GYRFALCON DIET: SPATIAL AND TEMPORAL VARIATION

EUGENE POTAPOV

Bryn Athyn College, Bryn Athyn, PA 19009, USA. E-mail: [email protected]

ABSTRACT.—The diet of the Gyrfalcon (Falco rusticolus) has been studied throughout the range (Potapov and Sale 2005). The species tends to be stenophagous in the majority of the range, espe- cially at the beginning of the breeding season. At this time, the diet is limited to the organisms which live on the snow surface, such as ptarmigan and . Most important for Gyrfalcons are two sympatric species of grouse, namely Willow Ptarmigan ( lagopus) and Rock Ptarmi- gan (L. muta). Although the ranges of these two grouse species mostly coincide, in some areas only one species is present; e.g., only Rock Ptarmigan in , and only Willow Ptarmigan in large areas of the flat of Russia. In the areas where both species are present, the Gyrfalcon tends to prefer Willow Ptarmigan. The timing of Gyrfalcon breeding in continental, non-coastal tundra is linked to the beginning of the grouse display. The timing of breeding in the coastal areas is linked to the arrival of waterfowl or seabirds, and varies from site to site. Seasonal changes in the ecosystem, such as snow melt, arrival of migratory , arousal from hiber- nation, and ice breakup on rivers and lakes, make more food available to the , and their diet diversifies. The Gyrfalcon is known to breed in the areas of the with cyclical abundance of such as , ground squirrels, and . In such areas, hares dominate the diet at the early stages of breeding since the others remain under the snow and are unavailable to the falcons. After Gyrfalcon chicks hatch, there is a variety of birds available for them as food, such as waterfowl, and . All play some part in the diet, the composition of which reflects habitat surrounding the nest site. Received 31 May 2011, accepted 6 June 2011.

POTAPOV, E. 2011. Gyrfalcon diet: Spatial and temporal variation. Pages 55–64 in R. T. Watson, T. J. Cade, M. Fuller, G. Hunt, and E. Potapov (Eds.). Gyrfalcons and Ptarmigan in a Changing World, Volume I. The Peregrine Fund, Boise, Idaho, USA. http://dx.doi.org/10.4080/gpcw.2011.0106

Key words: Gyrfalcon, Falco rusticolus, diet, prey, availability, season.

THE GENERAL RULES OF ECOLOGY propose that instability, especially if climate change reduces a predator which is dependent on just a single the availability of its primary prey, ptarmigan. species as a food source is vulnerable to local extirpation or even extinction. Conversely, This paper is a general review of the literature diversity of prey enhances stability of the pred- data on the Gyrfalcon diet with some addition ator. In this paper, I describe the diet of the of personal experience with the species in Gyrfalcon and its variation in space and time, North-East . Since we summarized the and speculate that the lack of prey diversity papers on the Gyrfalcon’s diet (Potapov and leaves the Gyrfalcon vulnerable to population Sale 2005), there have been a few new studies

55 –POTAPOV – published (Nystrom et al. 2006, Ganusevich and (Muir and 1984, Booms 2006, Gilyazov 2006, Labutin 2006) which are and Fuller 2003a,b), a large portion of the diet also incorporated into this review. Here we consisted of the (Lepus arcticus). analyze available published data by calculating The Gyrfalcons brought to the nests only Shannon’s diversity index young hares, which were delivered in parts. In s Eurasia, the Mountain Hare (L. timidus) was ( ′= – ) H ∑pi*1n pi mentioned in the diet of Gyrfalcons in Scandi- i=1 navia, Komi, Yamal and Yakutia (Huhtala et al. in the diets of Gyrfalcon. Theil’s index 1996, Hagen 1952, Kalyakin 1989, Kalyakin (T'=ln(Nspecies)-H' or the maximum possible and Vinogradov 1981, Kishinskiy 1958, diversity (log(N)) minus Shannon's diversity Koskimies and Sulkava 2002, Langvatn and index (Theil 1979) is used as a measure of the Moksnes 1979, Labutin 2006, Mikkola and lack of diversity. Theil’s index is increasingly Sulkava 1972, Pulliainen 1975, Shklyarevich used in economics (Haughton and Khandker and Krasnov 1980, Voronin 1987). The overall 2009) to interpret the inequality in distribution role of this species in the diet of Eurasian Gyr- of various parameters (e.g., wealth, shares, falcons was, however, much lower than that in resources, population). In biological studies, it North America, and rarely exceeded 4%. measures the distance between the observed distribution of the species of prey in a diet and In some regions, Gyrfalcons took various a theoretical uniform distribution of the prey ducks, waders and birds, as well as species. Samples of less than 50 items were small mammals. Lemmings and ground squir- excluded from the analysis. rels in particular were mentioned in some stud- ies in Arctic Canada (Muir and Bird 1984), Studies of the Gyrfalcon’s diet were tradition- Kola (Kishinskiy 1958), Alaska (Cade 1960) ally performed at the end of the breeding sea- and Karyakia (Kishinskiy 1980). son, or at the late stage of breeding, when large chicks were present in the nests. Frequent vis- The overall diversity index of prey in the diet its to nests were often impossible due to the of the Gyrfalcon varied from 0.42 to 2.36, vulnerability of the species, difficulty of reach- average 1.31±0.569 (SD), N=28 (Table 1). ing nesting areas, and short duration of the field season. During the early stages of nesting, The Theil index (lack of diversity index) in the especially at first breeding displays and egg diet of Gyrfalcon (T’) varied from 0.10 to 2.98, laying, researchers did not approach the nests average 1.231±0.628, N=28. to avoid disturbance. These constraints resulted in skewed diet data which mostly rep- INCUBATION PERIOD resented only the ‘developed brood’ stage. There were only two studies of the Gyrfalcon’s diet in the early stage of the breeding season DIET VARIABILITY ACROSS (the incubation period between egg laying and THE GYRFALCON’S RANGE hatching) in Iceland (Nielsen and Cade 1990) Several studies of the Gyrfalcon’s diet were and Komi, Russia (Voronin 1983). As an early carried out in various localities. The majority season breeder, the Gyrfalcon relied on a very of them were done during the breeding period limited number of available prey species. In (Table 1). The overall diet of the Gyrfalcon is most of its range the most important prey summarized in Figure 1. The Lagopus grouse species was either Willow or Rock Ptarmigan. dominated the diet in the majority of localities. The breeding range of the Gyrfalcon partially There were, however, some notable excep- coincides with the range of the hare: Arctic tions. In some places, such as Arctic Canada Hare in the Canadian High Arctic and Green-

56 –GYRFALCON DIET – mulative klyarevich and Krasnov 1980 klyarevich Suetens and Groenendael 1976 Suetens and Groenendael Gilyzov 1991 Gortchakovskaya 1945 Gortchakovskaya 1945 Small Passeri- Lepus Water- Charadrii- Lagopus Diet of the Gyrfalcon, expressed as percent composition of prey groups, in various parts of the species’ range. All data are cu in various parts of the species’ range. All data are groups, composition of prey as percent Diet of the Gyrfalcon, expressed Kharlov Island, Kola 12.8 87.2 0.60 0.50 39 Dementiev and Table 1. Table period. for most of the breeding LocalityN FinlandAlaska, USA NorwayTrondelag, Inari, N FinlandNorthern Finland CanadaCentral Arctic Kola, RussiaSalla, NE FinlandAlaska (all data combined), USA 82.2 Fjell, NorwayDovre sp. 34.5 90.9 85.5 RussiaSouthern Yamal, 73.5 80.7 0.4Enotekio, NW Finland 72.2 fowl RussiaYamal, 2.8 1.8 1.1 1.7Syatteivis, Komi, Russia 3.7 formes 2.1 1.1Bolshaya Rogovaya, Komi, Russia 51.2 38.3 39.9 2.0 0.8 7.9Iceland (Compiled) sp. 82.0 94.9 1.9 6.8Iceland mammals 0.4 5.1 78.3 1.2 formesIceland 15.5 2.2 Others 11.5 0.3 86.3 3.5 Island, CanadaEllesmere 4.6 4.0 H'Lower Kolyma, Russia 23.4 7.2 4.2 13.9 0.8 1.6Finnish lappland 43.9 6.5 3.9 57.2 4.4Lapland Reserve, Russia 13.9 T' 41.5 5.9 1.6 0.5 2.9 1.9 0.7 1.3 1.7 1.6 15.6 2.3Koryakia, Russia 3.9 12.6 N 3.1 4.4 48.2 5.5 1.1 0.3Bolshoy Litzkiy Island, Kola, Russia 36.0 0.9 31.8 72.4 1.4 13.6 Author 3.3 2.1 3.7 0.2 9.4 0.4 37.3 0.96 0.5 2.6 1.8 8.5 1.9 62.1Kharlov Island, Kola, Russia 2.18 7.2 0.87 6.7 19.0 1.20 3.3 4.4 3.1 1.46 67.2 1.2 90.4 27.1 1.25 0.51 478 0.8 2.98 1.93Khibin mtn, Reserve, Russia 10.4 1.58 11653 1.01 Langvatn and Moksnes 1979 30.4 5.6 5.9 2.9 1.79 2.44 8.6 880 Koskimies and Sulkava 2002 Alaska, USA 20.3 2.9 24.7 615 3.0 4.3 44.6 1.88 3.8 2.12 273 618 Poole and Boag 1988 Alaska Alaska, Colville, Seward, 79.7 Cade 1960 2.26 2.36range compiled, USA Mikkola and Sulkava 1972 White and Cade 1971 161 100.0 6.2 1.62 38.2 48.8 Denmark 0.2 1.69 Greenland, West 1.40 8.7 Huhtala et al. 1996 17.9 1.9 4.5 508 5.8 7.7 1.7 YakutiaTykkah, 697 594 Huhtala et al. 1996 0.77 RussiaOzhogina, Yakutia, 4.1 Kishinskiy 1958 0.8 1981 Kalyakin and Vinogradov 0.81 6.0 1.01 3.0 1.4 1.18 DenmarkGreenland, 3.4 35.0 1.49 24.9 1.39 1.75 3.8 50 216 0.2 2.4 69 14.3 1.24 1987 Voronin Hagen 1952 21.1 0.61 25.0 1.66 0.4 3294 Huhtala et al. 1996 2.12 Kalyakin 1989 1.18 0.97 4.0 1.14 23.7 13.9 811 58.3 44 30.8 3.0 0.3 1.7 554 1984 Muir and Bird 1987 Voronin 43.7 51.0 Bengtson 1971 44.4 7.9 42.9 6.0 1.65 8.4 0.9 1.7 1.70 0.66 14.7 1.13 1.22 18.4 1.52 1.01 20.5 59 8.4 126 1.08 1.37 0.93 10.5 2.19 Semenov-Tjan-Shanskiy and Sh 67 322 105 0.58 4.2 1980 Woodin Potapov unpubl, 1987-1995 0.42 Pulliainen 1975 117 0.97 37.4 Dementiev and 16.7 1.00 52 1.09 22.7 44.0 16.7 0.80 18.4 0.52 157 0 1.75 32.0 1.,92 Kishinskiy 1980 84 0.59 0.44 0.16 Cade 1955 0.10 4.0 832 24.0 18 2.53 126 Booms and Fuller 2003 0.67 Labutin 2006 Semenov-Tjan-Shanskiy 1959 0.36 1.53 36 24 White and Spinger 1965 1.07 Labutin 2006 0.03 25 1974 Summers and Green

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Figure 1. Overall diet composition of the Gyrfalcon during the breeding period. Updated figures from Potapov and Sale 2005 with additions.

land, and Mountain Hare in Eurasia. In some ber of prey species available to Gyrfalcons areas this species also was the only quarry increase considerably (Figures 2 and 3). The available for Gyrfalcons. Lagopus species dominated in the diet in the majority of the studies (Table 1), but the local diversity of prey in various regions varied sig- NESTLING PERIOD nificantly. In some areas, like Yamal Peninsula, The majority of the diet studies of Gyrfalcon Russia, North Finland, or Myvatn Lake, Ice- covered the nestling period of the breeding land, ducks played an important role (Bengt- cycle when chicks were in the nest. Most of son 1971, Kalyakin and Vinogradov 1981, the studies used analysis of prey remains and Kalyakin 1989). In other localities, like Kola pellets accumulated at the nests; some authors, (Russia) or Salla (Finland), the waders were however, supplemented this kind of data with very important (Huhtala et al. 1996, Kishinskiy direct observations. Voronin (1987) observed 1958). Close to the fledging period, a new shift a Gyrfalcon nest from 13 June to 12 July 1982 in the diet occurred: the young of the passer- from a hide. He reported that the Gyrfalcons ines start fledging, and being an easy target, brought two ptarmigan per day. Nevertheless, they immediately appeared in the diet of the apart from the Willow Ptarmigan, which Gyrfalcons. clearly dominated in the diet (86.2%), he reported that the diet contained Long-tailed POST-FLEDGING DEPENDENCE PERIOD Duck Clangula hyemalis (3.9%), Ruff Philo- machus pugnax (2%), Wood Sandpiper Tringa After leaving the nests, the chicks tend to stay glareola (3%), Water Arvicola amphibius together, while the parents still deliver food to (3.9%), and Muskrat Ondatra zibethicus (2%). them (Potapov and Sale 2005, Kalyakin 1981). During this stage of the Gyrfalcon’s breeding A study in Iceland (Nielsen and Cade 1990) cycle, the snow melts, ptarmigan stop display- registered a sharp increase in the number of the ing and start to lay eggs, and the summer waders and waterfowl in the diet in this period migrants arrive en-mass. As a result, the num- of the breeding cycle.

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Figure 2. Number of species in the diet of Gyrfalcons from the Lower Kolyma, Russia.

Figure 3. Breeding cycle of the Gyrfalcon (black lines, bottom), Lagopus grouse (grey lines, middle) and Arctic migrants (white lines, top). The polar night lines marked on the chart are equivalent to a latitude of around 70°N.

WINTER urophasianus (Garber et al. 1993), ducks (Dekker and Court 2003), and occasionally Kokhanov (1970) studied the winter diet of carrion and poultry (Tømmeraas 1988, 1989). Gyrfalcons in the Kola, Russia, a locality within the species’ breeding range. He men- tioned that the most important prey of Gyrfal- DISCUSSION cons were bird species common in a seabird The average prey species diversity in the diet colony where the Gyrfalcons were wintering: of Gyrfalcon (H’) was 1.31, which is lower Thin-billed Murre Uria aalge, Common compared to generalist predators. For example Somateria mollissima and King S. spectabilis Eagle Owl (Bubo bubo) diet had a Shannon Eiders, Atlantic Puffin Fratercula arctica, and diversity index of 1.935–2.325 (Penteriani et Northern Fulmar Fulmarus glacialis. In Ice- al. 2005). The average Theil index (1.28) was land, ptarmigan played an important role, with comparable to the Shannon index. This means some addition of waders in the coastal areas that the diet is skewed towards minimal num- (Nielsen and Cade 1990). While wintering out- ber of species and the Gyrfalcon tends to be a side the breeding range, Gyrfalcons fed on specialized predator. It is stenophagous at the medium- and large-sized birds, such as early stages of the breeding cycle as only few pigeons and partridges (Potapov and Sale prey species are available at the breeding 2005), Sage Grouse Centrocercus

59 –POTAPOV – ground at this time. These are mostly Willow and proved to be unbiased. A detailed study in and Rock Ptarmigan, followed in some locali- Iceland (Nielsen and Cade 1990) provided ties by the Arctic Hare. enough resolution by a relatively high fre- quency of nest visits and records of kills, as There is a marked difference in the utilization well as by a large enough sample size. of hare by Gyrfalcons of the New and Old World. The latter consumed many fewer hares It appears that the overall specialization of the than the North American and Greenland Gyr- Gyfalcons on ptarmigan is evident from March falcons. The use of Arctic Hare by Gyrfalcons to early June, when the falcons are strictly of the New World was not uniform across the stenophagous (see the timeline in Figure 3). At range. In some places, the proportion of hare in this stage, Gyrfalcons consumed the same pro- the diet was comparable to that of ptarmigan portion of ptarmigan males and females. Both (Booms and Fuller 2003a,b), but in other cases, sexes at this time are predominantly white and hares and other mammals dominated the diet snow cover is present. In May, Willow Ptarmi- (Muir and Bird 1984). These differences are gan males get their reddish neck coloration, evidently caused by the distribution of the Arc- while the male Rock Ptarmigan get their black tic Hare and its habitat preferences. The range stripe connecting their eye and . There of this species, which prefers to live in open was no marked difference in the abundance of areas without any shrub vegetation (Hoffman male and female ptarmigan in the Gyrfalcon’s and Smith 2005), coincides with the Gyrfal- diet at this stage either. In June, female ptarmi- con’s breeding range only in Arctic Canada and gan molt into summer plumage and keep a low Greenland. The abundance of hare changes profile while incubating. The males still con- from year to year, and in some years, its local tinue to display from time to time, and tend to density is high enough to support Gyrfalcons. be visible. At this stage the overall importance The question remains how Gyrfalcons manage of ptarmigan in the Gyrfalcon’s diet declines, to kill and deliver to their nests such a large as was clearly demonstrated by the study in prey item. Booms and Fuller (2003a) demon- Iceland (Nielsen and Cade 1990); however, the strated that, in Greenland, Gyrfalcons delivered Gyrfalcons’ predation pressure on male to the nests only small hares, and most fre- ptarmigan increases proportionally, as was quently only part of them. In all other places, shown in Komi Republic (Voronin 1983) and the hare overlaps with the Gyrfalcon’s breeding Canada’s North West Territories (Cotter et al. range only in shrubby tundra and forest-tundra, 1992). The proportion of other species, such as where its availability is somewhat limited. waders, ducks, and passerines in this period, increases in the diet. The majority of studies of the Gyrfalcon’s diet were cumulative, i.e., the pellets and prey Nielsen (1999) found that in the spring, species remains were picked up after the chicks had diversity in the diet of Gyrfalcons in Iceland left the nest, or were close to fledging. This showed a statistically significant decline when suggests that the feathers and other remains of ptarmigan population density increased. This small prey items could be blown away and means that if ptarmigan numbers are high, therefore underrepresented in the total count. there is no need for Gyrfalcons to spend time Small passerines seem to be underrepresented and effort hunting other species. In years of in the diet mostly for this reason. It is only pos- low ptarmigan density, the diet becomes more sible to eliminate such bias by visiting the nest diverse as the Gyrfalcons switch to other frequently (but at the risk of disturbing the species, mostly waders and ducks. This is birds) or to set up an automated imaging sys- especially visible in the diverse coastal habitats tem. Such a system was deployed in the diet (Nielsen and Cade 1990). In contrast, Huhtala study in Greenland (Booms and Fuller 2003c) and co-authors (1996) found no correlation

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