Demography and Populations

Measures Of Breeding Activity

Age At First Breeding

In Iceland, age at first breeding for one female was 2 yr, age at first breeding for one male was 4 yr (Nielsen and Cade 1990b). In captivity 3 pairs of birds with like-aged mates bred at 2 yr, 3 yr, and 4 yr of age (The Peregrine Fund) and Seifert (1982) had 1 pair that bred when female was 4 yr and male was 3 yr. Suggestion that Palearctic birds may occasionally breed in first year considered unlikely (Dementiev and Gladkov 1957, Cramp and Simmons 1980). Pairs do not necessarily attempt breeding every year (Cade 1960, Nielsen and Cade 1990b). Interval between breeding years varies and is dependent on food supply (Nielsen and Cade 1990b).

Clutch

Mean clutch size 3.72 ± 0.71 (range 1–5, n = 122 clutches from Alaska, Labrador, Greenland, and Iceland [WFVZ]). No geographic variation in clutch size documented, although clutch size declines as breeding season progresses (Barichello 1983). See also Cade et al. (1998) and Potapov and Sale (2005) for data outside North America.

Annual And Lifetime Reproductive Success

Over a 10-yr period in the Northwest Territories (NWT), Canada, 54% of territories were occupied each year, on average (Shank and Poole 1994). Over a 4-yr period in NWT, 23% of pairs occupying territories did not lay eggs (Poole and Bromley 1988b). Estimated combined egg and nestling mortality was 48%, giving overall annual productivity of 1.5 young/active nest (Poole and Bromley 1988b). Brood size averaged 2.54 young over 10-yr period over entire NWT; no temporal or spatial trends observed in brood size (Shank and Poole 1994). Little difference observed between brood size at first sighting and brood size at fledging, indicating most mortality takes place either at egg stage or early in nestling period (Cade 1960, Nielsen 1986). Over 10-yr period, however, 73.6% of occupied territories (43% of available territories) in NWT produced young (Shank and Poole 1994), so relatively small proportion of pairs fail completely.

Of 2 copulating captive females, paired as young birds and retained until their death, 1 produced 97 eggs with 47% fertility, 70% of which hatched; the other produced 90 eggs with 63% fertility, 79% of which hatched. Eggs and clutches were removed from these birds throughout the breeding season each year, so numbers represent maximal productivity (The Peregrine Fund). Fertility rates in captivity probably lower than in wild birds.

Life Span And Survivorship

Oldest wild bird recovered in Iceland was 12 yr old male (Cade et al. 1998); in the NWT, a banded female (re-sighted) believed to be 12 yr old, assuming age at 1st breeding was 3 yr (K. Poole pers. comm.). Three captive females averaged 12 ± 4.6 yr at death (The Peregrine Fund). Of 46 recovered birds banded as nestlings, 67.4% were juveniles, 93.5% were either juveniles or subadults (Nielsen and Cade 1990b). Of another 38 birds found dead or diseased in Iceland, 84% were < 1 yr old (Clausen and Gudmundsson 1981). Little survivorship data from N. America, but breeding adult survival estimated at 90% in Iceland; no information on first year survival, but possibly around 50% (Cade et al. 1998).

Disease And Body Parasites

One nestling in the Northwest Territories succumbed to an infestation of parasitic fly Protocalliphora avium at 10 d of age; infestations of dipteran larvae and fleas also observed (Poole and Bromley 1988b). Mosquitoes can also cause distress to young (TJC). Several nymphal ticks (Ixodes howelli) collected from 11 d old nestlings in Alaska (White and Springer 1965). In Iceland, nematode Capillaria contorta found in 36 of 38 birds; 13 birds died from these infestations, remainder only lightly affected. Small numbers of other parasites found in intestines in 12 out of 38 birds: Hymenolepis sp. (7), Plagiorchis elegans (2), Cladotaenia cylindracae (2), and Mesocestoides sp. (1) (Trainer et al. 1968, Clausen and Gudmundsson 1981). No hematozoa observed in the blood of 2 Greenland Gyrfalcons (Taft et al. 1998).

In Iceland, 8 of 13 birds dying from parasitic infections also had pneumonia. Corynebacterium murium isolated from 1 bird and C. pyogenes from another (Clausen and Gudmundsson 1981). Nonclinical bacterial isolates from wild birds include Escherichia coli, Streptococcus sp., Staphalococcus epidermis, Haemophilus aphrophilus, Proteus mirabilis, P. vulgaris, and Actinobacillus sp. (Cooper et al. 1980). Captive birds susceptible to avian cholera (Pasturella multocida, Williams et al. 1986), avian malaria (Plasmodium relictum) (Kingston et al. 1976), Aspergillosis (Aspergillus fumigatus), frounce (Trichomoniasis gallinas) (Hamilton and Stabler 1953), and pigeon herpes, with all being potentially fatal. Aspergillosis and West Nile virus most serious infections of captive Gyrfalcons (TJC). Nonclinical presence of Staphalococcus sp., non-hemolytic Streptococcus sp., and various gram-negative bacteria observed in captive birds (Kingston et al. 1976).

Causes Of Mortality

Weather probably a major cause of mortality in nest; snowfall negatively correlated with number of young per occupied nest (Nielsen 1986, Poole and Bromley 1988b), and nest abandonment often associated with, and attributed to, isolated events of severe weather. Nest sites with northern orientation may have higher success than those with southern orientation (Barichello 1983, Poole and Bromley 1988b; see Nest: microclimate). Starvation of nestlings also occurs (Cade 1960, Poole and Bromley 1988b). No record of predation on nestlings by other species.

Of 23 birds found dead out of the nest in Iceland, 8 (35%) were hit by cars, 7 (30%) hit other objects, 4 (17%) were shot, 2 (9%) were oiled, and 2 were found emaciated (Nielsen and Cade 1990b). Of 38 unbanded birds found dead or diseased in Iceland, 13 (34%) died from parasitic infections, 12 (32%) were shot, 8 (21%) died of trauma, 2 (5%) were oiled, 2 died of unknown causes, and 1 (3%) was poisoned (Clausen and Gudmundsson 1981). Several birds in se. Northwest Territories poisoned by strychnine-loaded caribou carcass (Kuyt 1980). Human-related causes of mortality may be lower in North America, where Gyrfalcon populations are more isolated.

Range

Initial Dispersal From Natal Site

No information on natal dispersal or philopatry in North America. In Iceland, two males found breeding 14 and 25 km from their natal site; two females bred 53 and 84 km from natal site (Nielsen 1991).

Fidelity To Breeding Site And Winter Home Range

Nest sites are traditional and may be used for generations, but little information on fidelity of individuals. Generally thought to be site faithful. In Iceland, 2 banded females remained faithful to sites for 3 and 4 yr (Nielsen and Cade 1990b) and in w. Alaska, 1 banded female remained faithful to site for at least 3 yrs (TLB unpub. data). Maximum known number of consecutive years for site occupation is 5 yr (Burnham and Mattox 1984, Poole and Bromley 1988b). One banded female in NWT observed 10 yr later on same territory, although fidelity to this site may not have been continual (K. Poole pers. comm.). In South Dakota, 1 subadult female established winter home ranges with > 50% overlap in 2 consecutive years (Sanchez 1993).

Dispersal From Breeding Site

Almost no information in North America; one banded breeding female bred 5 km (in a different historical territory) from nest where captured (TLB unpub. data). Breeding females recaptured in Iceland in same territory and within 5.9 km of previous nests (Nielsen 1991).

Home Range

One female with older nestlings remained within 3.2 km of eyrie during all activities; the male patrolled an area of about 200 km², at one point traveling up to 24 km from nest site. Range size probably varies annually and geographically with prey abundance (White and Nelson 1991). One breeding female harnessed with a satellite transmitter in Greenland ranged over 589 km² (Klugman et al. 1993). All radio-tagged wintering subadults in South Dakota (n = 4) established home ranges; mean maximum home ranges were 4,422 ± 956 km², high-use areas (85% harmonic mean) averaged 1,586 ± 263 km², and average range length was 32.3 ± 6.1 km. Two birds with adjacent ranges shared only 5–7% (247 km²) of their ranges, and another 2 had ranges with no overlap (Sanchez 1993). Range lengths of immatures averaged longer than those of subadults (n = 5, 94.9 ± 31.7 km) and generally showed little reuse of area. One immature did appear to set up a home range south of his study area (Sanchez 1993), and 1 immature in Washington established a home range similar in size to that of sub-adults in South Dakota (Dobler 1989).

Population Status

Numbers

Alaska. (White and Springer 1965, Roseneau 1972, Swartz et al. 1975, Swem et al. 1994). Total known pairs about 180, estimated pairs about 375–635; north (northern slope Brooks Range and Arctic slope), about 90 pairs known at 1/181 km², up to 9 in 38 km along rivers with suitable cliffs; west (between Brooks Range and Alaska Peninsula), about 56 pairs known, estimated about 132 pairs at 1/176 km²–1/1,000 km²; central (Alaska Range, Wrangell Mtns., southern slope Brooks Range), about 26 pairs at 1/212 km², largest region, most not surveyed; southwest (Aleutians and Alaska Peninsula), about 6 pairs known, estimated about 36 pairs; south/southeast (Gulf of Alaska and Pacific Ocean), about 3 pairs known, estimated about 30 pairs.

Yukon. (Mossop and Hayes 1994). Total known pairs about 240, estimated about 748, total estimated population 2,490–4,180 birds; North Slope, about 106 pairs known in 17,500 km² at 1/165 km², nearest neighbor distance 8.1 km, estimated about 188 pairs in 31,020 km²; southern Richardson Mtns., about 17 pairs known in 15,947 km² at 1/1,724 km², nearest neighbor distance 18.2 km, estimated about 90 pairs in 85,200 km²; Ogilvie Mtns., about 58 pairs known in 17,302 km² at 1/299 km², nearest neighbor distance 11.0 km, estimated about 184 pairs in 54,903 km²; Dawson Range, about 10 pairs known in 5,030 km² at 1/505 km², nearest neighbor distance 25.6 km, estimated about 155 pairs in 78,450 km²; Kluane Range, about 6 pairs known in 10,227 km2 at 1/1,695 km², estimated about 11 pairs in 18,906 km²; Macmillian Pass, about 7 pairs known in 10,965 km² at 1/1575 km², nearest neighbor distance 96.7 km, estimated about 28 pairs in 42,436 km²; Coast Mtns., about 36 pairs known in 10,023 km² at 1/279 km², nearest neighbor distance 12.4 km, estimated about 92 pairs in 25,550 km².

Northwest Territories. (Shank and Poole 1994). Estimated total pairs about 1,300, estimated total population about 5,000 birds; Queen Elizabeth I., estimated about 45 pairs in 17,000 km of coastline at 1/375 km of coast, mean internest distance 75 km; Low Canadian Arctic Is. estimated about 175 pairs in 26,000 km of coastline at 1/150 km of coastline, mean internest distance 50 km; mainland coast, estimated about 195 pairs in 8,500 km² at 1/175–1/875 km²; mainland interior, estimated about 450 pairs in 900,000 km² at 1/2,000 km²; Mackenzie and Richardson Mtns., estimated about 425 pairs in 150,000 km² at 1/350 km².

British Columbia. Fifteen breeding locations known, south to 57° 45’ N (Campbell et al. 1989).

Quebec. S. Quebec, about 15 pairs known; Ungava, about 35 pairs known; Hudson’s Bay coast and nw. islands, about 5–10 pairs known; n. Quebec, estimated population > 1,000 birds (M. LaPage pers. comm.). Most of Hudson Bay islands and much of mainland unsurveyed.

Labrador. 10–12 known pairs; estimated population much higher; surveys not conducted specifically for this species (J. Brazil pers. comm.).

Greenland. Koskimies (2006a) estimates 500-1,000 pairs, though many areas remain unsurveyed.

North America. Approximately 3,400 to 4,300 nesting pairs, based on estimates above; 2,925 to 3,875 more recently estimated (Potapov and Sale 2005). No information on size or status of non-breeding population.

Worldwide. Former estimate of 15,000–17,000 pairs (Cade 1982) too high based on overestimated range of 15–17 million km²); recent country by country estimates yield total of 7,880 to 10,900 breeding pairs (Potapov and Sale 2005). No information on non-breeding population.

Trends

No evidence of long-term population changes in North America (Fyfe and Grier 1972, Cade 1982, Mossop and Hayes 1994, Shank and Poole 1994, Swem et al. 1994), except for s. coast of Labrador and adjacent Quebec, where Gyrfalcons may have been more common breeders during the Little Ice Age, which did not end until mid-1800s (Audubon 1897, Townsend and Allen 1907); however, most of Nearctic range has not been surveyed or monitored. Some historical losses noted in Scandinavia (Cade et al. 1998), but see Koskimies (2006b).

Population Regulation

Breeding population size limited by presence of suitable nest sites and sufficient prey (Shank and Poole 1994). Size of breeding populations fluctuates widely among years (Swartz et al. 1975, Platt 1977, Nielsen 1986, Mossop and Hayes 1994, Swem et al. 1994). Population changes irregular, i.e., not cyclic in some areas (e.g., Alaska, Mindell et al. 1987, Mindell and White 1988, Swem et al. 1994) but cyclic in others (e.g., Yukon, Mossop and Hayes 1994). Size of breeding population correlated with ptarmigan numbers in most populations (Mossop and Hayes 1982, 1994, Nielsen 1986). In Iceland, total number of Gyrfalcons present in late summer and number of occupied territories were correlated with ptarmigan density with a 2 and 3-year time lag, respectively (Nielsen 1999). Reproductive success of individual nests mimics trend in population size (i.e., higher when occupancy is higher) in some areas (Mossop and Hayes 1994) but not in others (Mossop and Hayes 1982, Nielsen 1986, Shank and Poole 1994, Swem et al. 1994). Conflicting trends may reflect geographic variation in temporal stability of ptarmigan populations (Mossop and Hayes 1982) or availability of alternative prey (Mossop and Hayes 1994).

Specialization of Gyrfalcons on ptarmigan does not appear to influence ptarmigan population levels in some regions (Gudmundsson 1972). In Iceland, however, Gyrfalcon and ptarmigan numbers regularly fluctuate in a 10-yr cycle (Nielsen and Pétursson 1995), and Gyrfalcons influence the ptarmigan cycle by accelerating population declines, accentuating the amplitude of the cycle, and affecting the duration of the low periods of the cycle (Nielsen 1999). This suggests Gyrfalcon predation causes the ptarmigan population cycles in Iceland (Inchausti and Ginzburg 2002) and likely influences the cycles in Sweden (Nyström et al. 2006).

Reproductive success and timing are related to weather (Nielsen 1986, Poole and Bromley 1988b), but weather is not correlated directly with size of breeding population (Poole and Bromley 1988b). Geographic trends in population density correlate with higher summer temperatures and taller willows, which may reflect relative productivity of habitat (Shank and Poole 1994) and availability of winter cover for ptarmigan. Although sizes of local breeding populations vary annually, there is no indication that the Gyrfalcon population as a whole responds in similar manner.

Although Gyrfalcons have breeding requirements similar to those of Peregrine Falcons, Rough-legged Hawks, Golden Eagles, and Common Ravens, there is no evidence that interspecific competition influences size of Gyrfalcon breeding populations or their reproductive success (Cade 1960, Poole and Bromley 1988a). Conversely, these other species provide potential nest sites for Gyrfalcons. Sites where pairs depend on stick nests may be occupied less frequently than ledge sites because young Gyrfalcons destroy much of nest, requiring a renesting attempt by other species to maintain it (Burnham and Mattox 1984). Use of some nest sites by Gyrfalcons and other species in alternate years has been observed in Alaska (White and Cade 1971, Swem et al. 1994) and NWT (Poole and Bromley 1988a). Intraspecific competition may be important; in NWT, Gyrfalcons nesting close to each other have lower reproductive success than pairs nesting farther apart (Poole and Bromley 1988a).