Nature Notes From Crater Lake - Volume 24, 1993
Crater Lake Institute online library - www.craterlakeinstitute.com
Peregrine Falcons (Falco peregrinus) are crow-sized falcons that are distributed throughout the world. Their diet consists of other birds. Chemicals such as Dichloro Diphenyl Trichloroethane (DDT) tend to accumulate in their system because the peregrine is at the top of the food chain. DDT, which was banned in the United States in 1972, has caused thinning of egg shells and dehydration. The chemical continues to be a problem because the pesticide is still being used in Mexico and South America, where many peregrines or their food sources migrate.
The only known active peregrine eyrie in Oregon in recent years was at Crater Lake National Park. It was discovered in 1979, and remained active until 1983, when both adult birds disappeared. Although the birds in the eyrie successfully fledged young in 1979, they were unsuccessful in 1980. Each of the three eggs laid the second year showed high levels of a derivative of DDT. As a result, young peregrines were fostered into the nest in 1981 and 1982.
When both adult peregrines disappeared in 1983, a method of releasing birds into the wild known as hacking was initiated. Twelve young were hacked over the next four years. Roaming peregrines were seen at the hack site in all four years, and non-breeding peregrines were seen in other areas around Crater Lake. An active pair was present at the historic eyrie in 1986, but successful breeding did not take place.
Nesting again took place at Crater Lake in 1987. The nest was manipulated to ensure that the pair would successfully fledge young. Four eggs were removed from the nest, three of which hatched and were fledged in California. Two captive-bred young were fostered into the Crater Lake eyrie. Unfortunately, one of the young was killed by a great horned owl (Bubo virginianus Gmelin). The other bird successfully fledged.
The peregrines again used the historic eyrie in 1988. They laid four eggs, of which three hatched. Approximately twelve days later all of the young and the adult female were killed by a great horned owl. In order to ensure fledging of peregrines in the Crater Lake area it was decided to cross-foster young peregrines into a nearby prairie falcon nest. This effort was successful in fledging two peregrines that year.
It is believed that the male returned with an immature female in 1989 and 1990, but monitoring during those years was limited. In 1991 the historic eyrie was again active. An adult peregrine pair was successful in fledging three young without any manipulation. Of interest was the male of the pair being identified as a released bird due to the band on its leg. The spring and summer of 1992 was also a successful breeding season for the falcons. Two eggs were produced in the eyrie with both young subsequently being fledged.
The successful fledging of young peregrines during the past two years is very promising and is the result of much effort and patience. The site will again be monitored during 1993. Should the falcons continue to nest at the same eyrie, steps may need to be taken to monitor great horned owls in the area and to evaluate the effects of predation on the peregrines. Although the hatching success in recent years has been good, analysis of the eggs shows significant thinning and that the female was subjected to pesticide contamination.