Breeding

Phenology

Pair Formation

Figure 4. Evidence for year-round occupation of nest sites in Alaska (Cade 1960), Yukon (Platt 1976), and inland Northwest Territories (NWT) (Kuyt 1980, Norment 1985). On coastal mainland of NWT, however, no evidence of occupation prior to Feb (Poole and Bromley 1988b), perhaps because more severe weather conditions at coastal eyries or higher latitudes restrict hunting opportunities at winter solstice (Poole and Bromley 1988b, Nielsen and Cade 1990b). Sites occupied prior to breeding are generally same sites that are subsequently active during breeding season (Platt 1976, Poole and Bromley 1988b, Nielsen and Cade 1990b). In Yukon, first observations of paired birds in Feb (Platt 1976), about 1 mo after first observations of unpaired birds and 2 mo prior to egg-laying. In coastal NWT, first observations of paired birds in late Apr, about 2 wk prior to egg-laying (Poole and Bromley 1988b), but birds probably paired earlier (K. Poole pers. comm.). Courtship activities begin about 1 mo before egg-laying (Platt 1977).

Nest-Building

No nest-building per se. Scraping (pushing substrate aside with feet to make a shallow depression) begins early in courtship and continues until egg-laying.

First/Only Brood Per Season

Figure 4. Egg dates (ranges include possible renesting attempts): Yukon, 3–28 Apr (Platt 1977); coastal NWT, 21 Apr–30 May (Poole and Bromley 1988b); inland NWT, 20 Apr–2 Jun (estimated), Kuyt 1980, Norment 1985); Alaska, Apr to late May (estimated; Cade 1960). There is general trend for nesting to occur later at higher latitudes (Bromley 1986), although there is much overlap in egg dates among regions. Egg dates vary significantly from year to year (Poole and Bromley 1988b, Nielsen and Cade 1990b). Degree of synchrony within population also varies annually (Poole and Bromley 1988b). Chicks hatch after 34–36 d (Platt 1977). Males fledge at 45–47 d, females at 47–50 d (Poole and Bromley 1988b). Young independent ≥ 4 wk after fledging (Cramp and Simmons 1980, Nielsen and Cade 1990b, Britten et al. 1995).

Four records of renesting: 1 in Alaska Range (Cade 1960), 1 in NWT (Poole 1988a), and 2 in Yukon (Platt 1977). Renests occurred after failure (abandonment) of previous clutch (Platt 1977, Poole 1988a). Recycle time approximately 16 d for both wild (n = 1, Poole 1988a) and captive (n = 11 for 2 pairs, The Peregrine Fund) birds. Phenology similar to first brood, but post-fledging period may be shorter (Poole 1988a).

Second Brood Per Season

None; renesting only.

Nest Site

Selection Process

Unclear which sex chooses nest site, as unpaired birds of both sexes have been observed frequenting nest cliffs prior to breeding season (Platt 1977, Nielsen and Cade 1990b). Males seem to predominate (Platt 1977, Poole and Bromley 1988b), however, and have been observed to advertise for females (Platt 1977).

Microhabitat, Nest-Site Characteristics

Most (58–91%) nesting occurs in nests of other species, particularly Common Raven, Golden Eagle, and possibly Rough-legged Hawk; remainder of nest sites on ledges (Cade 1960, White and Cade 1971, Barichello 1983, Poole and Bromley 1988b, Nielsen and Cade 1990b) (Fig. 5). Will usurp newly built nest of ravens but not of eagles (Poole and Bromley 1988b, Nielsen and Cade 1990b). Most (> 80%) nest sites on precipitous cliff faces (Cade 1960). Mean nest height 4–30 m (Cade 1960, White and Cade 1971, Poole and Bromley 1988b). Most (85–94%) sites with overhangs (Cade 1960, Poole and Bromley 1988b, Nielsen and Cade 1990b, Obst 1994), except where this is an uncommon physical feature (e.g., Yukon and se. NWT, 33% sites with overhangs; Platt 1977, Kuyt 1980). Substrate varies with area: in Alaska roughly half ledges shale, half sandstone or conglomerate (Cade 1960; White and Cade 1971); in NWT, diabase (Poole and Bromley 1988b). In taiga of se. and nw. NWT, > 60% of nests in white spruce (Picea glauca) in nests of Common Raven and Golden Eagle, 5–8 m above ground and 1–5 m below tree top (Kuyt 1962, 1980, Obst 1994); in w. Alaska nests have been documented in old Common Raven nests in balsam poplar (Populus balsamifera) (Kessel 1989). In Alaska, Gyrfalcons have also been recorded nesting on artificial structures, including trans-Alaska oil pipeline, gold dredges, and sluice boxes (White and Roseneau 1970, Ritchie 1991).

Nest

Construction

None to speak of. Not known to construct stick nests in North America, but reported to do so in Russian Arctic (Cade et al. 1998). Both male and female scrape, which probably functions as much as a courtship ritual as “nest-building.” Stick nests of other species usually not added to or modified. Stick nests often destroyed during course of raising young.

Structure And Composition

Varies with species usurped; generally dead sticks, with little or no lining; eggs usually laid on bare soil or accumulated debris.

Dimensions

Varies with species usurped; outside dimensions roughly 0.6–1.2 m deep and wide.

Microclimate

Prefers sites not exposed to severe winds (Cade 1960). In Alaska, 62% of sites oriented northward (Cade 1960); in central NWT, no bias in orientation (Poole and Bromley 1988b, Obst 1994). In Yukon and se. NWT, most sites oriented south or west (Platt 1977, Kuyt 1980). This orientation may be preferable in sites lacking overhangs to keep them free of snow; snow-free sites not frequented preferentially during winter, but more likely to be occupied during nesting (Platt 1976). Young in nests with southern and western exposures, however, appear heat-stressed on sunny days (Fletcher and Webby 1977, Poole and Bromley 1988b). No information on insulative value of stick nests relative to ledge nests.

Maintenance Or Reuse Of Nests, Alternate Nests

Maintenance of stick nests dependent on other species. Between 1 and 3 alternative nest sites usually available within 1–1.4 km. Most, but not all, pairs change nest sites between years (Poole and Bromley 1988b, Nielsen and Cade 1990b, Obst 1994). Nest ledges reused over many years; carbon dating of accumulated feces at historical nest sites in Greenland revealed use over the past 2,500 years (The Peregrine Fund 2005b).

Nonbreeding Nests

Not known to occur.

Eggs

Data in this section from The Peregrine Fund, unpublished, except where noted.

Shape

Short elliptical.

Size And Mass

Mean length, 58.46 mm (55.66–61.54 mm); mean breadth, 44.95 mm (43.19–48.01, n = 2 clutches, 7 eggs from Mackenzie, Canada; 3 clutches, 11 eggs from Labrador; Western Foundation Vertebrate Zoology [WFVZ]). Mean fresh weight of 52 first-clutch eggs from captive birds, 61.99 g ± 2.87 SD; captive eggs average 4% shorter and narrower than wild eggs, which translates to 8.9% difference in mass. Eggs approximately 3.5% of female body weight overall, but no reported values available for individual females. No geographic variation in egg size. Egg size varies among clutches of different females, as does degree of variability (Poole and Bromley 1988b). In captivity, 1 egg (probably last laid) of 4-egg clutches generally smaller.

Color

Overall appearance ranges from almost white to uniform reddish brown. Base color white or yellowish white; variably spotted with cinnamon

Eggshell Thickness

Mean thickness, with membrane 0.429 mm ± 0.016 (0.409–0.470, n = 4 clutches, 14 eggs from Canada; collected 1864–1904 [WFVZ]). Mean weight of empty shell, 5.981 g ± 0.474 (5.355–6.615 g, n = 1 clutch, 3 eggs from MacKenzie, Canada; n = 2 clutches, 3 eggs from Labrador) [WFVZ]). No geographic variation in eggshell thickness or weight. See also Conservation and Management: effects of human activity.

Egg-Laying

Female becomes lethargic about 5 d before egg-laying. No information on time of day of laying for wild birds. Interval between eggs about 60 h (Platt 1977). In captivity, inter-egg interval decreases with subsequent eggs (Seifert 1982). As many as 13 replacement eggs laid by a single captive bird when eggs are pulled sequentially. Intraspecific egg-dumping not known to occur.

Incubation

Onset Of Broodiness And Incubation In Relation To Laying

No information on onset of broodiness for wild birds. Incubation typically begins with penultimate egg (Platt 1977) but may start sooner with onset of bad weather (TJC).

Incubation Patches

Both male and female have 2 paired lateral brood patches, although male’s are more poorly developed than female’s (Cade 1982).

Incubation Period

Incubation period 35 d for wild birds (n = 1), estimated at 35–36 d for 2 other nests (Platt 1977). Previous estimates of incubation period much lower, 28–29 d (Manniche 1910, Witherby et al. 1943, Cade 1960). Mean incubation period of captive birds 33.29 d ± 2.13 SD, n = 98, range 29–41, The Peregrine Fund). Incubation period 3–5 d longer for captive eggs from same clutch incubated artificially versus naturally (Seifert 1982).

Parental Behavior

Both sexes incubate, but males participate only 17–24% of time (Poole and Bromley 1988b). Length of incubation bouts of females about twice as long as those of males (females, 260.6 min ± 148.6 SD; males, 140.1 ± 58.4 min); only females incubate through night. Male interest appears to decline during course of incubation period (Platt 1977). Eggs are left uncovered for 2–4 min when adults change places, although during a period of -35°C temperatures, change-over was accomplished in 20–45 s, with one bird sometimes lying down next to other prior to change. Female does not always allow male to take over incubation, giving agonistic Chatter with out-stretched neck. Birds approach scrape walking in a horizontal position; at scrape, steps are slow and high with feet loosely clenched. Feet are worked beneath eggs as body is lowered with rocking motion and jerking back of head with beak pointed downward. Head movement is also performed as intention movement prior to incubation. Settling movements are repeated at irregular intervals during incubation, with bird generally changing orientation 90°. Incubating birds may “rim” scrape by scraping substrate inward with beak and piling pieces of debris nearby, creating a ridge around scrape surrounded by an area clear of debris (Platt 1977).

Hardiness Of Eggs

No specific information, but first and second eggs of clutch are left unattended in subzero and freezing temperatures for hours without apparent harm in both wild (Platt 1977, K. Poole pers. comm.) and captive birds (Seifert 1982).

Hatching

Data in this section from The Peregrine Fund, unpublished, except where noted otherwise.

Preliminary Events And Vocalizations

In captivity, pipping of eggshell occurs 46.8 ± 15.1 h prior to hatching (n = 94). From time of pip, clicking noises can be heard coming from egg (presumably from contact between egg tooth and eggshell). A soft complaining call (similar to Beg Call) often made by chick during hatching, and can be elicited by imitating adult’s Chup vocalization. No information on hatching in wild birds.

Shell-Breaking And Emergence

About 17.8% of eggs produced in captivity hatch between 0600 and 0759 h. No captive eggs hatched between 0100 and 0359 h, but most of hatching uniformly distributed throughout rest of day and night (n = 101). Duration of hatching process (once chick has begun to turn in shell) approximately 30–45 min (C. Sandfort pers. comm.). In wild birds, 6 broods hatched within 48 h, 1 hatched within 72 h, and several broods appeared to have hatched over up to 6 d from estimates of chick ages (Poole and Bromley 1988b). Such extended hatching periods may result from incubation starting before penultimate egg in very cold weather (K. Poole pers. comm.). In captivity, 8 complete clutches where all eggs were viable hatched over 95.6 ± 82.4 h (range 9.4–221.9 h).

Parental Assistance And Disposal Of Eggshells

No information on parental assistance. At least some, probably most, adults eat eggshells upon hatch (TLB). Addled eggs, however, are left in nest until ultimately crushed (K. Poole pers. comm.).

Young Birds

Data from The Peregrine Fund, unpublished, except where noted.

Condition At Hatching

In captivity, hatch weight averages 52.1 g ± 3.7 SD (n = 96). No linear measurements available for hatchlings. Thick primary down uniformly distributed in feather tracts, except sparser in posterior-medial portion of ventral tracts. Bill pale horn in birds that develop gray plumage but ranging from blackish to nearly colorless, gape pink, iris black-brown, cere, tarsi, and feet bare, pale yellowish-pink. Color of first down varies in relation to color of future feathers: lightest birds have pure white down and colorless talons as hatchlings; darker birds have a dark wash on down of head and/or back and dark talons; darkest birds have jet black talons and beak tips. Chicks hatch with eyes open, slitlike at first; in captivity, egg tooth retained for at least first week. Young are able to sit up on tarsi very shortly after hatching to beg food from adults, but gaping is not oriented at this age. Hatchlings respond to vibration or sound by food-begging. Hatchlings can move around sufficiently to find warmth.

Growth And Development

Weight of hatchlings doubles by about day 5. Second down begins to come in at day 8 in captive birds, emerging from its own follicles, not replacing 1st down as suggested by Dementiev (1960). First down-feathers are replaced in prejuvenal molt by emerging contour and flight feathers (TJC) During most rapid growth (6–27 d), females gain weight faster than males (59 g/d vs. 50 g/d, Poole 1989). Primary 7 emerges at about 11 d and grows in a linear fashion up to about 40–42 d, at about 2 mm/d (Poole 1989). No information on timing or sequence of emergence of contour feathers on different feather tracts, but young are down-covered until about 3 wk and feathered by about 5 wk. At < 1 wk of age, in 5°C weather, young often move partially out from underneath brooding female (Platt 1976). Completion of growth of flight feathers does not occur until after nest departure.

No observations of direct inter-sibling conflict though older nestlings compete for food. No specific information on timing of behavioral development. Young birds sleep lying down, by sitting on tarsi and lying forward on their ventral surface; feet may be extended out and back if nestlings are heat-stressed. When cold-stressed, sleep sitting up with head tucked, or seek out siblings. When older, also adopt adult sleeping postures, but generally prop themselves against some object. Gradually spend more time standing and less time sitting on tarsi. When able to stand, begin stretching (see Behavior: self-maintenance) and wing-flapping. Wing-flapping accomplished by leaning forward, grabbing substrate with feet, and flapping with wings held slightly above and behind back. Preening behavior begins before emergence of contour feathers. Young first fly at 45–50 d of age (Poole and Bromley 1988b) before flight feathers are hard-penned.

Parental Care

Brooding

Brooding begins during hatching (Bente 1981). Young are brooded almost continually (> 80% of time) for minimum of 6 d (Platt 1977) and maximum of 19 d (Poole and Bromley 1988b), with 10–15 d most typical (Jenkins 1978, Poole and Bromley 1988b). Brooding time subsequently drops precipitously (Jenkins 1978), with young brooded only at “night,” during rain showers, or briefly after feedings (Platt 1977). Brooding ceases completely as early as 11 d (Platt 1977) and as late as 27–32 d (Bente 1981, Poole and Bromley 1988b), with 16–25 d probably more typical (Jenkins 1978, Poole and Bromley 1988b). Male participation in brooding ranges from 0% (Platt 1977) to 5–25% (Jenkins 1978, Bente 1981, Poole and Bromley 1988b) and is greatest during first 5 d after hatching (Jenkins 1978). Brooding bouts of females averaged 43–97 min (Platt 1977, Bente 1981, Jenkins 1982); bouts of males about 28–54% of females’, with up to 6 bouts/d total for both sexes (Bente 1981, Jenkins 1982). Brooding by male generally occurs while female feeds (Jenkins 1978). Female sometimes carries or drags young nestlings by mouth.

Feeding

Feeding of chicks begins on day of hatch (Jenkins 1978, Poole and Boag 1988). Age when chicks begin to cast pellets unknown. Chicks able to stand on prey and pull it apart by 4 wk of age (Platt 1977), but female continues direct feeding (apportionment to chicks, not just delivery of food to nest) of chicks almost until fledging (Platt 1977, Jenkins 1978, Bente 1981).

Adults first deliver food to nest without feeding it to nestlings (indirect feeding) at 29–43 d; such deliveries account for only 6% of all feedings (Poole and Boag 1988). Males participate in only 2.3–9.1% of direct feedings (Jenkins 1982, Poole and Boag 1988, Booms and Fuller 2003b). Male supplies all prey (primarily via food transfer to female) for first 2–3 wk (73% overall, Poole and Boag 1988), at which time female begins to hunt (Platt 1977, Jenkins 1978, Poole and Boag 1988). Male delivery of food directly to nest (without transferring to female) varies from 0-62% of all prey deliveries (Jenkins 1982, Booms and Fuller 2003b). Type and size of food items same as those eaten by adults (see Food Habits), but up to 5 wk of age, ptarmigan are brought plucked, decapitated, and sometimes partially dismembered. By 6 wk, adults begin to bring decapitated, partially plucked ptarmigan that are then plucked on ledge (Platt 1977). Male may first feed on prey away from nest (see Food Habits: food capture and consumption).

Delivery of prey either fairly uniform throughout day (Fletcher and Webby 1977, Bente 1981, Poole and Boag 1988), or exhibiting peaks in late morning and evening with a significant lull in very early morning (Jenkins 1982, Booms and Fuller 2003b). Feeding rates partially dependant on size of prey. Peaks at 10–12 d (maximum 12 feedings/d, Poole and Boag 1988) and 18–20 d (maximum 20 feedings/d, Bente 1981). Feeding rates decrease after 25–29 d (maximum 6–20 feedings/d, Bente 1981, Jenkins 1982, Poole and Boag 1988) unless diet shift to small prey occurs (Booms and Fuller 2003b). Feedings average 6–13 min in length (Bente 1981, Jenkins 1982, Poole and Boag 1988, Booms and Fuller 2003b), with average of 84–218 min between feedings (Fletcher and Webby 1977, Platt 1977, Bente 1981, Poole and Boag 1988). Feedings by males last about half as long as those by females (Jenkins 1982). Adults appear to be able to adjust prey biomass to number of young (Poole 1988b). Adults apportion food fairly evenly among chicks (Platt 1977, Bente 1981). At 15 d, when chicks begin to compete actively for food, distribution may become more skewed (Platt 1977). Although some have described behavior of chicks during feeding as aggressive (Jenkins 1978), and chicks may or may not mob adults at feedings, no sibling aggression observed, even at ages near fledging (Platt 1977, Bente 1981). Degree of aggressiveness may relate to food availability as seen in other raptor species.

Nest Sanitation

Young defecate by backing away from center of scrape, bending forward as if stretching, and directing a stream of urine and fecal material away from scrape (Jenkins 1982). No information on frequency of excretion. Use of traditional nest ledges can cause excrement and nesting debris to become several meters deep over time (Burnham and Mattox 1984). Adults remove 0-21% of prey remains from nest after feedings (Platt 1977, Booms and Fuller 2003b). For information on invertebrates associated with nest sites, see Demography and Populations: diseases and body parasites, and causes of mortality.

Cooperative Breeding

Not known to occur.

Brood Parasitism

Not known to occur, although stray Canada Goose and Rough-legged Hawk eggs have been found in Gyrfalcon nests (TJC).

Fledgling Stage

No information on mass or linear measurements at fledging, though fledglings appear similar to adults in all but feather growth and mass. In the Northwest Territories, fledglings remain within 200–300 m of nest for 7–10 d. By week 2 they travel up to 1 km, still returning to nest regularly. By 20 d post-fledging, some young have moved from general vicinity of nest (Poole and Bromley 1988b). Fledglings associate with adults and siblings during fledgling stage, continuing to receive food from parents (Platt 1976, Fletcher and Webby 1977, Bente 1981).

Immature Stage

Little information. Immature birds become independent of parents 4–6 wk after fledging. Groups of birds sighted in fall are suggested to be comprised of immatures (Cade 1960, Platt 1976). Immature birds prey heavily on rodents, passerines, and young ptarmigan (Cade 1982). No evidence of immatures remaining on breeding territories during winter (Platt 1977, Nielsen and Cade 1990b) and no directional trends of post-fledging movements observed (Britten et al. 1995).