Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions from 19 to 23.

The peregrine falcon, a predatory bird indigenous to North America, was once in danger of extinction. In the 1960s, scientists discovered byproducts of the pesticide DDT in the birds' eggs, which caused them to be too soft to survive. The use of the pesticide had been banned in the United States, but the falcons were eating migratory birds from other places where DDT was still used. In order to increase the survival rate, scientists were raising the birds in laboratories and then releasing them into mountainous areas. This practice achieved only moderate success, however, because many of the birds raised in captivity could not survive in the wild.

There is now, however, a new alternative to releases in the wild. A falcon that has been given the name Scarlett chose to make her home on a ledge of the 33rd floor of a Baltimore, Maryland, office building rather than in the wild, and, to the surprise of the scientists, she has managed to live quite well in the city. Following this example, programs have been initiated that release birds like Scarlett into cities rather than into their natural wild habitat. These urban releases are becoming a common way to strengthen the species. Urban homes have several benefits for the birds that wild spots do not. First, there is an abundance of pigeons and small birds as food sources. The peregrine in the city is also protected from its main predator, the great horned owl. Urban release programs have been very successful in reestablishing the peregrine falcons along the East Coast. Though they are still an endangered species, their numbers increased from about 60 nesting pairs in 1975 to about 700 pairs in 1992. In another decade the species may flourish again, this time without human help.

What is the main topic of the passage?