Falco Rusticolus) in Relation to Climate Change
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BIOLOGICAL TRAITS OF THE GYRFALCON (FALCO RUSTICOLUS) IN RELATION TO CLIMATE CHANGE TOM J. CADE The Peregrine Fund, 5668 West Flying Hawk Lane, Boise, ID 83709, USA E-mail: [email protected] ABSTRACT.—Largest of the true falcons (Falco spp.), the Gyrfalcon (Falco rusticolus) is the north- ernmost diurnal raptor with a circumpolar breeding distribution restricted to subarctic and arctic zones between 55º and 82º N. Some Gyrfalcons migrate south into north temperate zones in win- ter, but others remain in northern latitudes wherever suitable prey occurs. A review of the Gyrfal- con’s ecological relationships and what is known about its population history reveals some vulnerability to the potential effects of climate change on arctic environments, but also some pos- sible mitigating adjustments. The Gyrfalcon relies on two ptarmigan species (Lagopus spp.) for 50-90% of its diet biomass, so it is likely that an effect of climate change on Gyrfalcons will be mediated through impacts on ptarmigan. The Gyrfalcon has trophic relations with other birds and mammals that may allow for adjustment to reduced availability of ptarmigan. The Gyrfalcon’s main prey has fluctuated drastically in numbers from year to year; in peak food years a maximum number of pairs nest, but in years with low prey abundance, few breed. Under climate change the 10-year population cycles of ptarmigan and hares and the 3-4 year cycles of microtine rodents exhibit lower peaks and less regularity. Whether these population changes will persist and what they portend for predators needs study. Historically the Gyrfalcon has been the earliest nesting raptor in the Arctic. Climate change is lengthening the arctic summer, but it is unclear how Gyr- falcon breeding phenology will be influenced by this change. It could be advantageous in spring and autumn by allowing new trophic relationships. Interspecific relations with other raptors nest- ing in the Arctic may be influenced by climate change. The Gyrfalcon and Peregrine Falcon (Falco peregrinus) are potential competitors for nesting sites and food. In West Greenland, where nesting peregrines have increased dramatically in the past 50 years and Gyrfalcons have decreased, it has been suggested that the increasing number of peregrines may be forcing gyrs to abandon territories by interference competition over nest-sites or by feeding so persistently on ptarmigan during the breeding season that numbers surviving through winter are insufficient to induce gyrs to lay. The Gyrfalcon and Saker Falcon (Falco cherrug) are allopatric populations of closely related groups of falcons. If climate change were to alter biomes so that breeding Gyrfalcons and Saker Falcons come together, interbreeding and extensive introgression of genes likely would occur. Such hybridization might help Gyrfalcons adapt to changed ecological conditions resulting from global warming. The Gyrfalcon has a long historical association with mankind. Captive propagation now provides many Gyrfalcons for falconry. This technique could provide offspring for replenishing wild populations, should the need arise. Wild Gyrfalcons use various, manmade structures for nesting, and this habit offers a possibility to establish pairs in areas with a good food supply but no natural nest-sites. Received 17 February 2011, accepted 31 May 2011. 33 –CADE – CADE, T. J. 2011. Biological traits of the Gyrfalcon (Falco rusticolus) in relation to climate change. Pages 33–44 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.2011.0104 Key words: Gyrfalcon, ptarmigan, distribution, abundance, arctic ecology, climate change. LONG ADMIRED FOR its variable plumage rang- northern proto-Gyrfalcons in a refugium from ing from nearly white to dusky gray and for its the southern sakers. Subsequent redistributions prowess as a hunter of grouse, the Gyrfalcon and isolations of gyr populations provided (Falco rusticolus) is the northern-most diurnal opportunities for local differentiations of white raptor with a circumpolar breeding distribution and dark birds respectively in refugia located restricted to subarctic and arctic zones from in northwestern Greenland and Ellesmere about 55ºN to 82ºN. Although some Gyrfal- Land and in coastal Labrador, while the ances- cons move southward into north temperate tral gray gyrs remained in temporally shifting zones in winter, others, particularly adult habitats south of the main ice masses in Eura- males, remain in subarctic and low arctic sia and North America (Johansen 1956, Palmer regions wherever suitable prey such as ptarmi- 1988, Flann 2003, Cade 2006). Following the gan and marine birds can be found. Adult end of the last glaciations Gyrfalcons with females and juveniles wintering farther south these geographically differentiated colors have switch from ptarmigan to waterfowl, shore- come together and interbred but have not yet birds, corvids, pigeons, and other grouse such achieved panmixia, thus producing the confus- as Sharp-tailed (Tympanuchus phasianellus) ing mix of plumage variations we see today. and Sage Grouse (Centrocercus urophasianus) in North America (Cade et al. 1998, Booms et These hypotheses based on morphology and al. 2008). historical biogeography appear to be consistent with some of the emerging findings of molec- ular phylogenetics of the Hierofalco group of PHYLOGENETIC RELATIONSHIPS AND ORIGIN falcons, and of other avian species, but not The Gyrfalcon likely evolved from a Saker with all of this work. For example, Wink et al. Falcon (Falco cherrug) or saker-like ancestor (2004) found genetic distances of 0.4% to after tundra biotas became established follow- 2.0% among the four falcon species in Hiero- ing the first major glaciation at the Pliocene- falco and estimated that this modest degree of Pleistocene boundary around 2 million years differentiation would have occurred in a period BP, although another idea is that this split from 200,000 to 1,000,000 years BP. They between the two kinds of falcons did not occur further noted that in other families of birds until after the last glaciation around 10,000 such small genetic differences indicate taxo- years BP (Nittinger et al. 2005, Potapov and nomic distinction at no more than the level of Sale 2005, Cade 2006). As first proposed by subspecies. Also, estimates of species diver- Johansen (1956), the Gyrfalcon may have sep- gence times for other groups of birds, based on arated from its ancestral stock at the end of the molecular systematics and “clocks,” indicate long Mindel-Riss interglacial period some 200 that most recent speciation occurred from 1 to to 300,000 years BP, when extensive steppes 5 million years ago (early Pleistocene to connected Central Asia with arctic grasslands, Pliocene) and that late Pleistocene isolations allowing falcons and many other open land- caused by glaciation occurred too recently to scape organisms to extend their ranges north- account for speciation events (Klicka and Zink ward. Renewed glaciations then separated the 1997). These findings are in line with the view 34 –GYRFALCON IN RELATION TO CLIMATE CHANGE – that the Gyrfalcon and saker are allopatric sub- eyries in West Greenland has revealed some species of one polytypic species or “allo- interesting details about these changes over the species” of one Hierofalco superspecies [see past 2,500 years (Burnham et al. 2009). also Potapov and Sale (2005) for historical views of O. Kleinschmidt, A. Kots, and R. RECENT CHANGES IN DISTRIBUTION Meinertzhagen]. Historically recent changes in the southern The more recent idea that the proto-Gyrfalcon limits of the Gyrfalcon’s breeding range may became isolated from the ancestral saker only be associated with lesser climatic events since after the last major glaciation some 10,000 the last major glaciation, such as the “Little Ice years BP (Potapov and Sale 2005) has been Age” from ca. 1300 to 1870 A.D. Two nesting supported by recent molecular phylogenetic pairs of Gyrfalcons were reported from the studies, which also emphasize the small degree coast of southern Labrador, a “black” pair and of genetic differentiation between Gyrfalcons young at the mouth of the Bras d’Or River, and sakers (Nittinger et al. 2005, 2007, Daw- Quebec, at 51º28’N in 1833 (specimens col- nay et al. 2007, Johnson et al. 2007). The “out- lected there were the basis of Audubon’s of-Africa” hypothesis of Nittinger et al. that a famous painting and description of the Black North African Lanner Falcon (Falco biarmi- Gyrfalcon Falco labradora, Audubon 1834); a cus) population was the ancestral source for all “light colored” pair also nested “for many of the Hierofalco “species” seems plausible, years” in the late 1800s at Henley Harbour, but the further suggestion that the Gyrfalcon 52º01’N (Townsend and Allen 1907, Todd and Saker Falcon were independently derived 1963). Based on several government reports from ancestral Lanner stock makes no sense (e.g., Goudie et al. 1993), currently Gyrfalcons from a comparison of phenotypic similarities apparently do not breed farther south in and differences within the Hierofalco group or Labrador than Harp Lake at about 55ºN inland from zoogeographic considerations of their from Hopedale. There is also a recent record of breeding ranges. These genetic studies confirm nesting on Long Island along the eastern shore the close relatedness of the saker and Gyrfal- of Hudson Bay at 54º53’N (Brodeur et al. con with detected genetic differentiation no 1995). The two nesting records from extreme greater than that of subspecies in many other southern Labrador may represent the last of a polytypic species of birds. The fact that none breeding group of Gyrfalcons that had of the molecular genetic studies has so far extended their range down the Labrador coast shown a complete separation of the Saker Fal- during the Little Ice Age, when arctic condi- con and Gyrfalcon indicates that they should tions existed further south than they do today.