A For most of the twentieth century, the peregrine falcon was a bird in retreat. Once common across Europe and North America, it had by the early 1960s vanished from vast areas of its former range. In the eastern United States the species was declared locally extinct; in Britain, surveys found that the breeding population had fallen by more than half in a single decade. The cause was not hunting, nor the loss of wild country, but a chemical that farmers had been assured was harmless.

B DDT, an insecticide introduced on a large scale after the Second World War, was remarkably effective against crop pests. It was also remarkably persistent. Rain washed it into rivers, where it was absorbed by small organisms and passed up the food chain, becoming more concentrated at each stage. Peregrines, which hunt other birds and sit at the top of that chain, accumulated the highest doses of all. The chemical did not poison the adults outright. Instead it interfered with the way the female’s body handled calcium, so that the shells of her eggs were formed too thin. Parents returning to the nest to incubate crushed their own clutches under their weight. Whole populations failed to reproduce for years before anyone understood why.

C The turning point came in 1962 with the publication of Rachel Carson’s Silent Spring, which brought the consequences of agricultural chemicals to a general readership for the first time. Restrictions followed — in the United States in 1972, in Britain over a longer and more piecemeal period — and with them came an ambitious programme of captive breeding. Young falcons raised in captivity were placed on artificial ledges and fed by handlers who kept themselves out of sight, so that the birds would not associate food with people. In the United States alone, more than six thousand peregrines were released in this way over three decades.

D What the recovery programmes did not anticipate was where the birds would choose to settle. Peregrines are cliff nesters. They require a high, sheer surface with a wide view, an overhang for shelter, and no easy route for ground predators to climb. A city, it turns out, supplies all of these in abundance. Cathedral towers, power station chimneys, bridge supports and the ledges of office blocks are, from a falcon’s perspective, simply cliffs of unusual regularity. By the 1990s, pairs were nesting in the centres of London, New York, Manchester and Montreal, often within a few hundred metres of thousands of people who had no idea they were there.

E The cities offered more than architecture. Urban pigeon populations, sustained by discarded food, provide a dense and dependable prey supply that does not migrate away in winter. Street lighting extends the hunting day well beyond dusk, and several studies have recorded urban peregrines taking prey at night — behaviour rarely observed in rural birds. City centres are also several degrees warmer than the surrounding countryside, which reduces the energy a bird must spend simply keeping warm. Perhaps most importantly, the traditional persecutors of the species — gamekeepers and egg collectors — are largely absent from the middle of a large city.

F The results can be measured. Urban pairs in Britain fledge, on average, slightly more young per nest than pairs on natural cliffs, and they begin laying earlier in the season. Yet the picture is not uniformly favourable. Fledglings taking their first flights from a tower block frequently come down in streets and car parks, where they are vulnerable to traffic and cannot easily be retrieved by their parents; a study in one German city attributed roughly a third of first-year deaths to collisions with vehicles and glass. Birds nesting on working industrial structures may be disturbed by maintenance, and legal protection is difficult to enforce when a nest sits on private commercial property.

G Conservation practice has adapted accordingly. Purpose-built nesting trays, filled with gravel to imitate a cliff ledge, are now installed on buildings across Europe and North America, often at the request of the building’s owners. Several cathedrals and universities have mounted cameras above these trays and stream the footage online, where nests attract audiences of hundreds of thousands during the breeding season. The educational value is considerable, though ecologists caution that a species which is thriving in the visible heart of a city can create a misleading impression of the health of birds of prey generally. Many of the peregrine’s rural relatives — the hen harrier and the golden eagle among them — remain in serious difficulty, for reasons that have nothing to do with pesticides and everything to do with land use and illegal killing.

H For all that, the peregrine’s return is among the clearest demonstrations that ecological damage is sometimes reversible. The recovery required a scientific explanation, a change in the law, decades of patient fieldwork, and finally a degree of tolerance from the human population of the cities the birds adopted. None of those four elements would have been sufficient alone.