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Little information exists on the movements of Gyrfalcons Falco rusticolus outside the breeding season, particularly amongst High Arctic populations, with almost all current knowledge based on Low Arctic populations. This study is the first to provide data on summer and winter ranges and migration distances. We highlight a behaviour previously unknown in Gyrfalcons, in which birds winter on sea ice far from land. During 2000–2004, data were collected from 48 Gyrfalcons tagged with satellite transmitters in three parts of Greenland: Thule (northwest), Kangerlussuaq (central-west) and Scoresbysund (central-east). Breeding home-range size for seven adult females varied from 140 to 1197 km2 and was 489 and 503 km2 for two adult males. Complete outward migrations from breeding to wintering areas were recorded for three individuals: an adult male which travelled 3137 km over a 38-day period (83 km/day) from northern Ellesmere Island to southern Greenland, an adult female which travelled 4234 km from Thule to southern Greenland (via eastern Canada) over an 83-day period (51 km/day), and an adult female which travelled 391 km from Kangerlussuaq to southern Greenland over a 13-day period (30 km/day). Significant differences were found in winter home-range size between Falcons tagged on the west coast (383–6657 km2) and east coast (26 810–63 647 km2). Several Falcons had no obvious winter home-ranges and travelled continually during the non-breeding period, at times spending up to 40 consecutive days at sea, presumably resting on icebergs and feeding on seabirds. During the winter, one juvenile female travelled over 4548 km over an approximately 200-day period, spending over half that time over the ocean between Greenland and Iceland. These are some of the largest winter home-ranges ever documented in raptors and provide the first documentation of the long-term use of pelagic habitats by any falcon. In general, return migrations were faster than outward ones. This study highlights the importance of sea ice and fjord regions in southwest Greenland as winter habitat for Gyrfalcons, and provides the first detailed insights into the complex and highly variable movement patterns of the species.
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During 2000–2004, 56 platform transmitter terminals (PTTs; Microwave Telemetry Inc., Columbia, MD, USA, and North Star Science and Technology, LLC, Baltimore, MD, USA) were fitted to Gyrfalcons in the Kangerlussuaq, Thule, Scoresbysund and Maniitsoq study areas, units weighing either 18 or 30 g. Falcons were captured using a bow net, dho-gaza net or a lure pole/bow net combination (for further description see Meredith 1961, Bub 1978). All PTTs were battery powered and attached as backpacks using Teflon ribbon (Fuller et al. 1995). Duty cycles were programmed to transmit from as frequently as 4 h on/27 h off to 7 h on/106 h off, depending on battery life. GPS locations were taken at all nest-sites or capture locations where birds were tagged. Each PTT recorded information on location, battery voltage, temperature and activity. Location data were used for tracking falcons, while temperature, battery voltage and activity sensor data were used to determine whether the PTT was functioning correctly.
Of the 56 PTTs, 29 were placed on adult Gyrfalcons (one adult was recaptured and fitted with a new unit, so n = 28 adults tagged), 22 on juveniles and four on nestlings (Table 1). In Kangerlussuaq, all units were placed on individuals at the nest in June and July. In Thule, all juveniles were tagged at a ringing station, while some adults were tagged at the nest and others at a ringing station between July and September. At Scoresbysund and Maniitsoq, all individuals were captured at ringing stations between September and November while presumably on outward migration. All PTTs weighed <3% of the body weight of the Gyrfalcons tagged. Individuals were identified by their five-digit Argos PTT ID numbers.
Table 1. Number of PTTs deployed and the location, sex and age of individual Gyrfalcons tagged in Greenland during 2000–2004.
Falcon movements were followed using the Argos satellite system (http://www.argos-system.org), which provides locations with an associated estimate of accuracy (location class, LC) based on the quality of the signal. Location class is divided into seven categories (in descending order of accuracy, LC = 3, 2, 1, 0, A, B and Z), Argos suggesting estimated accuracies of < 150, 150–350, 350–1000 and > 1000 m for LC 3, 2, 1 and 0, respectively. However, accuracy levels appear to be less than reported (Britten et al. 1999, Hays et al. 2001). In their study, Burnham (2008) collected data from a stationary PTT in Thule for 13 consecutive months and found average accuracy levels of: LC 1 = 0.9 km, LC 2 = 1.3 km, LC 3 = 2.3 km and LC 0 = 11.0 km. Data from PTTs were analysed using ArcView GIS 3.3 and Spatial Analyst (Environmental Systems Research Institute, Redlands, CA, USA) and the Animal Movement Extension designed for it (Hooge & Eichenlaub 2000). Sea ice maps were provided by the National Ice Center, National Oceanic and Atmospheric Administration, and figures were produced in ArcGIS 9.2. Maximum sea ice extent is shown in all sea ice figures. Areas along the ice edge commonly ranged from 10 to 50% ice cover, with the percentage usually increasing nearer land. Individual locations from PTTs were visually inspected to verify likely accuracy based on other locations from the same day (Fuller et al. 1998).
When adults were tagged at the nest-site, breeding home-ranges included all points obtained from PTT attachment, usually during rearing of young, until departure for a pre-migration home-range or for outward migration (Ganusevich et al. 2004). For adults tagged with PTTs during outward migration or while already on winter home-range, breeding home-ranges included all points obtained the following breeding season between the completion of return migration to the nest-site and departure for outward migration. Although in some instances it was not possible to verify that individuals bred, based on movements and dates it was possible to make inferences. Breeding home-ranges were estimated for Gyrfalcons with > 20 locations with LC 3–1 and described using 90% minimum convex polygons (MCPs) and fixed 50 and 95% kernels. We calculated a 36.6-km2 90% MCP ‘home-range’ size for the stationary PTT in Thule (LC 3–1 = 1006 locations), suggesting that PTT accuracy using LC 3–1 is sufficient to provide estimates of home-range size. If breeding home-range estimates included areas that encompassed glaciers or the Greenland Ice Cap, these areas were included because prey species have been shown to cross the Greenland Ice Cap both during migration and in the breeding season (Alerstam et al. 1986).
The start of migration was defined as the date at which Gyrfalcons began continuous movement from the breeding home-range in the general direction of likely wintering areas or vice versa (Berthold 2001). Migration distances were measured as Great Circle Distances (GCDs; WGS 84 datum) and the total length of routes taken by Gyrfalcons was calculated by summing the lengths of the individual flight segments along the migration route, beginning at the location of the nest, capture site or pre-migration area, depending upon the individual. Although not suitable for breeding home-range estimation, LC 0 was included for description of migratory movements and time spent on winter home-range based on the long distances travelled and time intervals (Britten et al. 1999, Green et al. 2002). The starting points for flight segments were chosen by taking the location with the highest quality location class, LC 3–0, from each duty cycle/transmission period (Fuller et al. 1998). If multiple locations with the same LC were available, the first to occur in the transmission cycle was used (Fuller et al. 1998). The overall speed of outward and return migration was determined by dividing the total of the segment lengths by the number of days spent on migration. Data from Falcons which appeared still to be on outward migration when their PTTs stopped functioning have been included as ‘incomplete migrations’ if they either completed more than 10 days of outward migration or travelled more than 250 km.
Departure dates from breeding areas could be determined accurately only for those Gyrfalcons tagged at the nest (n = 13) or which departed the nest after being earlier tagged at a ringing station. Falcons captured and tagged at ringing stations in Maniitsoq and Scoresbysund were possibly from further north and already on migration. Several Gyrfalcons that were tagged at ringing stations appeared to be already on their winter home-range, and partial outward migrations were not recorded. Additionally, two individuals tagged at ringing stations continued to have long-distance movements throughout the entire winter period, and were not included with outward migration data.
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Previous research using PTTs on Gyrfalcons is limited. Klugman et al. (1993) calculated an MCP breeding home-range size of 589 km2 for an adult female in Kangerlussuaq and McIntyre et al. (2009) described the dispersal patterns of 15 juveniles from nest-sites in Alaska. Data from our study provide the first detailed examination of movement patterns of both adults and juveniles outside the breeding season. The results revealed the first records of Gyrfalcons leaving Greenland, to both Canada and Iceland, and documented the largest winter home-ranges sizes ever calculated for raptors, including large areas over the open ocean and sea ice, far from land.
Home-range sizes of breeding Gyrfalcons in Kangerlussuaq, Thule and Ellesmere Island appeared to be similar, with no apparent differences between areas or sexes despite apparent differences in prey availability. In Thule, tens of millions of seabirds (Salomonsen 1950, Boertmann et al. 1996, Egevang et al. 2003) were available as prey within kilometres of almost all Falcon nest-sites, and a small breeding home-range was expected as a result. In Kangerlussuaq, limited numbers of land-based prey were observed close to Gyrfalcon nests, and a larger home-range was expected as a result of Falcons having to travel generally further for food. However, Rock Ptarmigan numbers have been shown to fluctuate cyclically throughout the Arctic (Gudmundsson 1960, Weeden & Theberge 1972, Watson et al. 1998, Moss & Watson 2001), including Greenland (Salomonsen 1950, Vibe 1967), and we do not know at what point in this cycle the local population was during our study. If marked changes in Rock Ptarmigan densities occurred in the Kangerlussuaq area, the size of Gyrfalcon breeding home-ranges could have been affected.
Data were recorded for two individuals for the entire approximate 5-month period they were on their breeding home-ranges. The range size for adult female 35243, which bred in Thule, increased in size each month, probably as a result of the growth and level of independence of young, as described by Newton (1979, 1986) for other raptor species. However, for adult male 35248 from northwest Ellesmere Island, range size decreased through the breeding period. Based on PTT locations the nest was located in a large fjord. Early in the season prey were probably scarce in this area, with adult Ptarmigan and adult Hares Lepus arcticus the only food sources, but as pack ice broke up other land-birds and seabirds would have become increasingly available, allowing the male to make shorter and closer foraging trips from the nest.
On average, adult Gyrfalcons departed Thule on outward migration on 21 September (n = 8) while juveniles departed on 23 September (n = 10). Using a calculated average fledging date of 2 August, juveniles spent a post-fledging period of approximately 7 weeks in the Thule area before departure (Burnham 2008). In Alaska, McIntyre et al. (2009) calculated an average departure date of 27 August for 15 juveniles tagged with PTTs, almost a month earlier than in Thule, with an average post-fledging period of 6 weeks, again shorter than in Thule. Part of the difference in departure dates of juveniles between the two areas results from latitudinal variation in timing of breeding, with the Alaska study being more than 12° further south. In addition, the high late season abundance of prey near where the juveniles were captured in Thule may have allowed them to further delay migration by up to several weeks. In particular, large numbers of Black Guillemots Cepphus grylle were observed, which Salomonsen (1950) considered to be the last bird species to depart the High Arctic on outward migration, remaining until sea ice formed in October or November.
Although it appears that adults departed within days of juveniles, there was no evidence that adults and young travelled together on outward migration. Even though large numbers of juveniles congregated and were observed together at the ringing station in Thule (up to 10), PTT data suggest that they travelled individually. However, at the Scoresbysund ringing station up to 13 different juveniles were caught in a single day, with several caught within an hour. Similar occurrences were reported by Manniche (1910) for northeast Greenland, who reported, ‘Often 4 to 5 individuals would appear at one time…circling around the mast-heads…watching for pigeons’. Whether this was a result of juveniles travelling in groups is unknown, and it is equally possible that large numbers of Falcons were travelling through the area independently of one another and were attracted to the ringing station by other individuals stooping/diving at the decoys.
Gyrfalcons tagged in Kangerlussuaq, Thule and Scoresbysund all migrated in a southward direction. Several individuals tagged in Maniitsoq appeared already to be on winter home-range, and it was not possible to determine if they were migrants. In Thule, Gyrfalcons most frequently crossed the ocean to Canada before turning south, although routes over the ocean and along the west coast of Greenland were also used. This was the first documentation of Gyrfalcons from Greenland migrating along the east coast of Canada. Todd (1963) described Gyrfalcons migrating annually along the east coast of Labrador, but believed that these birds were from Canada, doubting that they regularly crossed the ocean. Falcons crossing to Canada and travelling south along the east coast of Ellesmere Island, Devon Island and Baffin Island probably preyed upon the large number of seabirds that occur throughout this area (McLaren & Renaud 1982, Lepage et al. 1998). The lone Falcon that did not depart to the south was a juvenile male from Scoresbysund, which flew to the east, the last location being nearly equidistant between Iceland and the Faeroe Islands.
Falcons from Thule frequently made reverse migrations or even stopped for weeks at a time while on outward migration, with one individual reversing its migration over 550 km northwards. Although reverse movements or migration have been documented and described in a number of bird species (Åkesson et al. 1996, Berthold 2001), these are probably the longest recorded. In the Kangerlussuaq, Scoresbysund and Maniitsoq areas Falcons similarly travelled up and down the coast for many hundreds of kilometres.
The average speed of outward migration for Gyrfalcons ranged from 4 to 99 km/day (n =33). However, these numbers include incomplete outward migrations and might represent only small portions of longer journeys. For the three individuals that completed outward migrations from their breeding home-ranges, the speed of travel decreased with latitudinal nesting location, from 83 to 60 to 30 km/day. With so few complete outward migrations documented, it is difficult to make any general statements comparing one area with another. In the Thule area, and based on limited samples, it appears that males migrated faster than females irrespective of age, although further research in this regard is needed.
Distances travelled by the three Gyrfalcons with completely recorded outward migrations from breeding home-ranges in Ellesmere Island, Thule and Kangerlussuaq varied from 391 to 4234 km. Although the Falcons from Ellesmere Island and Thule wintered only about 125 km apart, the Falcon from Ellesmere Island took a more direct route, and despite nesting over 500 km northwest of Thule, it travelled approximately 1100 km less than the Thule bird on outward migration. Furthermore, the Thule bird spent almost twice as many days on outward migration (71 vs. 38) than the Ellesmere Island bird, despite having less far to travel. Although both these birds completed outward migration from the most northern study areas, they did not spend the most days on outward migration. An adult female tagged in Thule spent 122 days on outward migration with her PTT failing while still travelling south. This pattern was also apparent in other individuals for which we recorded incomplete outward migrations, and it appears that the number of days spent on outward migration and the distance travelled were not correlated, although too few data were collected to be sure.
Winter home-ranges of Gyrfalcons varied widely within Greenland, with birds tagged in Scoresbysund having significantly larger winter home-ranges than those on the west coast. Gyrfalcons tagged in Scoresbysund used extensive areas over the open ocean, sea ice and along the ice edge during winter months, with one Falcon spending up to 40 days offshore. Similar behaviour was not observed in west coast birds except during migration. On both coasts, movements while on winter range were not always in one direction, and could have been described as ‘pursuing’ or weather-related behaviour, as found in other migrants (Berthold 2001).
On the west coast, Gyrfalcon movement patterns varied from individuals having small and stationary winter home-ranges to continually moving up and down the coast for hundreds of kilometres. Some Falcons had large winter home-ranges that completely overlapped those with much smaller ranges. Additionally, some individuals had up to four winter ranges, spending between a few weeks and 3 months in an area before moving on.
Winter home-range sizes for Gyrfalcons on the east coast of Greenland were the largest so far documented for raptors, with only the Lesser Spotted Eagle Aquila pomarina having a reported winter home-range (25 000 km2) that approached the size of our estimated ranges for Gyrfalcons (Meyburg et al. 1995, 2004). Although ranges of individual Falcons frequently overlapped on the east coast, they did not use their entire winter home-range each month, but they did continually travel over long distances. Calculated minimum distances travelled by three Falcons while on winter home-range on the east coast were 5201 km for Falcon 49762 and 3864 km for Falcon 49773, with average daily speeds of travel of 28 and 21 km/day, respectively. Falcon 49764 travelled an even greater average distance each day, at 30 km/day. If data were available for every day these distances may have been much greater, as demonstrated for the Ivory Gull Pagophila eburnea (Gilg et al. 2010).
Two Gyrfalcons did not have typical winter ranges and were not included in winter home-range analyses. A juvenile female that was tagged in Scoresbysund spent most of the winter over the ocean and sea ice, almost continually moving, and occasionally used the east coast of Greenland and northwest coast of Iceland. An adult male tagged in Maniitsoq continued to move south along the coast throughout the entire winter, with multiple winter ranges or use areas.
It is likely that prey availability determines the movements of Gyrfalcons both during migration and while on winter home-range. While conducting surveys for ‘sea-associated’ birds in the Davis Strait and southern Baffin Bay during March 1981 and 1982, Mosbech and Johnson (1999) observed 16 Gyrfalcons on the sea ice. Falcons were observed along the coast and up to 300 km out to sea, frequently perched on or flying near large icebergs, with open water in the vicinity. They speculated that the Falcons were hunting Black Guillemots, which were using the open water around large icebergs to feed. It seems probable that Gyrfalcons regularly use sea ice and icebergs to rest on and hunt from during migration and while on winter home-range.
Many millions of seabirds and sea ducks winter or pass through the fjords and offshore areas of southwest Greenland and Iceland (Salomonsen 1950, Brown 1984, Durinck & Falk 1996, Merkel et al. 2002, Boertmann et al. 2004, Barrett et al. 2006). When compared with data from wintering Harlequin Ducks Histrionicus histrionicus and Common Eiders Somateria mollissima tagged with PTTs, Gyrfalcons tagged during this study overlapped within the same fjord regions in southwest Greenland (Mosbech et al. 2006, Chubbs et al. 2008). This combination of an abundance of seabirds and sea ducks provides wintering Gyrfalcons with a potentially rich food source, especially with their apparent ability to spend long periods of time living on the sea ice.
The large observed differences in winter range size amongst Gyrfalcons on the west coast probably result from some birds being territorial there. Individuals with small winter home-ranges, which probably have an abundance of food, are dominant and drive off other Falcons. Individuals that are not able to establish small winter home-ranges are left to wander, possibly being pushed from one area to another and relying upon a wider variety of prey, and thereby achieving larger winter home-ranges. In particular, juveniles may face more frequent movements than adults; with the single juvenile which provided data all winter covering a larger area than any of the adults, a pattern similar to that described by Marquiss and Newton (1982) for adult and juvenile Eurasian Sparrowhawks Accipiter nisus. In extreme situations, juveniles may even be forced out over the open ocean or sea ice (depending upon time of year), or even longer distances, such as to Iceland (e.g. Fig. 4, tag ID 49768). Although the ocean is home to large numbers of seabirds, these populations are likely to be scattered, with Falcons having to frequently make long daily flights. When compared with individuals with small home-ranges along the coast, which appear to forage over short distances and rest on protected cliff faces, these Falcons probably expend much more energy. It could be such individuals that occasionally turn up well south of the usual range in winter, including the northeastern USA and the British Isles.
The extreme difference in winter home-range size between Gyrfalcons on the east and west coast is probably a result of prey abundance and movement patterns. As can be seen in Figures 2 and 3, southwest Greenland remains ice-free throughout the winter, allowing seabirds and sea ducks continuous access to the shore and fjords to feed. On the east coast, sea ice gradually builds throughout the winter, eventually to encompass the entire coastline all the way to the southern tip of Greenland. As a result, seabirds are pushed towards the ice edge and sea ducks are probably driven to open coastline elsewhere (e.g. southwest Greenland or Iceland). As the amount of sea ice on the east coast increases, seabirds and Gyrfalcons are pushed greater distances from shore. Furthermore, as sea ice conditions are continually changing, sea birds must continually move along the ice edge, with Falcons accompanying them over long distances. Gilg et al. (2010) reported a similar pattern for Ivory Gulls, which followed the ice edge along the east coast of Greenland from July to December.
Perhaps the only other predator to behave in this way is another arctic nester, the Snowy Owl Bubo scandiacus, in which some satellite-tracked individuals spent up to 3 months on the sea ice (Therrien et al. in press). Snowy Owls also frequently travel long distances in winter, probably facing many of the same difficulties as Gyrfalcons in this harsh environment (Fuller et al. 2003, Therrien et al. in press). Individual Falcons probably adjust to environmental conditions as they find them, their daily movements reflecting a continually shifting prey supply.
Complete return migrations were recorded for six Gyrfalcons and generally appeared to be faster than outward migrations. Falcons commonly used the ice edge, probably as a result of the greater density of prey, with the timing and route of return migration for Falcons 35243 and 35248 very similar to those of other seabirds along the east coast of Canada (Tuck 1971, McLaren 1982, Renaud et al. 1982). The rapid speed of return migrations is probably influenced by the narrow breeding window available for Gyrfalcons in the Arctic, with Falcons that nest further north generally travelling faster and having a shorter breeding window than those that nest further south (e.g. Peregrine Falcons Falco peregrinus, Burnham 2008). A similar difference between spring and autumn migration speeds has been recorded in some other bird species, but not in all raptors (Newton 2008).
With limited light, at times as little as a few hours per day, and temperatures frequently below −20 °C, winter months in the Arctic are severe. To survive, Gyrfalcons must have a continual food supply that can be depended upon and captured in a relatively short period of time. While during the breeding season Falcons can spend numerous hours hunting in an area (24 h of daylight), periodic long-distance pursuing movements are necessary during winter in order to find large and abundant food sources that can be captured quickly. Although written to describe other migrants, Newton’s (2003) statement that migrants ‘mobile lifestyle enables them to exploit short-lived food-supplies at different places at different times, as they occur’ seems to fit the situation in Gyrfalcons. This ‘mobile lifestyle’ is probably the key factor that enables Gyrfalcons to survive the harsh winter weather during the non-breeding season in the Arctic.