Peregrine Falcons and Glen Canyon Dam

 

 

Description
Peregrine falcons are medium sized birds - 15 to 21 inches in from the head to the tip of the tail, wingspan 37 to 43 inches. As with most raptors females are larger than males, males weigh about one and a half pounds (660 grams), females two and a half pounds (1130 grams). Peregrines are beautiful birds, overall appearing blue or grayish blue in color. The head has a dark cap, face white with a vertical black stripe under each eye. Undersides are pale, with the back of the wings and tail predominately slate blue. Peregrines are carnivores, living almost exclusively on prey struck in the air, that is; birds and bats. With its reliance on flying animals as food the peregrine would seem to be something of a specialist. However the range of size in prey species is very large. They take quarry ranging in weight from 10 grams to 2000 grams. This range includes most species of birds and many species of bats.

Occurrence
One of the most widely distributed birds in the world, peregrine falcons breed on every continent except Antarctica. Nineteen recognized subspecies exist worldwide, including the North American Peregrine, Falco peregrinus anatum. They are nowhere an abundant species. In the western United States breeding density of the peregrine is thought to have historically been very low. No comprehensive surveys of peregrines were done prior to the DDT era, but records of bird observations since the early 1900's indicate that peregrines were rare in the intermountain region. In 1946 Richard M. Bond reported in "The Condor" that breeding density between the Rockies and the Sierra Nevada Range was "extremely low, and this surely is not due to lack of knowledge". Densities in the region were thought by Bond to be 1 pair per 20,000 square miles. No breeding pairs had been reported in the Grand Canyon, though an adult carrying prey had been seen in 1932.

Breeding habitat requirements
Peregrine breeding habitat throughout the world has been found to share three characteristics:
1. High, near vertical cliffs. Peregrines do not build nests, but lay their eggs on existing flat ledges in cliffs. Ledges in the upper third of cliffs seem to be preferred
2. A nearby source of water, most eyries (nest sites) are within sight of water.
3. An adequate prey base. Breeding pairs establish and defend territories. The size of these territories appears to be directly proportional to abundance of prey. In areas where prey species are abundant, nesting pairs will tolerate closer proximity to each other.

The population crash - DDT
After World War II an effective new pesticide came into use. DDT (dichloro diphenyl trichloroethane) was widely used in agriculture and pest control. In the eastern U.S. millions of acres were sprayed by the U.S. Department of Agriculture in an attempt to control gypsy moths and fire ants. Widespread deaths of songbirds resulted, as reported by Rachel Carson in her groundbreaking book - "Silent Spring".

DDT also had a more subtle impact. Peregrine falcons were laying eggs with abnormally thin shells, causing shell breakage and nesting failure. Peregrine populations in North America and Europe declined sharply. In the eastern United States peregrines simply ceased to exist. The eastern American peregrine, the "Duck Hawk" of American history, is gone forever.

The connection between DDT, nesting failure and thin-shelled eggs was first suspected by Derek A. Ratcliffe in England. In 1963 he published a paper, "The Status of the Peregrine in Great Britain", reporting eyrie desertion and suspecting insecticides as the cause. Publication of the paper raised interest in the problem. In the United States J.J. Hickey initiated a survey of historical eyries, funded by Kathleen Herbert. Of 133 known eyries all were abandoned. Subsequent research and experimentation proved DDT to be the causative agent in egg shell thinning in peregrines, bald eagles and other birds. Further research found DDT and it's byproduct DDE to be widespread environmental pollutants with far-reaching effects on living organisms. DDT use was restricted by law in Canada in 1970. In the United States the Environmental Protection Agency's administrator William Ruckelshaus banned the use of DDT for almost all purposes in 1972. With the passage of the Endangered Species Act in 1973, the American peregrine falcon (Falco peregrinus anatum) was listed as endangered.

Captive breeding and reintroduction
Concern over peregrines and DDT in the environment led to the proposal of an international conference. J.J. Hickey sought funding from the U.S. Departments of Agriculture, Interior, and Health, Education, and Welfare. Only the National Institute of Health contributed; 10,000 dollars. The National Audubon Society board of directors approved a motion by Roger Tory Peterson to donate 8,000 dollars, providing the additional funds necessary. The first international Peregrine Conference was held at Madison, Wisconsin in 1965.

A group of attending falconers and biologists met after the conference to discuss the (then) audacious concept of breeding peregrines in captivity for release into the wild, with the goal of preventing the possible extinction of the species. At the time captive breeding of birds of prey was considered by most experts to be impossible, although it had been accomplished by a few people on a limited scale. From 1967 on several individuals in North America began having success breeding peregrines. In 1970 the Peregrine Fund was founded by Dr. Tom J. Cade, a Professor of Ornithology at Cornell University. The initial purpose of the Peregrine Fund was to learn to breed and successfully release falcons into the wild. In 1973 the Peregrine Fund produced 20 young at its facility in Ithaca, New York. About 200 peregrines were being produced each year by 1975, half by private individuals. In 1984 the Peregrine Fund moved its facilities to Boise, Idaho, where it is today. The Peregrine Fund was responsible for the release of more than 4000 peregrines into the wild.

The ban on DDT made it possible for both surviving wild populations and birds released from captive breeding programs to produce viable young and increase their numbers. There are now at least 1,650 peregrine breeding pairs in the United States and Canada, well above the recovery goal of 631 pairs. On August 20, 1999 the anatum peregrine falcon was removed from the List of Endangered and Threatened Wildlife.

The Anatum
Releases in the western United States were confined to the endangered "anatum' subspecies, since some wild peregrines remained. The disjunct eastern population had been extirpated, so captive bred birds from other subspecies, and hybrids of subspecies, could be released. As a result the gene pool in the eastern group are not anatums, leaving western North America as the only place where the North American peregrine (Falco peregrinus anatum) exists in the world.

Releases did not occur in Arizona and southern Utah. The Peregrine Fund has stated that "In the Rocky Mountains, California and parts of the Pacific Northwest the incredible repopulation can be attributed to the release program, while populations in Arizona and southern Utah recovered naturally". The two states with the largest populations of breeding pairs of anatum peregrines are Arizona and Utah. The highest densities of breeding pairs in this region occur in the habitat influenced by Glen Canyon Dam.

Glen Canyon Dam
The Colorado River had carried an enormous sediment load, similar to the present Little Colorado in flood, resembling something closer to a mud flow than a river. A series of dams formed by lava flows into the Colorado River corridor over the last 1.8 million years have had the effect of settling out this sediment. Remains of sediment deposited in the lakes formed by these lava dams have been found in the Grand Canyon and along the shores of Lake Powell. Glen Canyon Dam has the same effect, when it started backing up the water in 1963 sediment began dropping out in the new lake. The clear water released by the dam now allows sunlight to reach the river bottom. The alga Cladophora glomerata now flourishes in the river, nourishing insect larvae that emerge from the river and providing food for an increased number of birds and bats. With the elimination of annual flooding vegetation along the river increased significantly, providing habitat and food for insects and birds. Waterfowl have also increased, feeding on the organisms thriving by the greatly increased aquatic food production. This combination of factors has resulted in more food for peregrines. 71 pairs of peregrines had been found nesting in Grand Canyon National Park by 1989.

The lake created by Glen Canyon Dam, Lake Powell, lies within Glen Canyon National Recreation Area. Bird records for Glen Canyon National Recreation Area record over 270 species, and large numbers of bats occur above the lake. Of the birds in the recreation area, (ignoring records of accidental or vagrant species) over 190 species fall into the range recorded to have been taken by peregrines, weighing from 10 grams to 1750 grams. 78 known peregrine breeding pairs had been located in GCNRA by 1996. Eyries are spaced as close as one mile apart, remarkable for this species. The study area did not include all of the suitable habitat inside the Recreation Area, Park Service personnel suspect that the total number probably exceeds 100 pairs. This population is commonly referred to as the Lake Powell peregrines. Nest productivity of the Lake Powell peregrines is very good. The U.S. Fish and Wildlife Service set a recovery goal for this species at 1.25 young fledged per active nest, the average for the Lake Powell peregrines for 1988 to1996 was 1.6 young per active nest. (Intensive surveys ceased after 1996) The U.S. Fish and Wildlife Service recovery plan also required a minimum goal of 46 breeding pairs in Arizona and 21 in Utah. With 78+ breeding pairs producing an average of 1.6 young, the Lake Powell peregrines alone exceed the recovery goal for the species for both Utah and Arizona.

Anatum peregrine falcons breeding in the area influenced by Glen Canyon Dam have the highest density in the contiguous United States, and among the highest in the world. The area is the stronghold of the anatum peregrine.

 

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