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Gyrfalcon Diet: Spatial and Temporal Variation

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Gyrfalcon Diet: Spatial and Temporal Variation 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 hare. Most important for Gyrfalcons are two sympatric species of grouse, namely Willow Ptarmigan (Lagopus 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 Iceland, and only Willow Ptarmigan in large areas of the flat tundra 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 birds, ground squirrel arousal from hiber- nation, and ice breakup on rivers and lakes, make more food available to the falcons, and their diet diversifies. The Gyrfalcon is known to breed in the areas of the Arctic with cyclical abundance of mammals such as lemmings, ground squirrels, and hares. 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, passerines and waders. 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 Siberia. 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 Greenland (Muir and Bird 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 Arctic Hare (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 passerine 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 Table 1. Diet of the Gyrfalcon, expressed as percent composition of prey groups, in various parts of the species’ range. All data are cumulative for most of the breeding period. Lagopus Water- Charadrii- Lepus Small Passeri- Locality sp. fowl formes sp. mammals formes Others H' T' N Author N Finland 85.5 1.1 7.9 0.3 0.8 2.3 2.1 0.87 2.98 11653 Koskimies and Sulkava 2002 Alaska, USA 90.9 1.8 0.8 4.4 0.3 1.8 0.51 2.44 618 White and Cade 1971 Trondelag, Norway 82.2 0.4 2.1 0.4 4.6 5.9 4.4 0.96 2.18 478 Langvatn and Moksnes 1979 Inari, N Finland 80.7 3.7 6.8 1.9 3.7 3.1 1.01 1.88 161 Huhtala et al. 1996 Northern Finland 72.2 1.1 5.1 4.0 13.9 1.1 2.6 1.25 1.79 273 Mikkola and Sulkava 1972 Central Arctic Canada 73.5 1.7 1.9 3.5 6.5 12.6 0.2 1.20 1.93 880 Poole and Boag 1988 Kola, Russia 38.3 2.2 13.9 0.7 36.0 3.3 5.6 2.26 1.69 697 Kishinskiy 1958 Salla, NE Finland 51.2 1.2 23.4 1.6 5.5 6.7 10.4 2.12 1.62 508 Huhtala et al. 1996 Alaska (all data combined), USA 34.5 2.8 2.0 57.2 3.1 0.5 1.46 1.58 615 Cade 1960 Dovre Fjell, Norway 94.9 0.5 0.9 1.9 1.9 0.81 1.49 216 Hagen 1952 Southern Yamal, Russia 39.9 15.5 4.2 1.3 31.8 4.4 2.9 2.36 1.40 594 Kalyakin and Vinogradov 1981 Enotekio, NW Finland 78.3 7.2 2.9 1.4 7.2 2.9 1.01 1.39 69 Huhtala et al. 1996 –G Yamal, Russia 43.9 15.6 9.4 1.2 20.3 8.7 0.8 1.75 1.24 3294 Kalyakin 1989 Syatteivis, Komi, Russia 86.3 3.9 3.9 5.9 0.61 1.18 44 Voronin 1987 YRFALCON DIET Bolshaya Rogovaya, Komi, Russia 82.0 11.5 1.6 1.6 3.3 0.77 1.18 50 Voronin 1987 57 Iceland (Compiled) 41.5 48.2 8.5 0.2 1.4 0.2 2.12 1.14 554 Bengtson 1971 Iceland 62.1 30.4 6.2 0.3 0.9 1.52 1.37 322 Woodin 1980 Iceland 90.4 3.8 5.8 0.42 0.97 52 Suetens and Groenendael 1976 Ellesmere Island, Canada 1.7 0.4 27.1 24.7 38.2 4.1 3.8 1.66 0.97 811 Muir and Bird 1984 – Lower Kolyma, Russia 72.4 19.0 8.6 1.01 0.93 105 Potapov unpubl, 1987-1995 Finnish lappland 67.2 3.0 17.9 3.0 3.0 6.0 1.22 1.08 67 Pulliainen 1975 Lapland Reserve, Russia 37.3 0.8 14.3 4.0 43.7 1.70 1.13 126 Semenov-Tjan-Shanskiy and Gilyzov 1991 Koryakia, Russia 44.6 4.5 51.0 1.00 0.80 157 Kishinskiy 1980 Bolshoy Litzkiy Island, Kola, Russia 13.6 79.7 1.7 3.4 1.7 1.65 0.66 59 Shklyarevich and Krasnov 1980 Kharlov Island, Kola 12.8 87.2 0.60 0.50 39 Dementiev and Gortchakovskaya 1945 Kharlov Island, Kola, Russia 4.3 7.7 35.0 30.8 1.7 20.5 2.19 0.58 117 Dementiev and Gortchakovskaya 1945 Khibin mtn, Reserve, Russia 100.0 0.59 0.10 126 Semenov-Tjan-Shanskiy 1959 Alaska, USA 25.0 13.9 44.4 16.7 2.53 0.36 36 White and Spinger 1965 Alaska, Colville, Seward, Alaska 48.8 6.0 2.4 42.9 1.09 0.52 84 Cade 1955 range compiled, USA West Greenland, Denmark 24.9 0.4 14.7 37.4 22.7 1.75 0.44 832 Booms and Fuller 2003 Tykkah, Yakutia 58.3 8.4 8.4 4.2 16.7 0 4.0 0.67 1.53 24 Labutin 2006 Ozhogina, Yakutia, Russia 21.1 23.7 7.9 18.4 10.5 18.4 1.,92 0.16 18 Labutin 2006 Greenland, Denmark 44.0 32.0 24.0 1.07 0.03 25 Summers and Green 1974 –POTAPOV – Figure 1.
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