-in-law Mortimer: “Nay, I’ll have a starling shall be taught to speak nothing but ‘Mortimer,’ and give it him, to keep his anger still in motion,” muses Hotspur. In 1877, this obscure Shakespearean reference to Sturnus vulgaris, the European starling, landed the bird a place on the list of animals and plants that were to join the human colonizers in the U.S. For in that year, drug manufacturer Eugene Schieffelin became president of the American Acclimatization Society, a group of idealists who saw it as their calling to “improve” North America by releasing “such foreign varieties of the animal and vegetable kingdom as may be useful or interesting.” And for some unfathomable reason, Schieffelin’s particular brand of acclimatization included the bringing into the U.S.A. of every bird ever mentioned in Shakespeare’s works.

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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 U.S. 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.

[Did you know the the word ‘quark’ is has a special link to a literary classic?]

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 roadside 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 10.8 cm. But as time went on, they found that the living birds’ wings grew shorter, by about 2 mm 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.

Excerpted from DARWIN COMES TO TOWN: How the Urban Jungle Drives Evolution by Menno Schilthuizen. Published by Picador April 3rd 2018. Copyright © 2018 by Menno Schilthuizen. All rights reserved.