In the middle of the 1900s the pesticide DDT made peregrine falcon eggshells thin, and the eggs broke while the parents sat on them. The cause was in the shell itself: DDE, the main breakdown product of DDT, prevents normal calcium deposition during eggshell formation, resulting in thin-shelled eggs that are susceptible to breakage during incubation. Populations laying eggs with shells that averaged more than 17 percent thinner than before DDT had such high rates of reproductive failure that the number of peregrine pairs declined. West of the 100th meridian, only 33 percent of historical peregrine nest sites in the Rocky Mountains were still occupied by 1965.
In 1970 there were 39 known pairs of peregrine falcons in the lower 48 states. The U.S. Fish and Wildlife Service listed the peregrine falcon as endangered in 1970, when the population in the eastern United States had completely disappeared and western populations had declined by as much as 80 to 90 percent. The United States banned the use of DDT in 1972, and by 1975 the population reached an all-time low of 324 nesting pairs in North America. The Peregrine Fund's first successful hatches in captivity came in 1973, with 20 young produced, and the first release was in 1974. According to its founder, it was not until 1980 that released falcons returned and successfully fledged young.
In southwest Canada, the northern Rocky Mountain States and the Pacific Coast States, populations were greatly depressed or gone, and over 3,400 young American peregrine falcons were released there. In the southwestern U.S. very few captive-bred birds were released and populations recovered naturally. By August 1999 The Peregrine Fund and others had released over 4,000 captive-raised peregrine falcons in 28 states, after 28 years of effort. In the five eastern recovery units, 193 pairs were counted in 1998, and the number of territorial pairs there had risen an average of 10 percent a year from 1992 to 1998. When the species was delisted in August 1999, the Fish and Wildlife Service said there were at least 1,650 peregrine breeding pairs in the United States and Canada, well above the overall recovery goal of 631 pairs.
The bird that came back lives across much of the world. Peregrine falcons live on every continent except Antarctica, and they like open country. Many peregrine falcons migrate, but some stay in one place all year. Peregrines that nest on Arctic tundra and winter in South America can fly more than 15,000 miles in a year. In a satellite-tracking study of 61 peregrines, the birds migrated an average of 8,624 km southward and 8,247 km northward. Home ranges of peregrine falcons have been estimated from 177 to 1508 square kilometers, and males and females regularly hunt up to 5 km from their nest site or territory.
Words to know
- DDE: DDT breakdown product
- calcium: shell mineral
- incubation: egg sitting
- fledged: learned to fly
- delisted: taken off list
