ARCTIC
VOL. 62, NO. 3 (SEPTEMBER 2009) P. 301 – 311

Change in Abundance of Pacific Brant Wintering in Alaska:
Evidence of a Climate Warming Effect?
David H. Ward,1,2 Christian P. Dau,3 T. Lee Tibbitts,1 James S. Sedinger,4 Betty A. Anderson5
and James E. Hines6
(Received 19 June 2008; accepted in revised form 28 October 2008)

Abstract. Winter distribution of Pacific Flyway brant (Branta bernicla nigricans) has shifted northward from lowtemperate areas to sub-Arctic areas over the last 42 years. We assessed the winter abundance and distribution of brant in
Alaska to evaluate whether climate warming may be contributing to positive trends in the most northern of the wintering
populations. Mean surface air temperatures during winter at the end of the Alaska Peninsula increased about 1˚C between
1963 and 2004, resulting in a 23% reduction in freezing degree days and a 34% decline in the number of days when ice cover
prevents birds from accessing food resources. Trends in the wintering population fluctuated with states of the Pacific Decadal
Oscillation, increasing during positive (warm) phases and decreasing during negative (cold) phases, and this correlation
provides support for the hypothesis that growth in the wintering population of brant in Alaska is linked to climate warming.
The size of the wintering population was negatively correlated with the number of days of strong northwesterly winds in
November, which suggests that the occurrence of tailwinds favorable for migration before the onset of winter was a key factor
in whether brant migrated from Alaska or remained there during winter. Winter distribution of brant on the Alaska Peninsula
was highly variable and influenced by ice cover, particularly at the heavily used Izembek Lagoon. Observations of previously
marked brant indicated that the Alaska wintering population was composed primarily of birds originating from Arctic breeding
colonies that appear to be growing. Numbers of brant in Alaska during winter will likely increase as temperatures rise and ice
cover decreases at high latitudes in response to climate warming.
Key words: Alaska Peninsula, abundance, Branta bernicla nigricans, climate warming, distribution, Izembek Lagoon, Pacific
brant, Pacific Decadal Oscillation, wintering
RÉSUMÉ. Au cours des 42 dernières années, la répartition de la bernache cravant du Pacifique (Branta bernicla nigricans)
s’est déplacée vers le nord en hiver, passant ainsi de régions faiblement tempérées à des régions subarctiques. Nous avons
évalué l’abondance et la répartition de la bernache en Alaska l’hiver afin de tenter de déterminer si le réchauffement climatique
contribue aux tendances positives au sein des populations d’hivernage les plus au nord. Les températures moyennes de l’air à
la surface en hiver se sont accrues d’environ 1˚C entre 1963 et 2004, ce qui s’est traduit par une réduction de 23 % du nombre
de jours atteignant le point de congélation et d’une diminution de 34 % du nombre de jours pendant lesquels la couverture de
glace empêche les oiseaux d’avoir accès aux ressources alimentaires. Les tendances caractérisant la population d’hivernage
fluctuaient en fonction des états de l’oscillation pacifique décennale en ce sens qu’elles augmentaient pendant les phases
positives (tièdes) et qu’elles baissaient pendant les phases négatives (froides). Cette corrélation vient appuyer l’hypothèse selon
laquelle la croissance de la population d’hivernage de la bernache en Alaska est liée au réchauffement climatique. L’effectif
de la population d’hivernage a été négativement corrélé au nombre de jours de vents forts en provenance du nord-ouest en
novembre, ce qui laisse croire que l’occurrence de vents arrières favorables à la migration avant le début de l’hiver constituait
un facteur-clé déterminant si une bernache migrait de l’Alaska ou y restait pendant l’hiver. Dans la péninsule de l’Alaska, la
répartition de la bernache en hiver variait énormément et dépendait de la couverture de glace, surtout à la lagune Izembek
particulièrement achalandée. Les observations de bernaches déjà marquées ont permis de constater que la population
d’hivernage de l’Alaska était principalement composée d’oiseaux provenant des colonies de reproduction de l’Arctique qui
semblent prendre de l’ampleur. Le nombre de bernaches en Alaska pendant l’hiver augmentera vraisemblablement au fur
et à mesure que les températures augmenteront et que les couvertures de glace diminueront en haute latitude en raison du
réchauffement climatique.

 U.S. Geological Survey, Alaska Science Center, 4210 University Dr., Anchorage, Alaska 99508, USA
 Corresponding author: dward@usgs.gov
  3
 Migratory Bird Management-Region 7, U.S. Fish and Wildlife Service, 1011 East Tudor Rd., Anchorage, Alaska 99503, USA
  4
 Department of Environmental and Resource Sciences, University of Nevada Reno, 1000 Valley Rd., Reno, Nevada 89512, USA
  5
 ABR, Inc. – Environmental Research & Services, P.O. Box 80410, Fairbanks, Alaska 99708, USA
  6
 Canadian Wildlife Service, Environment Canada, Suite 301, 5204 – 50th Avenue, Yellowknife, Northwest Territories X1A 1E2,
Canada
 © The Arctic Institute of North America
  1
  2

 302 • D.H. WARD et al.

Mots clés : péninsule de l’Alaska, abondance, Branta bernicla nigricans, réchauffement climatique, répartition, lagune
Izembek, bernache cravant du Pacifique, oscillation pacifique décennale
 Traduit pour la revue Arctic par Nicole Giguère.

INTRODUCTION
Rapid climate warming is occurring in the Arctic and subArctic (ACIA, 2004). Over the past 100 years, average Arctic air temperatures have increased by 1.4˚C, a rate that is
nearly twice the global average (0.7˚C; IPCC, 2007). Most
of the warming during this period has occurred since 1976,
at rates higher than at any other time during the past 1000
years (IPCC, 2001). As a consequence, most high-latitude
regions are experiencing increasing periods of abovefreezing temperatures and diminishing snow and ice cover
(Stone et al., 2004).
Climate affects the movement and distribution of fauna,
often directly through species-specific physiological tolerances to temperature and precipitation, but also indirectly,
through impacts on the availability and abundance of food
resources (Root, 1988; Hoffman and Parsons, 1997). In the
current global warming scenario (IPCC, 2007), the expectation is that species will respond to warming trends by shifting their distributions to higher latitudes or elevations (but
see Helmuth et al., 2002). Northward shifts in the breeding ranges of many species of birds have been reported in
Europe (Burton, 1995) and North America (Hitch and Leberg, 2007). In Britain, Thomas and Lennon (1999) found
that 59 species of birds had shifted the northern boundaries of their breeding ranges an average of 19 km northward
over a 20-year period (i.e., 1968 – 72 to 1988 – 91). Some evidence also points towards a poleward movement in winter
distribution of temperate species of birds in North America (Johnson, 1994; Root and Weckstein, 1995; LaSorte
and Thompson, 2007), although published accounts are
relatively sparse. Nevertheless, shifts in distribution of animals may be more evident in migratory birds than in resident birds because the migrators are more apt to move in
response to environmental and climatic changes (Parmesan
et al., 1999; LaSorte and Thompson, 2007). Also, shifts in
distribution are likely to be more apparent during winter
(January to March) because over the last 100 years, average
daily temperatures have increased more rapidly in that season than in other seasons (IPCC, 2007).
Pacific brant (Branta bernicla nigricans; hereafter brant)
winter along the Pacific coast of North America, over a wide
geographic range extending from Alaska (55˚ N) to Sinaloa,
Mexico (24˚ N). Until recently, however, more than 70% of
this population had traditionally wintered in Mexico (Reed
et al., 1998). Brant are currently exhibiting a northward
shift from their primary wintering areas in Baja California
that appears related to changes in the availability and abundance of eelgrass (Zostera marina), their primary food during the nonbreeding season (Ward et al., 2005). Numbers
of brant are increasing throughout their northern wintering

range, with greatest gains occurring in Alaska relative
to the other sites north of Mexico. In Alaska, brant are
increasing along the Alaska Peninsula at Izembek Lagoon
and adjacent embayments, where virtually the entire eastern Pacific Flyway population stages in fall prior to transoceanic migration to wintering areas (Reed et al., 1989,
1998; Dau, 1992). This increase in wintering numbers has
coincided with a general warming of temperatures in the
North Pacific (Zveryaev and Selemenov, 2000; Mantua and
Hare, 2002), suggesting that environmental conditions may
have changed for one of the northernmost-wintering populations of geese.
Here, we examine 42 years of observational data on
wintering brant in Alaska to determine whether climatic
conditions may be contributing to positive trends in this
population. Specifically, we assess winter use patterns and
long-term trends in abundance of brant along the Alaska
Peninsula to ascertain when this population increased and
to understand whether growth in the Alaska population was
influenced by environmental (e.g., ice-cover, temperature)
changes resulting from climate warming. We also examine the breeding origin of the birds in the Alaska wintering population to determine whether growth in the winter
population was related to increases in particular segments
of the breeding population.
METHODS
Study Area
We analyzed data collected on brant during winter
(1 December–15 March) between 1963 and 2004 at the
western end of the Alaska Peninsula (162.5 – 163.5˚ W
longitude), a narrow landmass that separates the Bering
Sea from the Pacific Ocean (Fig. 1). This remote region of
tundra and steep volcanic mountains is edged by shallowwater embayments containing extensive intertidal stands of
eelgrass (Ward et al., 1997). These areas are accessible to
feeding brant when tide height falls about 1.0 m below mean
lower low water (Ward and Stehn, 1989). In winter, diurnal
feeding opportunities are further limited by reduced periods of daylight. The average daily number of daylight hours
when eelgrass is available for foraging ranges from about
two hours in December to seven hours in March (Mason
et al., 2006). Nocturnal foraging may also occur (Lane and
Hassall, 1996), but it has not been detected on the Alaska
Peninsula.
Izembek Lagoon supports the majority of the birds
staging on the Alaska Peninsula in fall (> 80%) and
spring (100%) (Ward and Stehn, 1989). Most birds arrive

 Brant wintering in Alaska • 303

Ho

ok

Ba

y

Amak Is.

Iz

em

be

a
kL 4

3
2

1

Big Lagoon
St. Catherine’s
Cove

Bechevin
Bay

o
go

n
5

Alaska
Peninsula

6

Kinzarof Lagoon

� Cold
Bay

Morzhovoi
Bay

Unimak Is.

Sanak Is.
Caton Is.

0

12.5

25

km

FIG. 1. Wintering sites used by Pacific brant on the Alaska Peninsula.
Numbers one to six indicate the areas surveyed during aerial counts of brant
in Izembek Lagoon, and triangles mark locations where brant were counted
during the annual Christmas Bird Count of Izembek and Kinzarof lagoons.

at Izembek Lagoon by early October and depart by late
November (Reed et al., 1989). Spring migrants begin returning to the lagoon in early April (Mason et al., 2007). Winter distribution of brant on the Alaska Peninsula is largely
confined to four embayments (Dau and Ward, 1997):
Izembek Lagoon, Kinzarof Lagoon, Big Lagoon in northern Morzhovoi Bay, and northern Bechevin Bay (including Hook Bay and St. Catherine’s Cove; Fig. 1). Brant also
use intertidal areas adjacent to the Sanak Islands and Caton
Island (Jones, 1952; Dau and Chase, 1995), but survey data
there are limited by the remoteness of these islands, which
are located 75 km south of the study area. Winter observations of brant have been rare at other coastal areas on the
Alaska Peninsula, and when birds were observed, flock
sizes were small (< 100 birds) (Boden, 1994; Larned, 2000).
The climate in this region is maritime, but it becomes
more continental in winter when Arctic sea ice extends south
into the Bering Sea (Brower et al., 1977). Winter weather
is harsh and highly influenced by the position and strength
of the Aleutian Low and the Arctic High atmospheric pressure systems (Niebauer, 1983; Overland et al., 1999; Rodionov et al., 2005). Storms and high winds are frequent, and
the ubiquitous precipitation fluctuates almost continually
between rain and snow (Brower et al., 1977). Arctic sea ice
(concentrations of more than 70% cover) generally moves
south into the Bering Sea beginning in November, reaching an average southern extent at about 57˚ N in February
(National Ice Center, unpubl. data). Only twice in the last
30 years (winters of 1971 – 72 and 1973 – 74) has sea ice
extended south to the tip of the Alaska Peninsula (55˚ N).
Shorefast ice, however, is a regular feature of shallow-water
embayments in the region between November and April.

Shorefast ice forms initially in protected shallow estuaries
before expanding into the more exposed and deeper water
areas, where ice formation can be disrupted by winds and
currents. The extent of shorefast ice cover in estuaries on
the Alaska Peninsula is highly variable, and it is greater on
the north side of the peninsula (Brower et al., 1977).
Marine ecosystems of the North Pacific and Bering Sea
vary with interdecadal (15 – 25 or 50 – 70 years) climatic
shifts, commonly described by the Pacific Decadal Oscillation (PDO; Mantua and Hare, 2002). The PDO is a largescale ocean-atmospheric phenomenon that is measured
by an index derived from temporal (monthly) and spatial
changes in the sea surface temperatures and sea-level pressure in the North Pacific (above 20˚ N). Three climate shifts
(warm vs. cold phases) in the 20th century were described
by the PDO index; the most recent occurred in winter of
1976 (Minobe, 1997; Hare and Mantua, 2000). Analyses of
long-term biological and physical data provide evidence that
the North Pacific and Bering Sea experienced a cold phase
(negative PDO values) and a period of low productivity and
biomass in fish stocks and zooplankton between 1964 and
1977 (Anderson and Piatt, 1999; Hare and Mantua, 2000).
After this period, the region entered a warm phase (positive
PDO values) and a period of enhanced biological productivity and biomass. This warm phase has persisted through the
2000s except for short-term cold periods in 1989 – 91 and
1998 – 2001 (Hare and Mantua, 2000).
Environmental Data
To examine long-term variation in climatic conditions
within the winter range of brant on the Alaska Peninsula,
we obtained daily weather records (i.e., surface air temperature, precipitation, and surface wind direction and speed)
collected at the Cold Bay airport (55.13˚ N, 162.44˚ W)
between 1963 and 2004 from the National Climatic Data
Center (2004). The airport is near (< 13 km) to Izembek
and Kinzarof lagoons, which form the northeastern portion of the wintering range of brant on the Alaska Peninsula
(Fig. 1). From this dataset we used daily air temperature to
calculate freezing degree days (FDD) per year by summing
the daily mean air temperatures of days with mean temperatures below 0˚C. FDD were given a plus sign for purposes
of analysis and display. Additionally, we estimated the percent cover of shorefast ice in each embayment during the
periodic aerial surveys to census brant.
Population Indices
We assessed trends in population size of brant wintering
in Alaska using two long-term indices: aerial survey data
and Christmas Bird Count (CBC) data. Since 1986, systematic aerial surveys of wintering brant have been conducted
annually along the Alaska Peninsula. These replicate surveys (≥ 2 counts in most winters) include coverage of the
four main embayments used by brant in winter and provide
the most complete estimate of the total number of brant in

 304 • D.H. WARD et al.

Alaska (Pacific Flyway Council, 2002). Counts of brant at
Izembek Lagoon were divided further into six survey areas
(Fig. 1) to differentiate use within this lagoon. These areas
were delineated by physiographic features, such as tidal
channels, peninsulas, or islands. The CBC, which has been
conducted annually since 1963 (except in 1967, 1970, and
1971) at Izembek and Kinzarof lagoons, provides a longterm index of brant abundance in these lagoons. The CBC is
made in late December–early January by staff of the Izembek National Wildlife Refuge, who scan the central portion
of Izembek Lagoon and western Kinzarof Lagoon from
basically the same vantage points each year (Fig. 1). The
CBC data were correlated positively with the aerial survey
data (r = 0.49, n = 19, p = 0.03; Pearson product-moment
correlation coefficient test), indicating that the CBC was a
reasonably good index of the number of brant in the region
and a gauge to trends in the brant population on the Alaska
Peninsula before systematic aerial surveys began in 1986.
Trends were also examined in a third population index
for brant, the Pacific Flyway midwinter count, to examine
changes in Alaska winter population relative to the overall size of the population. The Pacific Flyway population
estimate (Pacific Flyway Council, 2002) is a compilation
of aerial survey and ground counts of brant collected from
Alaska to Mexico in mid-January each year. This estimate
has proven to be reasonably accurate for evaluating changes
in overall size of the entire population (Sedinger et al.,
1994).
Breeding Origin
We resighted color-marked brant at Kinzarof Lagoon
during February 1994, 1996, 1997, and December 1999 to
determine the breeding origin of birds in the wintering population. Adults and goslings had been marked previously
with uniquely encoded, colored tarsus bands (Sedinger et
al., 1997) at one sub-Arctic site, the Yukon-Kuskokwim
Delta (YKD), Alaska, where about 80% of the Pacific Flyway population breeds (Pacific Flyway Council, 2002), and
two Arctic sites, the Arctic coastal plain of Alaska (ACP)
and the western Canadian Arctic (WCA), including Liverpool Bay and Banks Island, Northwest Territories, Canada.
Tarsus bands were observed through variable-power spotting scopes during high ebb tides, when birds stood on sand
bars to preen and acquire grit (Ward et al., 2004).
Data Analysis
We used linear regression models to examine trends
across the 42-year study period in average daily winter temperatures, number of FDD, and estimated number of days
with extreme ice conditions. We were unable to use satellite imagery to measure the extent of shorefast ice because
such images were lacking for much of the western end of
the Alaska Peninsula during the study period. Therefore,
we estimated extreme ice conditions in Izembek Lagoon
by developing a model of FDD (Eicken, 2003). Extreme ice

conditions were defined as days when eelgrass beds became
inaccessible to brant at Izembek Lagoon or when shorefast
ice covered more than 70% of the eelgrass in the lagoon
as determined by observations of ice cover relative to the
known distribution of eelgrass in this lagoon (Ward et al.,
1997). To account for the considerable lag in the formation
and melting of ice caused by both wind and heat stored in
the water, we used a lag time of four days and calculated
a modified estimate of FDD using below-freezing temperatures with no above-freezing episodes longer than four
days (FDD-4d). We tested the relationship of FDD-4d to
our observed estimates of ice cover at Izembek Lagoon by
plotting the maximum ice cover for each year as a function
of the FDD-4d for the time interval prior to the observations. FDD-4d was a reasonably good predictor (F = 12.3,
df = 12, p < 0.001) of shorefast ice cover, explaining 67% of
the variance in the percent of peak ice cover in this lagoon
(Fig. 2). Using this relationship, we determined that FDD-4
values higher than 60 indicated more than 70% ice cover
for that day in Izembek Lagoon (Fig. 2). We then calculated
FDD-4d for all years and used FDD-4d values of more
than 60 to estimate the number of days with extreme ice
conditions on the Alaska Peninsula. We used the KruskalWallis test to compare average (± SE) daily temperatures
and number of days with extreme ice cover between cold
phases (< 1977, 1989 – 91, and 1998 – 2001) and warm phases
(1978 – 88, 1992 – 97, and 2002 – 04) of the PDO.
We assessed trends in the three population indices using
loglinear Poisson regression models in program TRIM
v3.53 (Pannekoek and van Strien, 2000; van Strien et al.,
2001). Trends were estimated using a times-effect model
adjusted for serial correlation and lack of model fit (overdispersion). Missing values in the CBC data were handled
by producing a model based on existing counts and using
this model to predict those values that were missing. Trends
were then calculated for the entire dataset using predicted
counts from the model in place of the missing values.
Brant counted during winter surveys represented birds
that had already decided to winter in Alaska. To assess the
influence of climatic variability on annual changes to the
winter brant population in Alaska (aerial survey data), therefore, we examined models of climatic parameters occurring
during fall staging (September–November) that may act as
cues to birds in deciding whether to stay in or migrate from
Alaska. For this analysis, we used the winter aerial survey
data as the dependent variable and explored the influence
of four climatic parameters during September–November:
mean surface air temperatures, FDD, values of the PDOindex (http://jisao.washington.edu/pdo/PDO.latest), and
the number of days of northwesterly (270 – 360˚) winds in
November (our proxy for weather conditions favorable to
migration). Dau (1992) showed that the majority of brant
migrate from Alaska in November, when strong Aleutian
Low pressure systems generate powerful tailwinds that can
aid southward migration. We defined a day of northwesterly
winds as one when average daily surface winds exceeded
18 km/hr, a minimum wind speed at peak departure of

 Brant wintering in Alaska • 305

A

80

20
0

0

50

100

150

200

FDD-4d
FIG. 2. Relationship between freezing degree-days with no above-freezing
episodes longer than four days (FDD-4d) and peak ice cover observed each
winter between 1986 and 2004 at Izembek Lagoon. FDD-4d values to the
right of the dashed line represent extreme ice conditions, when ice covered
70% or more of the lagoon and eelgrass was inaccessible to brant.

brant from Izembek Lagoon (Dau, 1992). We developed
candidate models from all possible combinations of the four
parameters and ranked models based on the Akaike Information Criterion adjusted for sample size (AICc; Burnham
and Anderson, 2002). We calculated AICc weights (wi) to
compare the relative support for each model and R 2 values
to indicate overall model fit. Finally, we calculated modelaveraged parameter estimates and unconditional standard
errors based on Akaike weights for parameters of interest
(Burnham and Anderson, 2002).
Using the winter aerial survey data, we evaluated the distribution of brant among the four embayments and within the
six areas of Izembek Lagoon by comparing means and overlap of 95% confidence intervals. The influence of ice cover
on the distribution of brant at Izembek Lagoon and the other
three embayments was tested using regression analysis.
To determine the breeding origin of birds, we used chisquare contingency tests to compare the distribution of
tarsus-marked brant from the different breeding areas. We
used only direct resightings (i.e., observations of individuals that were marked in the previous summer) to eliminate
uncertainty introduced by breeding dispersal (Lindberg et
al., 1998; Sedinger et al., 2008). The expected number of
tarsus-marked birds in the winter population was based
on the total number of brant tarsus-marked in the summer
preceding the winter observations. We assumed that adult
survival between banding and arrival on the Alaska Peninsula was similar among breeding populations, because
Sedinger et al. (2002) found adult annual survival was similar among sub-Arctic and Arctic breeding populations.
Because juvenile survival between banding and arrival on
the Alaska Peninsula differs between sub-Arctic and Arctic
populations (Ward et al., 2004), we adjusted the expected
number of juveniles in the winter population for differential seasonal survival using Ward et al.’s (2004) monthly
survival estimates of 0.68 for juveniles from the subArctic (YKD) and 0.94 for juveniles from the Arctic (ACP
and WCA) calculated over the 2.33 month period between
banding and arrival on the Alaska Peninsula. Because the
number of direct observations was small in any given year,
we conducted the analysis on data combined across the four
winters.

Cold

Warm

Cold Warm

B
50
40
30
20
10
0
-10
-20
-30

6
19 3
6
19 6
6
19 9
72
19
7
19 5
78
19
8
19 1
84
19
8
19 7
90
19
9
19 3
96
19
9
20 9
0
20 2
05

40

Warm

3
2
1
0
-1
-2
-3

19

60

Temperature (C)

Cold

Cut-off for extreme ice conditions
when eelgrass is inaccessible to brant

Number of days of
> 70% ice cover

Ice cover (%)

100

FIG. 3. Deviations from the 1963 – 2004 mean in (A) average daily surface
air temperatures and (B) number of days of extreme (≥ 70%) ice conditions
during winter at Izembek Lagoon. Vertical lines separate cold phases (before
1977, 1989 – 91, and 1998 – 2000) and warm phases (1978 – 97 and after 2000)
of the Pacific Decadal Oscillation.

RESULTS
Between 1963 and 2004, average daily surface air temperature in winter increased by 1˚C, resulting in a 10-day
reduction (23% decline) in FDD. Periods of warm temperatures (Fig. 3A) and reduced ice cover (Fig. 3B) were evident
after the 1976 climate shift except during the short-term climate shift of 1998 – 2001, when average daily winter temperatures dropped below the long-term mean. On average,
days with extreme ice cover occurred 1.5 times as often
(p = 0.03) before 1977 (mean = 41 ± 5 days, n = 14 years)
than after 1976 (mean = 27 ± 3 days, n = 28 years) (Fig. 3B).
Population Changes
A strong positive trend (p < 0.01; 7 ± 1% growth per
year) existed in the aerial counts of brant along the Alaska
Peninsula between 1986 and 2004 (Fig. 4A). The Alaska
wintering population was generally larger than predicted
during warm phases and smaller than predicted during cold
phases of the PDO. Mean number of brant from the CBC
data was significantly lower (p = 0.02) before 1977 (mean
= 110 ± 51 birds, n = 14) than after 1976 (mean = 1331 ±
413 birds, n = 28). Positive trends in the Alaska wintering
population were also apparent in the longer-term CBC data
(p < 0.01; 11 ± 1% growth per year; Fig. 4B). Where years of
the two indices overlapped, patterns of change in the CBC
index were similar to that of the aerial survey data. Gains in
the Alaska wintering population occurred despite an overall stable trend in the entire wintering population of brant in
the Pacific Flyway (p = 0.63; Fig. 4A).

 306 • D.H. WARD et al.

(Fig. 6B). Ice cover did not affect brant numbers similarly
among the four embayments (Fig. 6C). At Izembek Lagoon,
brant numbers decreased as ice cover increased (F = 5.27,
df = 1, p = 0.03, R 2 = 0.15), while at the other three sites
combined, brant numbers rose slightly with increasing ice
cover, though this change was not significant (F = 2.29, df =
1, p = 0.14).

A

Ln (brant count)

13
12
11
10
9

Breeding Origin

8

Over four winter seasons, we resighted 290 color-marked
brant, representing 227 individuals. A greater proportion of
the resighted brant were from the two Arctic breeding colonies than from the one sub-Arctic colony (samples pooled
across years: χ2 = 11.1, p = 0.004). Observations were
equally distributed between ACP and WCA, the two Arctic
breeding colonies where birds were marked; each accounting for 34% of the resightings.

B

Ln (brant count)

Cold

Warm

Cold

Warm

Cold Warm

10
8
6
4
2
NC

DISCUSSION

NC NC

19

6
19 3
6
19 6
6
19 9
7
19 2
75
19
7
19 8
8
19 1
8
19 4
8
19 7
9
19 0
93
19
9
19 6
9
20 9
0
20 2
05

0

Year
FIG. 4. Change in number of wintering Pacific brant counted during aerial
surveys in the Pacific flyway (Alaska to Mexico; A-top) and along the Alaska
Peninsula (A-bottom) between 1986 and 2004, and during the Christmas Bird
Counts at Izembek and Kinzarof lagoons between 1963 and 2004 (B). NC
indicates no count. Vertical lines separate cold phases (before 1977, 1989 – 91,
and 1998 – 2000) and warm phases (1978 – 97 and after 2000) of the Pacific
Decadal Oscillation. Horizontal lines indicate trends in survey data, either
stable (A-top) or increasing (A-bottom and B).

Our examination of variation in the number of brant
wintering on the Alaska Peninsula showed there is strong
support for its relationship to the number of days with
strong northwesterly winds in November. This parameter
was included in the five top models (combined AICc weight
= 0.87; Table 1) and when examined as a single variable,
explained the greatest proportion of the variation in the aerial survey data (R 2 = 0.27) of the four fall climatic variables
tested, nearly twice that of the next most important fall climatic parameter, FDD_fall (R 2 = 0.14). The population of
brant wintering in Alaska increased as the number of days
with strong northwesterly winds in November decreased
(model averaged estimate = -446.5 ± 176.7; note SE did not
overlap 0). On average, November had six fewer days with
strong northwesterly winds during warm phases of the PDO
(mean = 9 ± 1.0 days) than during its cold phases (mean =
15 ± 1.2 days) (Fig. 5).
Distribution Patterns
Brant were not distributed evenly among the four embayments between 1986 and 2004 (Fig. 6A). Mean numbers of
brant were greater at Bechevin Bay and Izembek Lagoon
than at the other sites. Within Izembek Lagoon, brant
tended to use area 5 the most, and areas 6 and 2 the least

The shift in climate to a positive (warm) phase of the
PDO after 1976 had a well-documented impact on the abundance and distribution of a variety of marine species (e.g.,
pandalid shrimp, Pandalus spp.; walleye pollack, Theragra
chalcogramma; Pacific cod, Gadus macrocephalus; kittiwake gulls, Rissa spp.; thick-billed murres, Uria lomvia;
northern fur seals, Callorhinus ursinus) in the Bering Sea
and Gulf of Alaska (Mantua et al., 1997; Anderson and Piatt,
1999; Hunt et al., 2002). Effects on organisms restricted to
estuarine ecosystems have not yet been investigated. Our
study provides evidence that the growth in the brant population wintering on the Alaska Peninsula was linked to this
same climate shift. This linkage is based on two lines of
evidence. First, although records are sparse, few (< 3000
birds) brant were detected wintering in Alaska prior to 1977
from CBC surveys (Fig. 4B), boat surveys by Jones (1952,
1955), and anecdotal accounts from residents of the region
(Dau and Ward, 1997). Second, numbers of brant increased
after 1976, and fluctuations in winter population indices
coincided with the phases of the PDO, increasing during
warm phases and decreasing during cold phases (Fig. 4).
We suggest that a large portion of the increase in the wintering brant population is related to a general improvement in
coastal environmental conditions for brant at the western end
of the Alaska Peninsula. Our findings of increased air temperatures, decreased number of FDD, and reduced extent of
shorefast ice are consistent with a general change in climatic
patterns that has resulted in warmer winters in southwestern
Alaska (Zveryaev and Selemenov, 2000; Hartman and Wendler, 2005). We contend that the milder winter conditions and
reductions in ice cover have improved the accessibility of
eelgrass and reduced thermoregulatory demands for wintering brant in Alaska. Accessibility (i.e., availability and abundance) of eelgrass is a major factor affecting movements,
distribution, and abundance of brant at other areas during

 Brant wintering in Alaska • 307
Table 1. Selection of models to assess the influence of climatic variability in fall on annual winter population size for Pacific brant on
the Alaska Peninsula, 1986–2004.
Model1 

K 2 

AICc3 

∆AICc4 

Days_NWwind_Nov, PDO_fall  
Days_NWwind_Nov  
Days_NWwind_Nov, AirTemp_fall  
Days_NWwind_Nov, PDO_fall, AirTemp_fall  
Days_NWwind_Nov, PDO_fall, FDD_fall  
FDD_fall  
Days_NWwind_Nov, FDD_fall  
PDO_fall, FDD_fall  
Days_NWwind_Nov, FDD_fall, AirTemp_fall  
FDD_fall, AirTemp_fall  
PDO_fall  
Days_NWwind_Nov, PDO_fall, FDD_fall, AirTemp_fall  
AirTemp_fall  
PDO_fall, FDD_fall, AirTemp_fall  
PDO_fall, AirTemp_fall  

4 
3 
4 
5 
5 
3 
4 
4 
5 
4 
3 
6 
3 
5 
4 

309.15 
310.76 
312.38 
312.52 
312.79 
313.81 
313.94 
314.37 
314.76 
314.88 
315.18 
316.16 
316.63 
317.25 
318.06 

0 
1.61 
3.23 
3.37 
3.64 
4.66 
4.79 
5.22 
5.61 
5.73 
6.03 
7.01 
7.48 
8.10 
8.91 

wi5 R 2
.401 
.179 
.080 
.074 
.065 
.039 
.037 
.029 
.024 
.023 
.020 
.012 
.010 
.007 
.005 

0.43
0.27
0.33
0.44
0.44
0.14
0.27
0.25
0.37
0.23
0.07
0.47
0.01
0.29
0.09

 Model parameters included Days_NWwind_Nov, number of days of northwesterly winds in November with average daily surface
wind speed over 18 km/hr and direction between 270˚ and 360˚; PDO_fall, average values of the Pacific Decadal Oscillation index
during fall (September–November); AirTemp_fall, average air surface temperatures during fall; FDD_fall, average sum of the freezing
degree days during fall.
2
 Number of parameters in each model.
3
 Akaike’s Information Criterion value adjusted for small sample size.
4
 Difference in AICc value relative to the model with the lowest AICc. A ΔAICc of 0 indicates the best model.
5
 The Akaike weight of each model.

  1

 
 
 
 

winter (Lindberg et al., 2007) and spring staging (Wilson
and Atkinson, 1995; Moore et al., 2004). Thermoregulatory
demands are relatively high for wintering brant in Alaska
(Mason et al., 2006), and undisturbed access to sufficient
amounts of eelgrass is likely crucial to their winter survival,
especially considering the access limitations imposed by
short daylight foraging opportunities and regular periods of
extreme ice cover in December and January. There are no
published data to indicate whether eelgrass accessibility or
brant survival has increased in Alaska. Nevertheless, Mason
et al. (2006), who compared the nutritional condition of wintering brant in Alaska and Mexico under the current climatic
conditions, found that physiological demands were similar
for the two wintering populations, indicating that the birds
were capable of employing either wintering strategy (i.e.,
winter residency or migration). Further, with the relatively
higher nutritious value of eelgrass in Alaska and its closer
proximity to breeding areas, recent milder conditions on
the Alaska Peninsula may provide a selective advantage for
brant to stay north (Mason et al., 2006).
The increase in wintering brant in Alaska is likely a
result of birds “short-stopping” on their southward migration flight and remaining north of their traditional wintering areas. This behavior is not uncommon in Arctic-nesting
geese and has been documented in association with, among
other factors, increases in food availability and temperature
at northern sites (see Calvert et al., 2005). Individual brant
also appear capable of changing their migration strategy
between years, as evidenced by the 15% (34 of 227) of individuals resighted in different years at either an Alaskan or a
southern wintering site (D. Ward, unpubl. data).

Our finding of a negative correlation between the size
of the Alaska wintering population and the number of days
with strong northwesterly winds in November corroborates
a recent theoretical evaluation of migration strategies of
Pacific brant. Purcell and Brodin (2007) found that changes
in the occurrence of tailwinds prior to onset of winter, in
combination with even a slight increase in survival (resulting
from milder winters), could cause a sudden shift in wintering strategy from a long-distance migration to either a shortdistance detour migration or winter residency on the Alaska
Peninsula. Together, these investigations emphasize the
importance of tailwinds for successful brant migration from
Alaska, an event that is characterized by a long period (ca.
two months) of slow fuel gain before departure and an initial
non-stop flight across the Gulf of Alaska (without the possibility of refueling) of at least 2000 km for detour migrants
and up to 5500 km for long-distance migrants (Ward and
Stehn, 1989; Dau, 1992). We anticipate that brant numbers
will continue to increase during winter in Alaska given climate predictions for increasing temperatures and decreasing ice at high latitudes (IPCC, 2007) and a northward shift
in the storm track of the Aleutian Low pressure system
(McCabe et al., 2001; Salanthé, 2006) that will likely reduce
the opportunities for wind-aided migration. As more brant
opt to winter in Alaska, more of the population will be put at
risk should mild winters be punctuated by any extended periods of severe cold weather and extreme shorefast ice cover,
as occurred in winter of 1991 – 92 (Fig. 3B). Such scenarios
are likely to become more frequent if predictions that couple
climate warming to greater climate variability and a rise of
extreme weather events prove accurate (IPCC, 2007).

 Warm

Cold

Warm

Cold

Warm

A

6000
5000
4000
3000
2000
1000

Year
FIG. 5. Frequency of days with strong (> 18 km/hr) northwesterly (270 – 360˚)
winds in relationship to cold phases (1989 – 91 and 1998 – 2000) and warm
phases (1986 – 88 and after 2000) of the Pacific Decadal Oscillation.

Breeding Origin
Most brant in the Alaska winter population were from
Arctic breeding colonies. We believe this interpretation to
be accurate for two reasons. First, while observations were
made at only one embayment, Kinzarof Lagoon, we regularly observed flocks of brant moving between Kinzarof and
Izembek lagoons and between Izembek and western embayments, which suggests that breeding populations were mixing among wintering locations on the Alaska Peninsula.
Second, we were conservative in our test of the relative
contributions of the different breeding colonies to the wintering population because in calculations of bands available
for observation from the respective populations, we used
the lowest survival estimate for birds from the larger subArctic colony and the highest survival estimate for birds
from the smaller Arctic colonies (Ward et al., 2004).
We cannot rule out that a portion of the increase in the
Alaska wintering population has been caused by growth
in the Arctic breeding population. Although this component of the brant breeding population, which represents
more than 15% of the overall Pacific Flyway population of
nesting brant (Sedinger et al., 1993; Pacific Flyway Council, 2002), has never been systematically surveyed, there is
indirect evidence that it has grown since the mid 1990s. For
instance, on the Arctic Coastal Plain of Alaska, numbers
of brant nests were stable from the mid 1970s to the early
1990s (Stickney and Ritchie, 1996), but increased in the
mid 1990s (Sedinger and Stickney, 2000). A similar pattern
of stability followed by a slight positive trend was evident
in the number of brood flocks in the Arctic between 1976
and 2005 (Flint et al., 2008). Consistent with the notion of
a growing Arctic population, survival of western Canadian Arctic brant improved for adults (> 15%) and juveniles
(> 40%) between the 1960s and 1970s and 1991 – 2001
(Hines and Brooks, 2008). Together these trends suggest that
the Arctic population has grown, particularly since the early
1990s, and that if these birds have a propensity to stay north
during winter (as band resightings suggest), it has contributed to the increase in the Alaska wintering population.
Our study shows that the western end of the Alaska
Peninsula has become an important wintering area for

0

B
Mean number of brant

20
18
16
14
12
10
8
6
4
2
0

1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004

No. days of NW winds
>18 km/hr in November

308 • D.H. WARD et al.

Izembek L. Bechevin B. Morzhovoi B. Kinzarof L.
2500
2000
1500
1000
500
0

C

18000
16000
14000
12000
10000
8000
6000
4000
2000
0

1

2

3

Area

4

5

6

Izembek L.
All other sites combined

0

20

40

60

80

100

Ice cover at Izembek Lagoon (%)
FIG. 6. Mean number (± 95% CI) of brant counted on 33 aerial surveys in four
embayments on the Alaska Peninsula (A) and within six areas of Izembek
Lagoon (B) between 1986 and 2004. Also shown is the relative pattern of brant
use at Izembek Lagoon and the three other wintering areas combined in relation
to shorefast ice cover in Izembek Lagoon (C). Locations of embayments and
survey areas within Izembek Lagoon are shown in Figure 1.

brant and that they are capable of adapting to the changing environmental conditions in the region. Since the end
of our study in 2004, numbers of wintering brant in Alaska
have continued to rise, highlighting the increased importance of the region. Currently, the Alaska Peninsula supports 25 – 30% (up to 40 000 birds) of the entire Pacific
flyway population of brant during winter and contains the
greatest concentration of brant outside of Mexico (Drut and
Trost, 2008). Given that significant numbers of brant now
regularly use the Alaska Peninsula for up to eight months of
the year, it would be prudent to determine the region’s carrying capacity for brant and assess patterns of local movements by birds in relation to the abundance and distribution
of eelgrass in all embayments and on the nearshore islands.
It is also important to understand the effects (whether positive or negative) on the region’s carrying capacity related
to increasing climate change pressures (e.g., increasing sea
levels, coastal erosion, temperatures, and CO2 levels) and
proposed nearshore and coastal development (e.g., oil and
gas development in the North Aleutian Basin; Minerals

 Brant wintering in Alaska • 309

Management Service, 2007). Finally, any threats to the
Alaska wintering population have implications for the entire
Pacific Flyway population; therefore, further monitoring
of brant in Alaska in winter is warranted, especially given
that this species is experiencing a long-term decline and is
of conservation concern across its range (Pacific Flyway
Council, 2002; Ward et al., 2005).
ACKNOWLEDGEMENTS
Funding for this project was provided by the Izembek National
Wildlife Refuge, U.S. Fish and Wildlife Service-Region 7, and
the U.S. Geological Survey-Alaska Science Center. We thank N.
Chelgren, T. Olson, D. Safine, B. Schulmeister, and L. Ziemba
for their assistance with the fieldwork. We are also grateful to D.
Derksen for his support. D. Douglas and H. Eicken kindly provided their expertise on the ice dynamics. This paper benefited
from discussions with P. Flint and from reviews by T. Alerstam,
T. Bowman, T. Fondell, J. Hupp, and two anonymous reviewers.

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