Peregrine Falcon

Falco peregrinus



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Figure 2. Annual cycle of breeding, migration, and molt in the Peregrine Falcon.

Note the extreme difference between timing in a northern migrant and a resident population. Thick line shows peak activity; thin line, off-peak.

Peregrine Falcon clutch, collected Battle Creek, Saskatchewan, May 1912

From collections of the Field Museum of Natural History, Chicago, IL. Photo by P. Lowther.; photographer Peter Lowther


Pair Formation

Related somewhat to latitude, but pairs at northernmost resident localities remain at eyries, even sitting side by side on ledge in Jan, particularly in locations where sufficient prey occurs (e.g., Aleutian Is.; S. K. Sherrod pers. comm., CMW). Banded birds (usually female) may remain at eyries in some urban areas (e.g., Milwaukee; G. Septon pers. comm.) in midwinter. Courtship and copulation seen in Sierra Madre Oriental, Mexico (where apparently resident) in late Feb (Lanning et al. 1977). Peregrines arrive at nest sites on lower Yukon River, AK, by 15 Apr (R. Ambrose pers. comm.) and Rankin Inlet, Nunavut, by 10 May where both sexes arrived simultaneously (Court et al. 1988b).


No nest built per se. Scraping in substrate begins early in courtship and continues until egg-laying; depending on latitude, 2 wk–2 mo (Nelson 1970b).

First/Only Brood Per Season

Figure 2 . No record of >1 brood fledged/yr in North America. Because of wide geographic distribution, large range in onset of laying. Latest in Arctic, earliest at southern latitudes; may vary at same latitude depending on conditions (i.e., maritime vs. inland, elevation). In Maryland, earliest record 12 Feb, but usually 25 Mar to 2 Apr (Wimsatt 1940). In s. California, first egg mid- to late Feb; n. California first egg usually in May but replacement clutches as late as Sep (B. Walton pers. comm.). Some dates extrapolated from fledging of young (based on average incubation period of 33 d, average nestling period of 44 d (see beyond and Sherrod 1983). In w. Greenland at 67°N, onset of egg-laying about last week in Jun, farther south at 60°N about 2 wk earlier (21 Jun; Falk et al. 1986). At similar latitude on upper Yukon River, AK (about 65°N), onset 11 May (n = 418 broods; R. Ambrose pers. comm.). In Aleutian Is., about 51°N, first week in Apr (CMW); at Langara I., British Columbia, about 54°N, 28 Apr (Nelson 1977b). In San Juan Is., Puget Sound, WA, about 48°N, about 3 Apr, but inland at Seattle, WA, at about 47°N, 3 wk earlier (C. M. Anderson pers. comm.). South- and central-coastal California (about 33–34°N) mid- to late Feb, but at >2,000 m (Yosemite Park region, about 37°50'N) late Mar to mid-Apr (J. Linthicum pers. comm.). At southern edge of range in Mexico (Sierra Madre Occidental), mean laying about 17 Mar, but variable, and young fledge 24 May–4 Jul (Lanning et al. 1977, WGH); onset of laying hard to determine because fresh eggs and 10- to 14-d-old young found at same time in nearby eyries in Baja California (Porter et al. 1988, M. A. Jenkins pers. comm.). Laying interval 48–72 h; interval to last egg longest.

Second Brood Per Season

None; renesting only (usually within 14 d of egg loss), but no data from southern end of range (Mexico), where presumably also renesting only. May renest 2 or 3 times if clutches removed early in incubation (Bent 1938b, Palmer 1988c). In California, when young 3 wk old removed from bridges; successful renesting occurred (B. Walton pers. comm.; see also Ratcliffe 1993).

Nest Site

Selection Process

In some migrants, male appears to arrive at nesting ledge first; in residents, both may remain together in nesting area. Male explores many ledges unless choices limited; then simply makes scrapes on ledge previously used. Usually makes several scrapes, female then selects one for egg-laying (Nelson 1977b, Ponton 1983). Same scrape used in Alaska in year following removal of pair from previous year (CMW). During recovery, some currently used scrape areas are same as those used 50 yr earlier (B. Walton pers. comm.; see also Ratcliffe 1993).

Microhabitat; Nest-Site Characteristics

Varies widely, often geographically, especially because of reintroduction (use of some artificial structures) and natural increase from low numbers. Many first-time breeders in expanding population select nontypical sites. Traditionally nest on cliffs ranging from about 8 to 400 m high; cliffs 50–200 m preferred. Lower size somewhat arbitrary as smaller cliffs merge into category of hillside (Bond 1946, Cade 1960, White and Cade 1971), upper limits often difficult to specify; e.g., nesting on rim of Grand Canyon, AZ (Ellis 1982a). On defined cliff, nesting ledge generally one-third way down face (33% on Colville River, AK [White and Cade 1971]; 40% in Greenland [Falk et al. 1986]).

Nest platforms include: tops of pingos in tundra; cut for roadbed in Alaska (R. J. Ritchie pers. comm.); Common Raven nests on electric-transmission tower; stone quarry; sugar-factory silo in Idaho; variety of buildings, churches, and bridges in metropolitan centers, usually aided by artificial nest box (Frank 1994, Bell et al. 1996, Cade et al. 1996b). In upper Mid-west (Mississippi River, shore of Lake Michigan, and elsewhere), occupied artificial nest boxes frequently placed on power plants (smokestacks or buildings; Septon et al. 1996, TJC). On Pacific Coast of Baja Cali-fornia in Bahía Sebastian Vizcaino, with essentially no relief, some use abandoned Osprey or Common Raven nests on boat-navigation channel markers (towers) about 7 m and 4 m tall (latter surrounded by water at high tide; Massey and Palacios 1994, Castellanos et al. 1997, J. B. Platt pers. comm.). On coastal British Columbia in Sitka spruce (Picea sitchensis) forests under tree roots on hillsides, and in 3 island groups in Queen Charlotte Is., 6 pairs using either abandoned tree nests (as low as 12 m) of Bald Eagle or cormorant (Phalacrocorax spp.) or on broken tree stub in natural cavity (Campbell et al. Campbell et al. 1977, Campbell et al. 1990a; R. W. Campbell pers. comm.); other reports of tree nests from islands in se. Alaska (Van Horn et al. 1982), need verification. In California, Peregrines used deserted Common Raven, cormorant, and Red-tailed Hawk nests on sandy coastal bluffs without cliffs (B. Walton pers. comm.).

Orientation varies by latitude or other habitat features, often cliff azimuth may be misleading as eyrie itself may face 90° at variance from cliff. Best cliffs offer updrafts. Sample of eyries (n = 22) on Colorado Plateau had mean azimuth of 101° ± 67 SD, and cliff had mean azimuth of 90° ± 71 SD (Grebence and White 1989). In w. Greenland (about 67°N), eyries face nearly due south; direction thought to be of theromoregulatory value by receiving maximum solar radiation (Falk et al. 1988). In Canada (62°49'N), 69% of eyries faced south or southwest (Court et al. 1988b). In warm, arid sw. U.S., 69% of nests cliffs (n = 95) faced some direction of north and east (Ellis 1982a, Grebence and White 1989); south- and west-facing nests usually had some boulder or shrub on eyrie ledge to produce afternoon shade.


Construction Process

On ledges, consists of scraping bowl in substrate, frequently initiated by male, but by both male and female. Falcon lies on breast and pushes feet backward to produce depression (see Fig. 18 in Nelson 1970b). Substrate consists of dirt, sand, fine gravel, or sometimes decomposed fecal material or decomposed lining materials of old stick nest. Male may construct several scrapes on same ledge or on different ledges. No material deliberately added, but bones and other debris may be pulled around sitting bird to form circle of material around edge of scrape. Scraping also occurs in stick nests of other birds. Behavior as much courtship ritual as “nest-building” (Wrege and Cade 1977).

Structure And Composition Matter

See above.


Substrate for scrape extremely variable, from small 30-×-30-cm spot on ridge in Arctic to potholes to large 4-×-4-m cavelike structures. Scrape (nest bowl) typically 17–22 cm in diameter and 3–5 cm deep; long-used sites have wider and deeper scrapes than newly formed ones.


No data. Some nesting scrapes in open on point of land or hillside completely exposed or with limited vegetation screening as in Arctic. On south slopes, in these conditions, female often just shades eggs/small young from heat and sun rather than incubating, and heat stress of both adults and young is evident (Enderson et al. 1973, TJC, CMW). Generally, scrape is in larger ledge with shading, sheltering, or overhangs, and trend to south- or west-facing orientation in high latitudes but more random directions in lower latitudes. Presumably orientation or other micro-features of eyrie protect young from temperature extremes shown in Prairie Falcon (Williams 1985c) and Gyrfalcon (Clum and Cade 1994).

Maintenance Or Reuse Of Nests, Alternate Nests

Considerable attachment to one nest location, but alternates frequently selected on same cliff or within a few kilometers (TJC, WGH, CMW). In California, some pairs moved together to alternate cliffs up to 9 km; with increase in numbers, pairs now each cliff in same year (B. Walton pers. comm.; see also Ratcliffe 1993).

Nonbreeding Nests

None recorded.



Between elliptical and short elliptical.

Size And Mass

From D. W. Anderson, converted from centimeters; SD not available. Data given as length × breadth (mm); a clinal tendency to greater length and volume north to south. Tundrius: Greenland, 51.0 × 41.2, 42.83 (n = 16 eggs); e. Arctic, 51.2 × 40.7, 41.96 (n = 30 eggs); w. Arctic, 52.7 × 40.7, 43.19 (n = 16 eggs). Taiga and temperate Canadian anatum: Maritime, 52.0 × 40.9, 43.04 (n = 81 eggs); central taiga, 52.2 × 40.9, 43.20 (n = 31 eggs); w. subarctic, 52.2 × 40.9, 43.20 (n = 50 eggs). Pacific Northwest pealei: 53.7 × 41.3, 45.32 (n = 130 eggs). W. U.S. anatum: Pacific Northwest, 53.1 × 42.0, 46.35 (n = 99 eggs); s. California, 52.9 × 41.2, 44.43 (n = 586 eggs); Baja California, Mex-ico, 53.1 × 41.1, 44.38 (n = 105 eggs). Central and e. U.S. anatum: Great Plains, 53.4 × 41.7, 45.95 (n = 133 eggs); Great Lakes region, 54.1 × 42.0, 47.22 (n = 49 eggs); e. U.S., 53.9 × 41.6, 46.15 (n = 399 eggs). Egg mass at laying in captive falcons about 45.5–47.3 g, but loses about 0.17 g/d; averages 16.3% loss of mass to hatching (Burnham 1983). Eggs produced in captivity smaller than wild eggs.

Color And Surface Texture

Fairly smooth without gloss. When fresh, ground color varies from pale creamy to brownish or reddish overlaid with dots, spots, and blotches of various warm browns to deep reds and purples; great variation. Generally deeper and richer color than other large North American falcons.

Eggshell Thickness

Within-clutch variation (cap-tive-laid eggs) may account for 67% of variation while between-clutch thickness accounts for 26% of variation (Burnham et al. 1984b). Degree of thinning resulting from chlorinated hydrocarbons (DDE) varied from region to region, but onset of reduced productivity usually associated with population averages of >18% thinning. In sample of California eggs, 201 prethinning eggs had mean thickness of 0.365 mm ± 0.049 SD; 287 eggs 1978–1985, mean thickness 0.300 ± 0.0174 SD (L. F. Kiff pers. comm.).

During DDT era, mean shell thickness in North American populations decreased by 12% in Queen Charlotte Is., up to 29% in New Jersey; other known populations with decreases >18% occurred in Arctic and interior Alaska, Ungava in Quebec, Colorado, Rocky Mtns., and California. Worldwide lowest de-crease recorded in Australia (>5%), but none of 30 sampled populations showed no shell-thinning in that period (see Cade et al. 1988, Ratcliffe 1993, and Johnstone et al. 1996 for reviews of eggshell-thinning in relation to pesticide contamination).

For large sample of pre-DDT western anatum (n = 573 eggs; L. F. Kiff pers. comm.): mean shell thickness 0.365 mm ± 0.023 SD (range 0.311–0.434; SE of mean 0.002) and shell mass 4.26 g ± 0.394 SD (range 3.253–5.502; SE of mean 0.032). For a sample of pre-DDT eastern anatum (n = 94; Anderson and Hickey 1972), shell thickness was 0.375 mm.

Clutch Size

Clinal, with smaller mean of 3.0 in Arctic (although quite variable; perhaps function of weather and thus food) to 3.72 in midlatitudes; then smaller (3.3) again southward into Mexico; slightly larger clutch size in Aleutian Is., with mean of 3.8 (mode of 4.0; CMW). Occasionally 5 or 6 eggs; in 1.0% of 282 clutches (Hickey 1969, Palmer 1988c).


First-time layers start later in breeding season (Nelson 1977b). Female becomes lethargic about 5 d before egg-laying. Interval between eggs about 48 h, but may be >72 h. In temperate latitudes, at least, clutch may be replaced in about 2 wk if first clutch lost. In captivity, by removing clutches as laid, a female may lay up to 16–20 eggs (TJC, cited in Palmer 1988c), or 12–16 eggs if taken one by one as laid (Weaver and Cade 1991). In California, 4 complete clutches removed from same female by egg collectors (B. Walton pers. comm.).


Onset Of Broodiness And Incubation In Relation To Laying

In temperate latitudes usually begins with penultimate egg, but in high latitudes and cold climates may begin after first or second egg, depending on conditions, with staggered hatching (Court et al. 1988b).

Incubation Patches

Both sexes have paired lateral brood patches. Less well developed in male. Belly area may function as patch also but less edematous and vascular than breast (TJC).

Incubation Period

According to A. Hagar (in Bent 1938b), 33–35 d, not 28–29 d, as commonly stated. Based on captive falcons, about 33.5 d (range 33–34, n = 42; Burnham 1983). Direct observation of identified eggs in wild near San Francisco, CA, was 37 d (B. Walton pers. comm.), possibly related to long or frequent periods of interrupted incubation.

Mean hatching dates related to latitude or temperature. At Thule, Greenland (76–77°N), hatching estimated 9–13 Jul (young 17–21 d old on 30 Jul; W. Burnham pers. comm.). In w. Greenland at about 67°N, 14 Jul; in s. Greenland at about 60°N, 4 Jul (Falk et al. 1988). At 62°N, in Nunavat, mean hatching 9 Jul. At mid-latitudes, in Puget Sound, WA (about 49°N), lays 3–7 Apr while inland at Seattle, WA (about 47°N), starts 3 wk earlier (C. M. Anderson pers. comm.). Near southern edge of range in Mexico (continental and Baja California), hatching date late Apr (Lanning et al. 1977, Porter et al. 1988); 1 nest in Cuba had estimated hatch date of about 1 Apr, laying about 18–19 Feb (based on 2-wk-old chicks on 14 Apr 1999; Regalado and Cables 2000).

Parental Behavior

Based on 4,200 film-h (70,000 frames) in interior Alaska, males incubated about 33% of time, with attentive periods of 2–3 h; female attentive period was about 4 h (Enderson et al. 1973); in same region, adults attended at ledge 99% of time (n = 24 nests) until young 11 d old, then most ledge visits were to deliver food or feed young (Palmer et al. 2001). Nelson (Nelson 1970b) suggested that for the Pacific Northwest male, incubation was 30–50% of the time. Extreme in New Mexico was male incubating as much as 87% of daylight period (average 63% for 18 d) and female as little as 12% (average 37% for 18 d; Clevenger 1987). Adults change position about every 30 min and may shift eggs then (Enderson et al. 1973). Parents seen moving displaced egg into nesting scrape by rolling it with bill (H. B. Tordoff pers. comm.) and frequently move young by picking them up by the nape to transport them.

Hardiness Of Eggs Against Temperature Stress; Effect Of Egg Neglect

Higher than normal egg temperatures during early incubation cause early fatality, whereas lower than normal at that time cause late embryo fatality. Low temperatures late in incubation have less effect; eggs may hatch at higher or lower than optimal body temperature (about 37°C), but such embryos frequently show abnormalities (Burnham 1983). Even advanced eggs can remain uncovered for several hours without injury to embryo; frequent interruptions of incubation result in longer than normal incubation (TJC).


Preliminary Events And Vocalizations

Few data from the wild. From blind near eyries on coastal British Columbia (F. p. pealei), Nelson (Nelson 1970b) could hear chick peeping inside egg before hatching began, becoming louder during hatch; initial pip of shell occurred >72 h before chick broke free completely. In artificially incubated eggs, 24–48 h before pip, air cell in egg expands and starts extending down one side of egg toward narrow end; normally chick makes pip inside air cell. In 500 artificially incubated eggs of F. p. anatum, mean time from pip to hatch was 47.8 h (range 11.8–84.8, mode 48; data from C. Sandfort and The Peregrine Fund archives). During this period, chick periodically works to break up area around initial pip but rests most of time. Human imitation of parental chip call stimulates chick to vocalize (Weaver and Cade 1991).

Shell-Breaking And Emergence

Often chick creates an opening in break-up area around pip before final breaking open of shell begins; then, looked at from blunt end of egg, chick makes a counterclockwise turn inside shell, at same time breaking a line around circumference of egg near blunt end by thrusting egg tooth against shell. Chick turns intermittently, breaking a portion of shell with much vocalization, then rests, and turns again. This last stage of hatching takes 15–60 min (Burnham 1983, Weaver and Cade 1991). Artificially incubated eggs hatch most frequently during morning hours: 40% of 500 eggs found hatched between 06:00 and 12:00; 24% between 12:00 and 18:00; 19% between 18:00 and 24:00; 17% between 24:00 and 06:00 (The Peregrine Fund data files; values somewhat biased toward morning and afternoon hours by times of observation).

Eggs generally said to hatch synchronously (i.e., 24–48 h for clutch of 4) in temperate and low-latitude regions, incubation beginning with last or penultimate egg (Ratcliffe 1993). On Yukon River, AK, 1 clutch of 4 eggs hatched at intervals of 10, 60–72, and 110 h, for total hatching time of about 7.5–8 d; last-hatched chick from this and 1 other asynchronous hatch died in few days (Enderson et al. 1973). At Rankin Inlet, Nunavut, staggered hatching was prevalent and associated with 7% decrease in brood size (Court 1986); about half of last-hatched young in broods of 4 died (Court et al. 1988b). Beginning incubation before penultimate egg and staggered hatching likely at high-elevation nests, too, as incubation early in laying cycle appears to be response to cold temperature.

Parental Assistance And Disposal Of Eggshell

Adults may remove eggshells and sometimes eat them (NJC). Some shells may remain in nest several days until presumably broken or accidently knocked from nest. Addled eggs remain in nest until ultimately crushed. Some intact eggs from previous year may be found in eyries (NJC).

Young Birds

Data herein from Palmer 1988c, Marchant and Higgins 1993, and Clum et al. 1996 .

Condition At Hatching

Semialtricial, nidicolous; covered with off-white (prepenne) down; bill and feet pinkish to pale gray; eyes closed; mass 35–40 g. If eyes open with food-begging first day, they are slitlike. Obtains 2 downy plumages.

Growth And Development

Five days after hatch, mass has doubled, sits with relative ease, and open eyes more round. At 6–8 d, second down (mesoptile or preplumulae) starts to emerge, first on humeral and alar tracts but no down visible on belly at 6 d, although on legs and belly at 8 d; also second down well out on wings and sheaths of primaries breaking skin on wings. By 10 d of age, second down complete and uniform and outer rectrices breaking skin. At 10 d, primaries growing at 2–3 mm/d, rectrix sheath not yet split.

At 14 d, second down dense and long, rectrix sheath about 2 mm and typically ninth primary emerges from sheath. By d 17, contour feathers start to push out prepennae and only pale (buffy) tips of rectrices have emerged but growing at about 2 mm/d (since day 13). At 20 d, while still with heavy coat of second down, contour feathers visible on margins of wings, tail, and faintly around eyes. By 30 d, young appears about half down-covered and half feathered; while side of face well feathered, crown still covered with down. At 35 d, while mostly feathered, large conspicuous patches of down around legs, underwings, and on crown. At 40 d, almost fully feathered with traces of down on crown and underwings and outer several remiges; rectrices not fully grown but bird capable of weak flight.

Formula for age (d) of nestling at any stage of development: (Wing length in cm + 0.84)÷ 0.69 (NJC).

Onset of thermoregulation variable, may be dependent on climate, but once full feather coverage (not fully grown), perches alone at night. In Greenland, young began to wander out of nest scrape proper ≥10 d after hatching, but most of time spent there until 24 d, after which time also less bodily contact with one another (Hovis et al. 1985).

Parental Care


Begins during hatching; young brooded >80% of time in Greenland up to age 10 d; amount gradually decreased to 20 d; not brooded thereafter (Hovis et al. 1985).


In Greenland, time adult spent feeding young about 9% to 5 d of age, about 3–9% to 20 d. Food delivery rate ranged from 1/43 to 1/75 min to about 20 d of age and length of feeding bouts ranged from about 3.0 to 6.7 min/bout to 20 d (Hovis et al. 1985). In Alaskan taiga, A. G. Palmer, D. L. Nordmeyer, and D. D. Roby (pers. comm.) found delivery rates/nestling declined with increase in brood size in broods of 1–3 but not between broods of 3 and 4. On biomass basis, mean delivery rate/brood ranged from about 20 g/h in broods of 1 to 60 g/h in broods of 4. New hatchlings fed only on muscle without bone; diet same as that of adult after 2–3 wk (see Food habits: diet, above).

Nest Sanitation

Young in first few days of life simply squirt excreta, but after several days, defecate by backing away from center of scrape, bending forward as if stretching, and directing a stream of uric acid and fecal material away from scrape. Use of traditional ledges can cause excrement and nesting debris to become several meters deep over time; dating of debris (Australia) suggests some debris and excreta as old as 16,000 yr (Olsen 1995). Adult removes some material (e.g., old carcasses) with bill from ledges.

Carrying Of Young

See Incubation, above.

Cooperative Breeding

Rare under normal population structure. Two females at Reelfoot Lake, TN, reported to incubate same clutch alternately (Spofford 1947a); also ob-served in 2001 at building site in Edmonton, Alberta (G. Court pers. comm.). On Langara I., British Colum-bia, 1 female paired with 2 males, but 1 male did not transfer food or copulate; observations suggested female behavior by pairing with second male was aimed only at holding his territory in addition to her own (Nelson 1990).

Brood Parasitism

Not known to occur. One case of Peregrine on Colville River, AK, incubating 3 of its eggs in Rough-legged Hawk nest that also contained 2 eggs of that species. Only male Rough-legged Hawk was present near nest; Peregrines may have killed female to usurp nest (J. H. Enderson and CMW unpubl.). D. Anderson found Prairie Falcon reared by Peregrines in Gulf of California, Mexico (B. Walton pers. comm.).

Fledgling Stage

Within 10 d of first flight, young pursue adults to solicit food. Flight progresses from Butterfly-Flight (1–2 d after first flight) to Flutter-Glide (3–9 d) to Powered Flight (15–25 d). Butterfly-Flight appears to be weaker form of Flutter-Glide associated with in-complete development of flight feathers and pectoral muscles. Pursuits gradually become more sustained and range farther from nest cliff. Adult pursuit is accompanied by Begging vocalization. During first 2 wk of flight, young birds' pursuit of parents takes precedence over most other activities. Young will even pursue parents during territorial defense (Sherrod 1983).

As young become more aggressive toward food-delivering parents, adults sometimes begin to drop both dead and live birds in air. Young pursue and catch these items. Has been interpreted as parental training of young to hunt, but may simply be way for parents to avoid being mobbed by hungry young (Sherrod 1983).

In migratory populations, dependency may con-tinue until onset of migration 5–6 wk postfledging. Period of dependency longer in nonmigratory pop-ulations (9–10 wk postfledging). Some level of parental feeding occurs during this time. Dispersal of hacked birds occurred (on average) after 30 d and 33 d on the wing for the male and female, respectively. Most hacked birds had dispersed by 6 wk after fledging, though records of later dispersal exist (58 d, 75 d, and 237 d postfledging); also, some young disappearing in first week after fledging later found breeding (TJC). Timing of dispersal may be related to level of food provisioning to hacked and wild birds. Adults observed to become aggressive toward offspring late in dependency (Sherrod 1983).

Immature Stage

Groups of immatures on migration are suspected of being siblings (Cade 1960, Sherrod 1983). Sherrod also cites 5 instances of immatures begging from adults on migration and 4 occasions where migrating adults allowed immatures to take food from them. Assumption of these observations is that offspring would beg only from their parents and only parents would allow offspring to rob them of food. However, food-begging by first-year birds and giving of food by adult to first-year birds have been seen in Jan and Feb in neotropical nonbreeding grounds (J. L. B. Albuquerque pers. comm.). Some year-old young tolerated at breeding sites in California and unrelated year-old young beg food from breeding adults (B. Walton pers. comm.).

Recommended Citation

White, C. M., N. J. Clum, T. J. Cade, and W. G. Hunt (2002). Peregrine Falcon (Falco peregrinus), version 2.0. In The Birds of North America (A. F. Poole and F. B. Gill, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA.