ECOLOGY AND BIOLOGY OF GYRFALCONS IN GREENLANDKURT K. BURNHAM1,2,3 AND WILLIAM A. BURNHAM2,^
1

Edward Grey Institute of Field Ornithology, Department of Zoology, University of Oxford,
South Parks Road, Oxford, OX1 3PS, UK
2

The Peregrine Fund, 5668 West Flying Hawk Lane, Boise, ID 83709, USA

3High Arctic Institute, 603 10th Avenue, Orion, IL 61273, USA (current address).
E-mail: kburnham@higharctic.org^

Deceased

ABSTRACT.—We studied two breeding populations of Gyrfalcons (Falco rusticolus) approximately
1,200 km apart in Greenland. During 1998–2006, we collected data from a population in the central-west (Kangerlussuaq), and during 1994–2006 and 2008–2010 from a population in the northwest (Thule). Gyrfalcons in Kangerlussuaq bred approximately 18 days earlier than those in Thule,with egg laying, on average, starting on 17 April and 5 May, respectively. Reproduction was similar between areas, with no significant difference between the number of young produced at occupied or successful nests. Years in which the Kangerlussuaq population bred later showed fewer
successful nests, while the lay-date advanced significantly for the Thule population during thisstudy. Diet varied between areas, with the more coastal population in Thule feeding primarily on
Dovekies (Alle alle) and the more inland Kangerlussuaq population feeding primarily upon RockPtarmigan (Lagopus muta) and passerines. Plumage color in Thule was > 99% white, while nearly
equal numbers of white and grey adults were observed in Kangerlussuaq. No changes in plumage
color were observed in Kangerlussuaq from the 1950s onward; however, an unexplained difference in plumage color between sexes was documented during this study. The Gyrfalcon populationin Thule appeared relatively stable, whereas the Kangerlussuaq breeding population was likely
cyclical, and possibly declining since the early 1990s. Based on the relatively long-term data setsavailable, these populations are ideal for continued monitoring to determine what effects futureclimate change may have on Gyrfalcon populations. Received 6 April 2011, accepted 20 July 2011.
BURNHAM, K. K., AND W. A. BURNHAM. 2011. Ecology and biology of Gyrfalcons in Greenland.
Pages 1–20 in R. T. Watson, T. J. Cade, M. Fuller, G. Hunt, and E. Potapov (Eds.). Gyrfalcons
and Ptarmigan in a Changing World, Volume II. The Peregrine Fund, Boise, Idaho, USA.
http://dx.doi.org/10.4080/gpcw.2011.0209
Key words: Gyrfalcon, Greenland, Peregrine Falcon, nesting chronology, cyclic population, prey,
reproduction.

1

– BURNHAM AND BURNHAM –Greenland and Russia lag far behind” thatfrom other locations.

GYRFALCONS (Falco rusticolus) have historically been reported breeding along much ofGreenland's ice-free coastlines from PearyLand (83° N: Manniche 1910, Koch 1925,
Johnsen 1953) to Kap Farvel (60° N: Rafn
1933, Salomonsen 1950) at the southern tip(Figure 1). Most records were of sightings
made and specimens collected prior to 1950.
In the Zoological Museum, Copenhagen, Denmark, there are more than 429 Gyrfalcon specimens that were collected in Greenland alone,primarily to investigate variation in plumage
color. Gyrfalcons were commonly shot and/or
noted during polar expeditions and collecting
trips (e.g., Sabine 1819, Walker 1860, Hall1876, Hagerup 1891, Stone 1895, Chapman
1899). Explorers and collectors documented
locations where falcons were observed breeding or migrating and helped to establish therange and migration timing of the species inGreenland.

To address this lack of knowledge, we collectedlong-term data from both Low and High Arctic
breeding populations in central-west (Kangerlussuaq) and northwest (Thule) Greenland,
respectively (Figure 1). We compared data
between areas and considered historical data
where available. We analyzed changes inbreeding biology over time to look for potential
shifts possibly associated with climate change.
METHODS

We studied populations of breeding Gyrfalcons
in central-west (Kangerlussuaq, 66.5–67.5° N)
Greenland from 1994–2006 and northwest(Thule, 75.9–77.6° N) Greenland from 1994–
2005 and 2006–2010 (Figure 1). These locations are geographically separated byapproximately 1,170 km with limited or noice-free land existing between the two sites.

Beyond these records from early explorers, little information exists on Gyrfalcons in Greenland other than the occasional anecdotal
mention in the literature and observations collected on individual nests or over small portions of a breeding season (e.g., Fletcher andWebby 1977, Fowles 1981, Cabot et al. 1988,
Booms and Fuller 2003). Not until the early
1970s did investigations of Gyrfalcon ecology
in Greenland begin in the Low Arctic, andeven then, the Gyrfalcon was of secondaryinterest to the Peregrine Falcon (Falco peregrinus) (Burnham and Mattox 1984). Only
during one four-year period (1990–93) did
systematic surveys for nesting Gyrfalcons takeplace (central-west Greenland); however,results from these surveys were difficult tointerpret (W. Mattox, unpubl. data, M. Yates
unpubl. data). In High Arctic Greenland (~70°
N or more, see Bliss 1979), summer information was restricted to notes made by early
explorers, birding surveys/expeditions, or single seasons spent observing particular nests(e.g., Stone 1895, Cabot et al. 1988). In areview article about the Gyrfalcon, Cade et al.
(1998) commented that “fresh data fromThe Kangerlussuaq study area is considered
Low Arctic and is located in one of the largest
ice-free areas within Greenland. The area isbisected by numerous lakes, rivers, streams,
and rolling valleys (Figure 2). Vegetation is
primarily sedges and grasses, with willow(Salix) up to 2 m in height. Small passerines,
for example, Lapland Longspurs (Calcarius
lapponicus), Snow Buntings (Plectrophenax
nivalis), Northern Wheatear (Oenanthe
oenanthe), and others, occur in abundance,with Rock Ptarmigan (Lagopus muta), waterfowl, and shorebirds also present. For additional information on the Kangerlussuaq
study area, see Burnham and Mattox (1984)
and Burnham (2008).
Further north, the Thule study area is almost
entirely coastal, with the largest ice-free margin
extending approximately 26 km from the sea.
The area is considered to be High Arctic, withsparse, prostrate vegetation. Cliffs are almost
always sea-facing (Figure 2). The most numerous bird species in the Thule area is the2

– GYRFALCON BIOLOGY IN GREENLAND –Peary Land

Thule
Study Area
BaffinBay
Uummannaq
Area
Disko
Bay
Kangerlussuaq
Study AreaN
Kap Farvel

0 km
600 kmFigure 1. Map of Greenland showing study areas

Figure 2. Photos of the Kangerlussuaq (A) andand locations mentioned in the text.

Thule (B) study areas showing typical habitat.

Dovekie (Alle alle), which has an estimatedbreeding population of 33 million pairs in theThule area (Egevang et al. 2003). Additionally,
Thick-billed Murres (Uria lomvia), Blacklegged Kittiwakes (Rissa tridactyla), and Black
Guillemots (Cepphus grille) occur in large
numbers, with lesser numbers of passerines andRock Ptarmigan. For further information on theThule study area, see Burnham (2008).

search, individuals with first-hand knowledgeof birds in Thule were contacted. Locations
identified by local hunters and those found in
the literature were first surveyed, with additional nesting locations found during systematic surveys of the coastline.
Occupied sites were those where one or moreterritorial adults and/or young were observed.
To be considered successful, one or morechicks had to be observed. If only adults were
present, they had to be seen on multiple visits
to the site or to act territorially toward
researchers to be considered as occupants. In
the case of breeding, nests were rappelled or
climbed into, prey remains were identified, andyoung were banded. Age of young was determined by counting backwards from the esti-

In the Kangerlussuaq area, most Gyrfalcon
nest sites were previously located by Burnham
and Mattox (1984), M. Yates (unpubl. data),
and W. Mattox (unpubl. data), with most discovered incidentally while surveying for Peregrines. New nest sites were also located during
this study while traveling between known nesting sites. In addition to an historic literature3

– BURNHAM AND BURNHAM –mid-June when young were usually around 20
days old. Surveys in Thule typically occurredfrom mid-to-late July through mid-August,when young were frequently at least 30 days
old and in some cases had already left the nest.
Surveys in Thule were usually conducted byboat, and due to the late break-up of ocean
pack-ice, earlier surveys were not possible.

mated average age of nestlings or fledglings.
We assumed a four-egg clutch with an egg laid
every other day, with incubation beginning on
the third egg and a 35-day incubation period(C. Sandfort pers. comm., Platt 1977). Pooleand Bromley (1988) reported that males
fledged at 45–47 days and females at 47–50
days. We considered 47 days as an average
fledging age.

Surveys and Occupancy: Kangerlussuaq.—
From 1998–2006 (n = 9), 51–96% (mean =80%) of known Gyrfalcon nesting locations
were checked for occupancy in Kangerlussuaq(Table 1). During this period territorial Gyrfalcons were documented on 99 occasions and an
additional 17 new nesting sites were located,giving a total of 74 known sites. The occupancy rate at previously known nest sites
ranged from 10–27% (mean = 17%). When
including new nest sites, a high of 15 sites
were occupied in 1999 and 2000, and a low of
five in 2003 (Table 1, Figure 3).

While multiple nests on the same section ofcliff (up to 0.5 km long) were considered to be
the same nest site it is apparent that some Gyrfalcon nesting territories have alternate nest
sites. Seven territories in Kangerlussuaq had
multiple adjacent cliffs with nest sites (one tofour) that were never used simultaneously during the entire study. This makes a strong casethey were alternative nest sites within the same
territories, although this cannot be proven withcertainty as the adults were not banded. If true,
the total number of known nesting territories
available for breeding would be greatly
reduced and nest sites could be grouped
together by territory for analyses.

While it is possible that sites with single, nonbreeding pairs, or failed pairs of Gyrfalcons
were missed, most locations where reproduction occurred were probably discovered. From
1998–2000, a limited number of nest sites surveyed by helicopter were also surveyed byother falcon researchers on foot during thesame period. In total, 51 known Gyrfalcon
nests were surveyed by both methods, withresults between helicopter and foot surveys
being the same.

Depending upon weather conditions, we visited nests with breeding Gyrfalcons more thanonce per season to record the number ofnestlings/fledglings. However, remote locations and inclement weather frequently precluded follow-up visits. We banded Gyrfalcons
with a standard Danish numbered lock-on
band on one leg and an alpha-numeric band
(ACRAFT, Edmonton, Canada) on the other.

Little information is available on the failure rateof Gyrfalcon nests in Kangerlussuaq during thisstudy. However, in 1999 and 2000, surveys ofknown Gyrfalcon nest sites were conducted in
the early spring (April, when egg laying
occurred) in an attempt to determine the proportion later failing, with eight and six nests
found occupied in the two years, respectively.
Of those nests found occupied in the early
spring, four were no longer occupied at a repeat
survey in June 1999, and one was no longer
occupied at a repeat survey in June 2000. Eggs
were not observed at any of these nest sites in

RESULTS

Surveys and Occupancy.—Surveys for breeding Gyrfalcons took place from 1998–2006 in
Kangerlussuaq and from 1993–2006 and2008–2010 in Thule. Results from Kangerlussuaq and Thule surveys should be consideredminimal estimates, with some nests likely failing before they were checked the first time,
resulting in a bias towards successful nests.
Surveys in Kangerlussuaq were primarily conducted by helicopter and occurred in early-to-4

– GYRFALCON BIOLOGY IN GREENLAND –Table 1. Nest site occupancy by Gyrfalcons in Kangerlussuaq, Greenland, from 1998–2006.
Year

Known
Gyrfalcon
nest sites

Known
sites
checked (%)

Known sites
occupied whenchecked (%)

Total sites
occupied (newsites found)

1998

57

43 (75%)7 (16%)

10 (3)

10

1999

60

54 (90%)11 (20%)15 (4)

13

2000

64

55 (86%)10 (16%)15 (5)

15

2001

69

59 (86%)

11 (19%)13 (2)

12

2002

71

37 (51%)

10 (27%)10 (0)

6

2003

71

47 (64%)5 (11%)

5 (0)

3

2004

71

59 (81%)9 (15%)

10 (1)

7

2005

72

71 (96%)7 (10%)

9 (2)

9

2006

74

69 (91%)

12 (17%)12 (0)

915

Figure 3. The number ofoccupied (open circles)
and successful (filledsquares) Gyrfalcon nest
sites in Kangerlussuaq,
Greenland, from 1998–
2006.

12

Nest Sites

Total
successful
sites

9

6

3

0

1998

2000

2002

2004

2006

Year

Surveys and Occupancy: Thule.—Surveys forbreeding Gyrfalcons took place from 1993–2006 and 2008–2010 (n = 17 years) in Thule,with territorial pairs of Gyrfalcons documented
77 times at 25 locations (Table 2). With theaddition of a new research boat in 2001, ourcoastal survey area approximately tripled.
From 2001 onward, our survey area remained
relatively constant, with annual variation in
nest sites checked for occupancy based oninclement weather and summer sea ice.

the early spring and, although adults appeared
to be incubating in some instances, pairs may
have moved to other nests in the vicinity. In
1999 and 2000, four nest sites that were notoccupied in April were found to be occupied bybreeding pairs of Gyrfalcons when they werere-checked in June. This would be expectedfrom the large range of egg laying dates
observed in Kangerlussuaq (>40 days in someyears, see later section on nesting chronology).
Based on these poor and inconclusive results,
and safety concerns for falcons, early springsurveys were discontinued.
5

– BURNHAM AND BURNHAM –Table 2. Nest site occupancy by Gyrfalcons in Thule, Greenland, from 1993–2006 and from 2008–2010.
Year

Known
Gyrfalcon
nest sites

Known sites
checked
(%)

1993

–

–

1994

–

1995

Known sites
occupied whenchecked (%)

Sites observed
occupied (newsites found)

Observed
successful
sites

–

–

0

–

–

(1)

1

1

1 (100%)1 (100%)

2 (1)

2

1996

2

1 (50%)

1 (100%)

1 (0)

0

1997

2

2 (100%)2 (100%)

2 (0)

2

1998

2

2 (100%)2 (100%)

2 (0)

1

1999

2

2 (100%)1 (50%)

3 (2)

2

2000

4

3 (75%)

3 (100%)

3 (0)

2

2001

4

4 (100%)1 (25%)

4 (3)

4

2002

7

7 (100%)6 (86%)

8 (2)

8

2003

9

4 (44%)

1 (25%)

2 (1)

2

2004

10

10 (100%)

7 (70%)

9 (2)

9

2005

12

12 (100%)

6 (50%)

11 (5)

8

2006

17

16 (94%)5 (31%)

5 (1)

5

2008

18

16 (89%)6 (38%)

8 (2)

7

2009

20

16 (80%)5 (31%)

7 (2)

7

2010

22

14 (64%)5 (36%)8 (3)8being used for nesting by Gyrfalcons, and inalmost all instances, previously used ledges or
potholes were within several kilometers.

During surveys for nesting Gyrfalcons inThule, few if any occupied nests were likely tohave been missed. Adults and juveniles were
almost exclusively bright white in plumage andin general were obvious to the unaided eyewhen observed on cliffs, even at long distances.
From 1993 to 2003, no obvious use of alternative nest sites occurred, and all breeding took
place on ledges or in potholes. Beginning in
2004 and continuing to 2010, it appeared likelythat pairs began using alternate nesting sites
within breeding territories; however, adults
were not ringed and it was not possible to determine the specific movements of individuals.
During this period, the use of Northern Raven(Corvus corax) nests by Gyrfalcons in theThule area was observed for the first time, withmultiple raven nests used in most years after
2004. The increase of new nesting sites after
2004 was primarily a result of these raven nests

Results from the Thule area provide the firstdata on the local Gyrfalcon population, but
despite 17 years of research, only limited conclusions can be drawn on occupancy. Not until
2001 did surveys begin on a large enough scale
to provide data on a number of nests each year.
It appears that the Thule population was relatively stable, apart from 2003 and 2006, when
only two and five occupied sites were found,respectively. The indicated low number ofoccupied nests in 2003 resulted from fewer
previously known nest sites surveyed (seeTable 2), and it does not appear that the population was peaking or crashing at any point
during this study.

6

– GYRFALCON BIOLOGY IN GREENLAND –Table 3. Nesting chronology of Gyrfalcons in the Kangerlussuaq (1998–2006) and Thule (1994–2010)
study areas, Greenland. Hatch-dates were calculated from estimated age of young observed in thenest (see text), with data included from 66 nests in Kangerlussuaq and 40 in Thule.
Kangerlussuaq mean (min–max, median)Thule mean (min–max, median)Lay-date

17 Apr (28 Mar–9 May, 18 Apr)

5 May (24 Apr–24 May, 4 May)

Hatch-date

26 May (6 May–17 Jun, 27 May)

13 Jun (2 Jun–2 Jul, 12 Jun)

Fledge-date

12 Jul (22 Jun–3 Aug, 13 Jul)

30 Jul (19 Jul–18 Aug, 29 Jul)

121

A

116

Julian date

Occupancy of nest sites is difficult to compare
between Kangerlussuaq and Thule. Within the
Kangerlussuaq area, the majority of Gyrfalcon
nest sites were documented by others prior tothis study, which provided us with significant
overall knowledge of the study area. In Thule,we had no previous knowledge of nest site
locations, and each year required new andextended surveys.

111

106

101

96
1998

1999

2000

2001

2002

2003

2004

2005

2006

Year

B

136

131

Julian dateReproduction: Kangerlussuaq.—During 1998–
2006, a minimum of 236 young were produced
at the 99 occupied Gyrfalcon nest sites locatedin Kangerlussuaq (2.38 young per occupiedsite). At successful sites (n = 84), where at least
one chick was observed, an average of 2.81
young were produced, with the number ofyoung per nest ranging between one and five.

126

121

The numbers presented here are minima as it
was frequently impossible to ascertain the total
number of young. In some cases, chicks were
too young, too near fledging, or cliffs were too
overhung for the nest to be entered safely.
When young were small, less than one week
old, adult females frequently did not leave thenest when approached by helicopter. Observations were thus limited to a mass of nestling
down under the front of the female’s breast, orin some cases, nothing was visible under thefemale, in which case, we noted the female as
incubating/brooding but did not count the nest
as successful. To some extent, this explains thedifference between occupied (n = 99) and successful (n = 84) nests, with some nests having
not actually failed, but reproduction being
unknown. Given the wide range of nesting
dates (some years up to 42 days between nests,

116
1994

1996

1998

2000

2002

2004

2006

2008

Figure 4. The mean (±SE) of hatching dates

recorded in Kangerlussuaq (A) from 1998 to2006 and in Thule (B) from 1994–2006 and2007–2010. No data available from Thule for
1996, 1998, and 2007.

7

2010

– BURNHAM AND BURNHAM –of successful nests found each year and theaverage date that breeding was initiated; fewer
pairs of Gyrfalcons produced young in years oflate breeding (Figure 5). No significance wasfound between the total number of occupied
nests and timing of breeding (P > 0.05, r2 =0.27). In Thule, from 1994–2010 (no data for
1996, 1998, and 2007), the average lay-date
for the Gyrfalcon population significantly
advanced (P < 0.05, r2 = 0.32, Figure 6), withthe average lay-date for the final year of thestudy being six days earlier than for the entire
17-year study period as a whole (29 April vs.
5 May).

see section below on nesting chronology), thesenumbers should be viewed with caution, as a
sizeable portion of chicks were very youngwhen their age was noted and some proportion
likely did not survive to fledging. Numbers
given here for productivity should not be interpreted as numbers of chicks fledged.
Reproduction: Thule.—Using the same criteria
we used for sites being occupied and successful in the Kangerlussuaq area, we located 77
occupied territories in Thule that produced
over 179 chicks, for an average of at least 2.38
young per occupied nest. At the 69 successful
sites, an average of 2.63 young were produced.
The number of young produced at individual
nests ranged from one to five, with young overhalf-grown at all but one of the successful nest
sites when first visited. Young in Thule wererarely banded, with young being too nearfledging to safely enter the nest, or already flying in many cases. No females were observed
incubating during surveys in Thule.

In general, Gyrfalcons in Kangerlussuaq
showed a much wider range of dates during
which reproduction occurred. In Kangerlussuaq, fledging dates spanned a 43-day period(22 June–3 August) compared to a 31-day
period in the more northern Thule area (19July–18 August) (Table 3). In one instance in
Kangerlussuaq, two nests were visited on thesame day (within 20 km of each other), with
one nest containing pipped/hatching eggs andthe other nest having young that were almost
fully feathered and ready to depart the nest
(approximate 42-day difference).

Nesting Chronology.—Gyrfalcons in Kangerlussuaq began nesting approximately 18 days
earlier than those in Thule, with the first egg
laid on 17 April (SD ± 9.3) and 5 May (SD ±7.0), respectively (Table 3, Figure 4). Calculating forward, young departed the nest around 12
July in Kangerlussuaq and 30 July in Thule(Table 3). We were unable to determine the ageof young at 22 nests in Thule as they had
already fledged. The majority of these nests(77%) were surveyed from early August
onward (after the mean fledging date), but datafrom these nests would have likely had little
impact on calculated nesting chronology. Basedon research in Canada and Iceland (Cramp andSimmons 1980, Nielsen and Cade 1990,
respectively), showing that young Gyrfalcons
become independent of adult care at an averageof ≥4 weeks after they left the nest, independence would be reached in mid-August in
Kangerlussuaq and early September in Thule.

Prey.—Jenkins (1978), Burnham and Mattox(1984), and Booms and Fuller (2003) havedocumented prey species delivered to Gyrfalcon nest sites in Kangerlussuaq. It was determined that Gyrfalcons prey on virtually all
small- to medium-size birds and mammals in
the area. During this study, ptarmigan remains
were the most common prey item in Gyrfalcon
nests, e.g., one 2005 nest, with four, approximately 38-day-old nestling Gyrfalcons, contained 55 ptarmigan and 2 Long-tailed Duck
(Clangula hyemalis) sterna. Arctic Hare
(Lepus arcticus), Snow Bunting, and LaplandLongspur remains were also found, but inmuch less frequency than ptarmigan remains.
Burnham and Mattox (1984) emphasized theapparent importance of passerines (from 1972
to 1981) as food to supplement ptarmigan andhare when their numbers were few.

In Kangerlussuaq, a significant relationship (P< 0.05, r2 = 0.48) existed between the number8

– GYRFALCON BIOLOGY IN GREENLAND –Figure 5. Significant

Productive nests

16

relationship (P > 0.05, r2 =0.48) between the averagedate of breeding (Julian
hatch-date shown here) andthe number of successful
nests. This suggests that latebreeding Gyrfalcons produce
fewer broods and that overall
production is lower in years
when the population as awhole nests later.

12
8
4
0
141

143

145

147

149

151

153

155

Julian date

176

Figure 6. Negative trend overtime showing that the averagedate breeding was initiated inThule significantly (P < 0.05, r2= 0.32) advanced (became
earlier) during the 17-year
study period. Julian hatchdate shown here.

Julian date

172

168

164

160

1995

1998

2001

2004

2007

2010

Year

nests. For example, one nest site locatedapproximately 60 km up a long fjord and further from any breeding seabird colony yielded
mainly Arctic Hare leverets, Rock Ptarmigan,and passerine remains.

In Thule, prey remains were collected at 40
Gyrfalcon nest sites during the 17-year study.
Dovekie remains were present in 75% of thenests. Fourteen nests had exclusively Dovekie
remains, the only prey species for which thatwas the case. Rock Ptarmigan remains were
found at 12 nests (30%), Black Guillemot at 10(25%), Arctic Hares at 6 (15%), Long-tailed
Ducks at 5 (13%), Black-legged Kittiwakes at
3 (8%), passerines at 2 (5%), and Common
Eiders (Somateria mollissima), Snow Geese(Chen caerulescens), and Thick-billed Murresat 1 (3%). Although prey transects were notconducted, there was an obvious positive correlation between potential prey seen in theareas of nest sites and prey species remains in

Plumage Color Variants.—During 1998–2006
in Kangerlussuaq, the plumage color of 137
adults was recorded as 71 (52%) “white,” 16(12%) “silver,” and 50 (36%) “grey.” Observations of adult females at nests were more frequent (80 vs. 57) than males, as might be
expected (Table 4). The ratio of white and grey
females was nearly even (43% vs. 41%), but
65% of adult males were white compared toonly 30% grey.

9

– BURNHAM AND BURNHAM –Table 4. Plumage color of adult Gyrfalcons

between prey abundance and nesting density(see Newton 1979).

observed in Kangerlussuaq from 1998–2006.
Adult females

Adult malesWhite

34 (43%)37 (65%)

Silver13 (16%)3Grey

33 (41%)17 (30%)

Total

80

57

Rock Ptarmigan have been shown to havecyclic population fluctuations throughout the
Arctic (Gudmundsson 1960, Weeden and Theberge 1972, Watson et al. 1998, Moss and Watson 2001), and specifically in Greenland(Salomonsen 1950). Based on hunter kills,
Vibe (1967) found ptarmigan populations tofluctuate in Greenland on approximately 11year cycles from 1886 to 1954. Nielsen (1999)
reported a 10-year cycle for ptarmigan in Iceland and found that Gyrfalcon populations
lagged behind by 3 years. Mossop and Hays
(1982, 1994) reported that, in the Yukon Territory, the Gyrfalcon breeding population fluctuated on a 10-year cycle, probably following
population fluctuations of Willow Grouse(Lagopus lagopus). Additionally, Nyström etal. (2005) showed a significant relationshipbetween the number of Gyrfalcon chicks
fledged each year and ptarmigan populationlevels. Alternatively, Mindell et al. (1987) and
Mindell and White (1988) found no relationship between Gyrfalcon and ptarmigan populations in Alaska, and Shank and Poole (1994)
found the same for Canada’s Northwest Territories (but see Mossop 2011, Barichello andMossop 2011).

(5%)

More than 285 juvenile and adult Gyrfalcons
were observed during 1993–2010 in Thule, of
which only two had grey plumage (an adultfemale and her daughter), and one silver. The
remainder of the Gyrfalcons had very whiteplumages and many showed no visible dark pigmentation on their tails, even as juveniles.
Repeat sightings may have occurred from one
year to the next in both study areas as few adults
were ringed and it was not possible to tell individuals apart. Color phases of individuals were
expected to remain consistent throughout theirlives, as evident from captive birds.
DISCUSSION

The world population of Gyrfalcons was estimated by Cade (1960, 1982) at 15,000–17,000
pairs. More recently Potapov and Sale (2005)
estimated the world population at 7,880–
10,900 pairs by totaling country estimates. For
Greenland alone, Mattox (1970) estimated 850
pairs, Cade (1982) 500–1,000 pairs, and mostrecently Boertmann (2008) estimated the population to be no more than 500 pairs. Based onthese estimates, Greenland contains a possible
5–10% of the world Gyrfalcon population.

In Kangerlussuaq, where ptarmigan remains
were found most frequently in Gyrfalcon nests
from 1998–2006, it is difficult to knowwhether a response relationship between Gyrfalcons and ptarmigan exist. However, basedon our data, it appears a peak in occupied Gyrfalcon nest sites occurred in 1999 and 2000,
followed by a steady decline and then increase(Figure 3). This pattern would be expected if
indeed the Gyrfalcon population was fluctuating with a cyclic ptarmigan population. Unfortunately, no recent information is available on
the annual fluctuations of the ptarmigan population in Greenland, with historic data based on
casual observations, local inhabitants, andhunter kills (Salomonsen 1950, Vibe 1967).

One of the most important influences on thebreeding density of birds is food supply (Lack
1954, 1966, Newton 1979, 1998). Ratcliffe(1969) showed that for three inland Peregrine
Falcon populations in the United Kingdom
the average breeding home range per pair foreach area was correlated with primary-productivity and food. Numerous other raptor
species have shown significant relationships

10

– GYRFALCON BIOLOGY IN GREENLAND –period, Gyrfalcons were observed at 13, 22, 7,
and 6 nest sites, respectively, and all these sites
were included in our study. However, between
the earlier study period and our study period,
the overall size of the study area increased,with a new and very productive valley to thenorth of the original study area included in thesurveys from 1998 onward. The data from theoriginal study area in both periods are given in
Table 5. Data in the “initial study area” encompassed all nest sites known from 1990 to 1993,
in addition to other nest sites found later, but
within the same area/boundaries.

Prey differences between Kangerlussuaq andThule were apparent primarily in the large
numbers of songbirds in Kangerlussuaq frommid-May to mid-September, and in the tens of
millions of seabirds present in northwest
Greenland from mid-May to August–September. In general, Gyrfalcons appeared to preyupon all suitable species present locally in theirbreeding areas.
In both Kangerlussuaq and Thule, it seems
probable that Gyrfalcon numbers, or nesting
density, were regulated to prey, and specificallyptarmigan. Of primary importance is the preyavailability when the Gyrfalcons first arrive on
territory in spring; Rock Ptarmigan, Snow
Buntings, and adult Arctic Hares are present in
Kangerlussuaq, and Rock Ptarmigan and adult
Arctic Hares in Thule. It is likely that RockPtarmigan are the single most important preyspecies early in the season, as shown by Nielsen
and Cade (1990) in Iceland, and are thus essential to Gyrfalcon breeding in Greenland.

Only from 1990–1993 and from 1998–2006,
when surveys were conducted specifically for
Gyrfalcons, were occupancy records comparable. Using occupied and successful nest sites
located within the initial study area (Table 5),
there appeared to be two possible Gyrfalcon
population peaks: in 1991 and 1999–2001. The
high in 1991 was followed by a large and immediate drop, while the high from 1999 to 2001
was followed by a similar rapid drop. The timeinterval between population peaks and lows
appeared to be 8 and 10 years, respectively, andfollowed a cyclical pattern similar to that as
described by Nielsen (1999) and Mossop andHays (1982, 1994) for Gyrfalcon populations
heavily dependent upon ptarmigan. Although
the initial peak and decline from 1991–1993
was slightly more severe than for the period
1999–2003, this may simply be a function ofpeak height and trough depth varying from one
cycle to another, as has been observed in otherspecies which fluctuate cyclically.

Population Changes.—Although there areinsufficient data to state accurately whether apopulation change in Gyrfalcons has occurredin Greenland, some regional decreases may
have occurred, particularly from the Uummannaq District south to the southwestern tip ofGreenland. For this region, comparing thenumber of Gyrfalcons reported by Bertelsen(1921) in the early 1900s with that of Burnham
et al. (2005) in 2000, almost 100 years later,
fewer pairs were located during the later survey. Additionally, in the Disko Bay area, nobreeding Gyrfalcons were located by Burnham
and Sherrod in 1974 (Burnham and Mattox
1984), despite Salomonsen’s (1950) statementthat the Gyrfalcon breeds “rather locally in
S.W. Greenland but in the Disko Bay region it
increases considerably in number,” referencing
Bertelsen (1932) and Fencker (1929).

Despite a doubling in the number of nest sites
known and surveyed, and improved survey
techniques (foot, small planes, and some helicopters in the early 1990s versus primarily
small helicopter during this study), the numberof occupied nests has decreased in the Kangerlussuaq area from the earlier to latter study
period (Table 5, Figure 7).

Only four years of previous surveys were donein the Kangerlussuaq area specific to Gyrfalcons, from 1990 to 1993 (M. Yates unpubl.
data, W. Mattox unpubl. data). During
thisInteractions Between Gyrfalcons and Peregrines.—The Peregrine Falcon population in11

– BURNHAM AND BURNHAM –Table 5. Nest site occupancy by Gyrfalcons in Kangerlussuaq, Greenland, from 1990 to 1993 and from1998 to 2006 in the “initial study area.” (M. Yates unpubl. data for 1990–1993, Mattox unpubl. data for
1990–1993).
Year

Occupied (% ofchecked occupied)

Successful

Known Gyr sites
checked (%)

Known
Gyrfalcon sites

1990

13 (52%)

725 (100%)

25

1991

22 (69%)1232 (100%)

32

1992

7 (18%)

3

38 (100%)

38

1993

6 (15%)040 (100%)

40

1998

8 (18%)

8

45 (78%)58

1999

11 (20%)1054 (92%)59

2000

11 (20%)1155 (90%)61

2001

11 (20%)1055 (90%)62

2002

8 (25%)

4

32 (52%)62

2003

5 (12%)

2

42 (68%)62

2004

7 (13%)

6

53 (85%)62

2005

6 (10%)

6

60 (97%)62

2006

8 (14%)

7

58 (94%)62

Figure 7.

60
50
40

Nest sites

Changes in thenumber ofsurveyed (filled
boxes), occupied(open circles), andsuccessful (opentriangles)
Gyrfalcon nests in
Kangerlussuaq
from 1990–1993
and 1998–2006.

30
20
10
0
1990

1992

1994

1996

1998

Year

12

2000

2002

2004

2006

– GYRFALCON BIOLOGY IN GREENLAND –and we were never convinced that Gyrfalcons
were dominant over Peregrines. Rather, on
several occasions, Peregrines were observed
driving adult Gyrfalcons away from nests
and/or fledged young, on occasion even striking the Gyrfalcon and locking feet. Similar
observations were made by Fowles (1981)
who commented that “The large falcon tended
to look clumsy and confused as it attempted toevade the agile stoops of the…Peregrines.”
The Peregrine is typically very aggressive
toward other species, including other Peregrines (Tordoff and Redig 1997).

the Kangerlussuaq area has dramaticallyincreased over time, from 34 known nest sites
found from 1972 to 1981 (Burnham and Mattox 1984) to over 170 known nests sites in2006 (Burnham 2008). There is little doubt
that, at the same time the ratio of nesting Peregrines to Gyrfalcons has also changed from areported ratio of nearly 1:1 during 1972–1981(Burnham and Mattox 1984) to between 4 and
8:1 by the early 1990s (calculated using Mattox 1998). Using information on occupancy
collected by Mattox (1998) during 1988–1997,
the last 10 years of large scale surveys for
Peregrines made in the Kangerlussuaq area, anaverage of 84.7% (median = 85, SD ± 6.7) ofchecked sites were occupied. If occupancy
remained similar during this study, thenapproximately 144 Peregrine nests were likely
occupied each season, making the ratio ofPeregrines to Gyrfalcons in the Kangerlussuaq
area between 10 and 29:1, depending upon thenumber of occupied Gyrfalcon nests each year.

Other than by direct aggression and displacement, how might Peregrines influence breeding Gyrfalcons? Direct displacement isprobably difficult for Peregrines because the
Gyrfalcons begin breeding earlier in thespring. But indirect displacement may occur
through the Peregrine’s influence on theptarmigan population. As shown by Rosenfield
et al. (1995), fledgling Rock Ptarmigan canmake up a significant portion of the diet at
Peregrine nests in Kangerlussuaq. When Gyrfalcons arrive at nest sites, and throughout
much of their incubation, the prey species
available are primarily ptarmigan, adult hare,
and Snow Buntings. Of the three, ptarmigan isprobably the most important. As Peregrine
numbers have increased in Kangerlussuaq, so
has their probable impact on ptarmigan, reducing the number surviving the summer andautumn seasons, and reducing the number ofbirds available in the spring.

Although perhaps ultimately driven by climatechange, might the probable Gyrfalcon population decline be associated with the increase in
Peregrines? Salomonsen (1950) states, “The
number of Gyr-Falcons in the low-arctic
region is probably to some extent checked by
the Peregrine…” He made the statement
because of overlap between the species in foodand nest sites. In Kangerlussuaq, territorial
Peregrines and Gyrfalcons were observed nesting on the same cliff ≥25 times from 1983–
2006 (W. Mattox unpubl. data and this study).
During 1972–1982, Burnham and Mattox(1984) never observed the two species nestingon the same cliff. At a cliff where a pair ofGyrfalcons and Peregrines nested simultaneously in 2001, the adult female Gyrfalcon was
found below the cliff unable to fly, evidently
because of concussion. After being held in captivity for a short time she was released at thenest site, but disappeared within days, hersmall young dying shortly after. The Peregrines were very aggressive, and successfully
reared their own young. Aerial encounters
between the species were seen several times,

A similar situation with Peregrines and Hen
Harriers (Circus cyaneus) holding Red Grouse(Lagopus l. scoticus) populations at low levels
has been described in Scotland (Redpath andThirgood 1999). Again, both raptors were supported at high density mainly by other prey,
but had their major impact on the more vulnerable grouse. Neither raptor in Scotland was asheavily dependent on grouse as GreenlandGyrfalcons were on ptarmigan. Likewise, in
Norway, Selås (1998) studied the effect of Red
Fox (Vulpes vulpes) density on Goshawks

13

– BURNHAM AND BURNHAM –approximately the same age in both areas. On
several occasions in Kangerlussuaq, we found
the remains of dead young from the previous
year, in some cases fully feathered, below thenest site.

(Accipter gentilis). Through a natural experiment where Goshawk, fox, and grouse populations were monitored over several decades in
a 650-km2 area, it was determined that Red
Fox influenced Goshawk breeding densities
negatively by limiting the number of grouse.
We suggest that this type of relationship is
occurring between Peregrine, Gyrfalcon, andptarmigan populations in Kangerlussuaq.

Breeding Chronology.—As expected, the more
northern Thule population bred seasonallylater than the Kangerlussuaq population to thesouth, with similar differences observed
between Peregrine populations in both areas(Burnham 2008, Burnham et al. in press).
Greater nesting synchrony was also observedin the starting dates for Gyrfalcons breeding in
the north versus south, as described for otherraptors by Newton (1979).

Reproduction.—Reproduction may have varied
regionally and annually, but we do not haveenough information to show this in Greenland.
As mentioned in the results, in both the
Kangerlussuaq and Thule areas, it was frequently difficult to determine the total numberof young at Gyrfalcon nest sites. Therefore,
most numbers provided are minima. Additionally, the difference in reproduction between
occupied and successful nests was likely underreported, because surveys took place late in thebreeding season; at nest sites that had failed,adults may have already left the area, biasing
the results towards successful nests. Further,although Gyrfalcon reproduction has beenshown elsewhere to fluctuate between years in
response to prey densities (White and Cade
1971, Nyström et al. 2005), it was not possible
to determine if this was the case in Greenland.

For Kangerlussuaq, fewer successful nests
were found in years when breeding was initiated later in the season (Figure 5). Similar
trends have been observed in numerous otherraptor species, including Sparrowhawks(Accipiter nisus), Peregrines, and Bald Eagles(Haliaeetus leucocephalus) (Newton and Marquiss 1984, Herbert and Herbert 1965, Broley
1947, respectively). Why the Gyrfalcon population in Kangerlussuaq bred later in someyears is unknown. The availability of prey andinclement weather have been shown to be themost common causes of variation in nesting inmost birds (Lack 1954, 1966, Newton 1979,
1998), but without more information on eitherof these variables, particularly ptarmigan density, no conclusions can be drawn.

Potapov and Sale (2005) reported a mean
brood size of 2.6 young (SD ± 0.91, n = 364)
across the entire range of the Gyrfalcon. In
Kangerlussuaq, Burnham and Mattox (1984)
reported an average of 2.6 young per successful nest (n = 20) from 1972–1981 for nests
they could observe. During this study, slightly
more young per successful nest were producedin Kangerlussuaq (2.81) compared to datafrom Burnham and Mattox (1984), while analmost identical number of young per successful nest were found in Thule (2.63). Chicks inThule appeared to be 2–3 weeks older on average when surveys occurred, with many sites inThule having young near to fledging or alreadyfledged. Accordingly, the observed differencein nest success would likely have been moresimilar had surveys occurred when young wereDuring this study, the lay-date of Gyrfalcons inThule significantly advanced (Figure 6).
Advancement of lay-date has also been
observed in numerous other species of birds(e.g., Crick et al. 1997, Crick and Sparks 1999,
Dunn and Winkler 1999, Both et al. 2004) andis considered to be one of the most likely
effects of climate change on bird species overtime (IPCC 2007). From north Greenland,
including the Thule area, temperatures havebeen increasing since the late 1900s (Whitfield
et al. 2004). In particular, Mountain et al.
(2001) and Serreze et al. (2000) found spring14

– GYRFALCON BIOLOGY IN GREENLAND –during the latter period and 69% of surveyedsites occupied vs. 20% and 16%, respectively,
with similar numbers of successful sites
observed between periods. However, more
previously-used nest sites were known andsurveyed during the latter period, with morethorough and improved surveys not finding
more nesting falcons. The population in Thuleappears to be stable, with only minor fluctuations occurring during this study, but withbreeding pairs increasingly moving throughout
the study area and using alternate nest sites.

temperatures to be increasing throughout thisarea, allowing for a milder and earlier spring.
Already this change in temperature has beenaccompanied by an increasing Peregrine population in the Thule area (Burnham 2008,
Burnham et al. in press) and is likely alsoenabling Gyrfalcons to begin nesting earlier.
Plumage Color Variants.—The observed
plumage color variation in Kangerlussuaq andThule is comparable with findings reported
elsewhere for Gyrfalcons, with a higher proportion of white falcons in the north. Slightly to thesouth of the Kangerlussuaq area, Salomonsen(1950) reported a 50:50 ratio of white and darkplumage Gyrfalcons. In Kangerlussuaq during
1972–1977, the plumage of 29 adult Gyrfalcons was recorded as 16 (55%) white and 13(45%) grey (W. Burnham unpubl. data). During
1998–2006, we documented plumage color of
137 adults as 71 (52%) white, 16 (12%) silver
and 50 (36%) grey. During the 1970s, silver
was included with grey, and if we lump silver
and grey Gyrfalcons together our ratio is nearly
identical. Based on these results it appears noshift in plumage color has occurred in the past
50–60 years.

The Gyrfalcon population in Kangerlussuaq
began nesting approximately 18 days earlier
than the population in Thule. No large differences were observed in reproduction, with the
Kangerlussuaq sample producing slightly moreyoung per successful nest but equal numbersof young per occupied nest. In years whenbreeding began later in Kangerlussuaq, fewer
successful nests were found. The initiation ofbreeding advanced in Thule during our study
period, likely in association with earlier andwarmer springs.
It is difficult to determine with certainty what
effect climate change is having on Gyrfalcons,
although it is clear that it is the driving force
behind large-scale ecosystem change. Current
observed climate change and future predictions
suggest that Peregrine densities in Greenland
will continue to increase, while at the same time
their range will continue to move north into
areas previously occupied only by Gyrfalcons.
In contrast, Gyrfalcons, adapted to live in thecold Arctic conditions, may well retreat frommany areas where they now occur. Continued
research is needed to determine what effects, ifany, both a warming climate and an expanding
Peregrine population will have on Gyrfalcons inGreenland and throughout the Arctic.

The observed difference in plumage color
between breeding females and males in
Kangerlussuaq was unexpected, with 43% offemales and 65% of males being white. To ourknowledge this is the first instance in Gyrfalcons where such a large difference wasobserved between sexes within the same area.
More research is needed to determine why thisis occurring.
CONCLUSION

These are the first long-term studies ever conducted on Gyrfalcon populations within
Greenland. Based on information collected byothers and data from this study it appears thatthe population in Kangerlussuaq is at best stable, and may be slowly declining. Population
peaks were observed in both 1991 and in 1999
and 2000 in Kangerlussuaq, with approximately one-third fewer occupied sites found

ACKNOWLEDGMENTS

We thank I. Newton and J. Burnham for comments and suggestions. Additional thanks to
the Greenland Home Rule Government forproviding permits for this research. Logistical15

– BURNHAM AND BURNHAM –BOERTMANN, D. 2008. Grønlands Rødliste–2007. (From Danish by D. Boertmann).
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support was provided by Polar Field Services,
the United States Air Force, Kangerlussuaq
International Science Support, VECO (now
CH2M HILL), 109th Air National Guard, andthe US Department of the Interior Bureau ofLand Management. For many years of critical
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