Disturbance At Eyries
Much as described for Prairie Falcon (Steenhof 1998). Pairs vary greatly in responsiveness to human activities, depending partly on individual characteristics, partly on period of breeding cycle, and partly on environmental circumstances (Cade 1960). Pairs in remote locations most reactive; those in urban areas or frequently visited sites become habituated to close human activities. In past, some historical eyries were abandoned because of human encroachments or increased levels of nearby activity (Hickey Hickey 1942, Hickey 1969; Bond 1946), but not a major factor in population change. Also, temporarily abandoned eyries sometimes reoccupied after disturbance ceased (Bond 1946, Herbert and Herbert 1965), in some cases after many decades (Ratcliffe 1993). Rock-climbing and activity of researchers at eyries not usually detrimental when reasonable precautions taken (Olsen and Olsen 1978, Cade et al. 1996b; see working bib-liography of Porter et al. 1987, for >80 references to effects of disturbances).
Shooting, Trapping, And Egg-Collecting
Before legal protection, these activities accounted for loss of many hundreds of falcons and their eggs. Often shot at famous migration passes (e.g., Fisher's I., NY; Hawk Mountain, PA; Cape May, NJ; Whitefish Point, MI); however, systematic persecution as practiced by European gamekeepers was rare, occasionally done by pigeon fanciers (Bent 1938b). Nonetheless, rein-troduced urban population in greater Los Angeles, CA, area in 1990s received considerable pressure from pigeon fanciers with 14 shot, adults and young, at 1 territory over 4 yr, but with no loss of eyrie occupancy (B. Walton pers. comm.). Small numbers of nestlings/fledglings and first-year migrants also taken by falconers in period 1920s through 1960s before protection under Endangered Species Act in 1970 and Migratory Bird Treaty Act in 1972, and afterward, in case of F. p. pealei . No evidence that losses from these activities depressed number of occupied nesting territories, except in isolated, local circumstances. Reputed cases of impact from taking nestlings include: (1) lower Hudson River, NY (Herbert and Herbert 1965); (2) 40-km stretch of river in Alberta (Dekker 1967); and (3) Langara I., British Columbia (Dekker Dekker 1969, Dekker 1972), but overriding effects of pesticides and/or reduced food supply were operative in these places at same time.
Pesticides And Other Contaminants/Toxins
From late 1940s to early 1970s, massive, continentwide-indeed, nearly global-use of organochlorine pesticides, particularly DDT and HEOD (dieldrin, aldrin) in agriculture and forestry and for human disease control resulted in bioaccumulation of toxic residues in prey species, which in turn contaminated falcons, causing both lethal and sublethal effects (see review papers in Cade et al. 1988). Few fatalities documented in North America (Reichel et al. 1974, Peakall et al. 1990), but HEOD considered important cause of death among British Peregrines in 1950s-1960s and in some other parts of Europe (Ratcliffe 1993); circumstantially implicated as lethal factor in North America (Nisbet 1988). Most significant sublethal effect of DDE (persistent environmental residue of DDT) was repro-ductive malfunction resulting from abnormally thin eggshells. First identified in Great Britain (Ratcliffe 1967), this thin-eggshell syndrome was quickly confirmed as widespread condition of North American Peregrines, coincident with North American use of DDT beginning in late 1940s (Hickey and Anderson 1968; Cade et al. Cade et al. 1971, Cade et al. 1988; Court et al. 1990; Johnstone et al. 1996). Direct correlation exists between concentration of DDE residues in egg contents (reflects circulating levels in laying female) and eggshell thickness, r- values ranging from -0.57 to -0.75 in different geographic samples (Cade et al. 1971, Peakall and Kiff 1988, Newton et al. 1989, Court et al. 1990).
Peregrine population declines in 1950s-1970s associated with population averages for eggshell-thinning of >18% and residue levels of 15-20 ppm wet weight DDE in egg contents (Hickey and Anderson 1968, Peakall et al. 1975), and with associated reduction in breeding success (reviewed by Fyfe et al. 1988).
In arctic Canada from 1981 to 1985, eggs from failed nests had shells averaging 20.2% thinner than normal, while eggs from successful nests averaged 16.1% thinner (Court et al. 1990). Affected populations showing these thinning correlations in North America include e. U.S. (New Jersey, Massachusetts), n. Alaska, interior Alaska, Ungava, n. Quebec, Colorado and Rocky Mtns., and California, as well as others world-wide (Peakall and Kiff 1988). Subsequent population recovery in all these regions following 1980s associated with reduced pesticide residues in prey and in eggs and body tissues of Peregrines, increased eggshell thickness, and increased productivity (Cade et al. 1988, Peakall et al. 1990, Enderson et al. 1995a, Mesta 1999).
PCBs, mercury, and lead are other environmental contaminants often implicated in bird morbidities and fatalities, but none known to have exerted population effects on Peregrines in North America (Dement et al. 1986, Stone and Okoniewski 1988, Peakall et al. 1990). Peregrines occasionally killed by eating birds poisoned by strychnine or other persistent toxic chemicals (see Porter et al. 1987 for specific references).
Collisions With Stationary/Moving Structures Or Objects
Urban-dwelling Peregrines killed or injured by flying into windows or other features of buildings while chasing prey, occasionally by collision with moving vehicles, including aircraft at airports; sometimes strike wires; recently fledged young sometimes fall down chimneys or are killed by air-conditioning equipment or other machinery on tops of buildings; young in nests on bridges often fall into water, significantly reducing productivity at such sites (Barclay and Cade 1983, Cade and Bird 1990, Frank 1994, Bell et al. 1996). In California, electrocutions and wire strikes common in nonurban areas, as well (B. Walton pers. comm.). For other accidental injuries and fatal-ities, see Porter et al. 1987 .
Degradation Of Habitat
Difficult to assess impact on Peregrine because species is so catholic in use of wide range of habitats and landscapes, including those highly modified by humans. Most affected by loss or modification of nesting places, which are limited in number and often nonreplaceable (e.g., cliffs, ledges, special trees, towers, buildings; see Steenhof 1998 for parallel with Prairie Falcon), but has latitude to switch among alternate nesting places in same territory where >1 exists (e.g., from building to stone quarry in Salt Lake City, UT). Unlike Prairie Falcon (Steenhof 1998), agricultural practices not usually detrimental to Peregrine, except use of pesticides, as species readily preys on birds attracted to cultivated landscapes, and owing to hunting style no doubt benefited from conversion of closed-canopy forests to agriculture, as occurred in ne. U.S. in 1700s and 1800s. Migratory and wintering Peregrines, and some breeders, favor wetland areas that support concentrations of waterfowl and shorebirds; loss or degradation of these habitats is no doubt detrimental to the species. Systemic changes in ecosystem functions (e.g., impact of warmer water temperature on marine food web and consequent reduction in plankton and associated seabirds that serve as food for Peregrines in Queen Charlotte Is.; Nelson and Myres 1976) can exert major impact on Peregrine populations (see J. Thiollay 1988 and Ratcliffe 1993 for other examples).
Deleterious Impacts Of Research
Careless methods of field study can result in injury or death of adults and young, or abandonment of eggs (no known case of abandoning young), but such impacts not known to have measurable effect on long-term population stability. Inspection of nest site just before or during laying likely to cause falcons to abandon that site and renest elsewhere on same cliff or in same territory. Sudden appearance of human, or helicopter, near nest can frighten sitting bird to leave so quickly that eggs or recently hatched young may be kicked out of the nest (Cade 1960). Prolonged investigations that keep parents off eggs or downy young for varying periods, depending on climate and age of young, can cause overchilling or overheating and death. Attempts to band young >4-5 wk old can result in premature departure from nest, increasing their vulnerability to predators (Cade et al. 1996b). Trapping adults or flying juveniles sometimes results in injuries; placing identification markers or telemetry transmitters on falcons can reduce survivability. Need for information derived from disturbing procedures must be balanced against risks of injury or fatality resulting from study methods. Fyfe and Olendorff (Fyfe and Olendorff 1976) and Olsen and Olsen (Olsen and Olsen 1978) provide guidelines for minimizing impacts of human disturbance at eyries (see also Cade et al. 1996b).