By the middle of the twentieth century, the peregrine falcon had all but vanished from the skies of eastern Canada. The cause proved difficult to establish at first, but investigators eventually attributed the collapse to DDT and related pesticides, which accumulated progressively along the food chain and concentrated in the tissues of the birds that fed on contaminated prey. The chemical interfered with the way the females formed their eggs, leaving the shells so exceptionally thin that they fractured beneath the weight of the brooding parent. One clutch after another failed, and within a few decades the breeding population east of the prairies had dwindled almost to nothing. A raptor that had once been widespread and familiar became, across much of the country, a bird that most people would never see.

The recovery that followed owed little to chance and a great deal to deliberate effort. After DDT was banned throughout Canada in the early 1970s, a network of universities and government biologists set about rebuilding the population from the handful of birds still held in captivity. Chicks raised in these programs were placed in sheltered boxes on cliff faces and, more and more often, on the ledges of tall buildings, where they were fed until they were strong enough to fly and hunt on their own. The method, borrowed from the old traditions of falconry, made it possible to release young falcons with no parent present, and over the years thousands were returned to the wild in exactly this manner.