Stempniewicz_06_Polar Bear Predation Gulls Barnacle.pdf

ARCTIC

VOL. 59, NO. 3 (SEPTEMBER 2006) P. 247 – 251

Polar Bear Predatory Behaviour toward Molting Barnacle Geese

and Nesting Glaucous Gulls on Spitsbergen

LECH STEMPNIEWICZ 1

( Received 12 September 2005; accepted in revised form 4 November 2005 )

ABSTRACT. A polar bear ( Ursus maritimus ) was observed stalking and chasing a flock of adult flightless barnacle geese ( Branta

leucopsis ) in a coastal bay in Hornsund, southwest Spitsbergen. Before chasing the geese, the polar bear appeared to make use of

the cover provided by a rocky pier in order to swim close to them. Immediately after that unsuccessful attempt, the bear plundered

a glaucous gull ( Larus hyperboreus ) nest situated on coastal rock and captured three nestlings. These observations are the first

documented instances of a polar bear hunting for barnacle goose and glaucous gull and provide additional evidence of the use of

terrestrial prey by polar bears during the season of minimal ice extent.

Key words: polar bear, Ursus maritimus , barnacle goose, Branta leucopsis , glaucous gull, Larus hyperboreus , predation,

Hornsund, SW Spitsbergen

RÉSUMÉ. Un ours polaire ( Ursus maritimus ) a été observé en train de traquer et de pourchasser une bande de bernaches nonnettes

( Branta leucopsis ) coureuses adultes dans une baie côtière de Hornsund, dans le sud-ouest de Spitsbergen. Avant d’avoir chassé

les bernaches, l’ours polaire semblait s’être servi d’un quai rocailleux pour se cacher et nager près des bernaches. Immédiatement

après cette tentative échouée, l’ours a pillé un nid de goélands bourgmestres ( Larus hyperboreus ) situé sur un rocher côtier et a

capturé trois oisillons. Ces observations, les premiers exemples de bernaches nonnettes et de goélands bourgmestres pourchassés

par un ours polaire, fournissent d’autres preuves selon lesquelles les ours polaires ont des proies terrestres pendant la saison de

l’étendue minimale de glace.

Mots clés : ours polaire, Ursus maritimus , bernache nonnette, Branta leucopsis , goéland bourgmestre, Larus hyperboreus ,

prédation, Hornsund, sud-ouest de Spitsbergen

Traduit pour la revue Arctic par Nicole Giguère.

INTRODUCTION

mainly subadults (Lønø, 1970; Derocher et al., 2000; Dyck

and Daley, 2002).

Polar bears have also been observed to prey on eggs,

nestlings, and adults of colonial nesting seabirds and

waterfowl, including Brunnich’s guillemot ( Uria lomvia )

(Donaldson et al., 1995), little auk ( Alle alle )

(Stempniewicz, 1993), snow goose ( Anser caerulescens )

(Abraham et al., 1977), Canada goose ( Branta canadensis )

(Smith and Hill, 1996), and pale-bellied brent goose ( Branta

bernicla hrota ) (Madsen et al., 1989), as well as on terres-

trial species such as willow ptarmigan ( Lagopus lagopus )

(Miller and Woolridge, 1983). In addition, polar bears

have been observed to consume vegetation, both marine

(e.g., Laminaria) and terrestrial (e.g., grass, berries,

Cochlearia) (Russell, 1975; Derocher et al., 1993; own

observations). However, the significance of waterfowl

and vegetation components in their diet is considered

minor (Knudsen, 1978; Lunn and Stirling, 1985; Ramsay

and Hobson, 1991; Ramsay et al., 1991; Hobson and

Stirling, 1997).

This paper describes observations made of a polar bear

stalking and chasing flightless adult barnacle geese ( Branta

leucopsis ) at sea, and plundering the nest of a glaucous gull

Polar bears ( Ursus maritimus ) are marine predators of the

Arctic sea ice. They feed primarily on ringed seal ( Phoca

hispida ), bearded seal ( Erignathus barbatus ), and harp

seal ( Phoca groenlandica ), and less commonly on beluga

whale ( Delphinapterus leucas ), narwhal ( Monodon

monoceros ), and walrus ( Odobenus rosmarus ) (Lønø, 1970;

Stirling and Archibald, 1977; Smith, 1980; Lowry et al.,

1987; Smith and Sjare, 1990; Rugh and Shelden, 1993;

Stirling and Øritsland, 1995). In many Arctic regions,

summer ice melt is known to limit the access of polar bears

to seals (Stirling et al., 1999), forcing the bears to fast

during summer and early autumn (Watts and Hansen,

1987) for periods of up to several months. However, the

bears do feed opportunistically on alternative foods. Polar

bears have been observed to hunt large terrestrial mam-

mals such as caribou ( Rangifer tarandus ) (Derocher et al.,

2000; Brook and Richardson, 2002) and muskox ( Ovibos

moschatus ) (Ovsyanikov, 1996). Moreover, polar bears

are both scavengers, feeding on carrion and anthropogenic

organic waste, and occasionally cannibalistic (especially

adult males), attacking cubs and younger or weaker bears,

1 Department of Vertebrate Ecology and Zoology, University of Gdansk, Legionów 9, 80-441 Gdansk, Poland, e-mail: biols@univ.gda.pl

248 • L. STEMPNIEWICZ

FIG. 1. Polar bear localizing barnacle geese resting on the inshore islet after approaching them from the opposite side of the flat rocky pier, at Gnållberget, Hornsund,

Spitsbergen, on 23 July 2005. Photo by L. Iliszko.

( Larus hyperboreus ) in Hornsund, SW Spitsbergen. The

observations were made opportunistically, during a seabird

study project.

climbed the rock, causing the chicks to abandon the nest for

the water. The bear then entered the water and caught and

consumed all three chicks. Following this event, the bear

swam away toward the Treskellen peninsula, 10 km away,

where it plundered a hut later the same day.

DESCRIPTION OF OBSERVATION

On 23 July 2005, I observed an adult male polar bear

along the seacoast at Gnållberget, Hornsund, southwest

Spitsbergen (77˚00 ' N, 15˚28 ' E). The weather was calm,

warm (> 10˚C) and sunny, with very good visibility. The

polar bear was observed for approximately 1.5 hours

(starting at 0930 local time) as it explored the beach and

rested in a hollow dugout in the gravel.

At 1100, the bear noticed a flock of 65 flightless adult

barnacle geese resting on a flat rock 50 m from shore. It then

moved slowly toward the sea, entered the water, and swam,

with only the top of its head protruding above the water

surface. The bear appeared to be stalking the birds, using the

flat, rocky pier as cover to avoid detection. When it arrived at

the islet on the side opposite the geese, it stopped and raised

its head to look for the birds (Fig. 1). At that moment, the

geese noticed the bear, then entered the water and swam away

in a close group toward the open sea. The bear immediately

rushed after the geese. During most of the chase, it was about

4 to 5 m behind them (Fig. 2), and a couple of times, the bear

speeded up, attempting to reduce this distance. The geese

responded with wing flapping and running across the water

surface to maintain their distance from the bear. At one point,

a rapid charge from the bear separated two geese from the

flock. The bear immediately followed them, but they rejoined

the flock. After swimming for several hundred meters toward

the open sea, the flock turned back to the shore, making a

loop, and then repeated this maneuver. In the shallow water

close to the beach, the bear made a sudden but unsuccessful

rush on the flock (Fig. 3). In total, the pursuit lasted for 30

minutes, but the bear did not manage to capture any geese.

The polar bear continued to swim along the shoreline and

soon encountered a small, rocky island inhabited by a pair of

breeding glaucous gulls with three-week-old young. The bear

DISCUSSION

The likelihood that a polar bear can capture a healthy,

adult barnacle goose at sea is presumably low. The ob-

served attempt of hunting could have been successful if

there had been young birds in the group chased. Adult

geese periodically become flightless when they are molting

their primary feathers. During this time, as well as during

the rest of the chick-guarding period, they keep close to a

water body (tundra ponds, lakes, or sea), to which they can

escape immediately when in danger. Such behaviour is

particularly efficient against predation by the arctic fox

( Alopex lagopus ), which avoids swimming, but only flight-

less adult birds may find it effective against polar bear

predation. Goslings swim much slower and have less

endurance than adults. On a few occasions, I observed

tourists kayaking along the shoreline and scaring away

barnacle geese. Each time, the goslings could not keep up

with the escaping adults for more than a few minutes, and

then lagged behind. Given the swimming speed of the

polar bear that I observed and the duration of its pursuit,

goslings would have a high likelihood of capture by polar

bears. A polar bear chasing a flock of geese in the water for

a long enough time has the potential to capture a large

proportion of the goslings.

In contrast to the Polish Polar Station area, where bears

are actively hazed and driven away by station personnel, at

Gnållberget (10 km away) they are generally left undis-

turbed. Most of the 70 pairs of barnacle geese near the

Polish Polar Station had two or three young. At Gnållberget

the situation was similar during the area survey at the

beginning of July. However, in the second half of the

month, only 80 adult geese and no goslings were observed.

POLAR BEAR PREDATORY BEHAVIOUR • 249

FIG. 2. Polar bear chasing a flock of flightless barnacle geese at sea, in Gnållberget, Hornsund, Spitsbergen, on 23 July 2005. Photo by L. Iliszko.

FIG. 3. Polar bear charging in the shallow water, at Gnållberget, Hornsund, Spitsbergen, on 23 July 2005. Photo by L. Iliszko.

This suggests that most goslings at Gnållberget may have

been lost to predation by polar bears.

The energetic cost to polar bears of taking terrestrial

prey can be high, for example, in the case of hunting

caribou, which are highly vigilant and well adapted to run

at high speeds for a long distance (Brook and Richardson,

2002). This cost is especially high when food items are

small and provide little energy. Lunn and Stirling (1985)

predicted that because of the high cost of running, a bear

chasing snow geese on tundra would not receive a net gain

in energy unless it caught a goose within 12 seconds. A

spectacular example was polar bear predation of a little

auk colony (Stempniewicz, 1993). A bear had to overturn

boulders weighing several hundred kilograms to get ac-

cess to one nest, capturing at most one adult and one

nestling bird with a combined weight of 0.25 kg.

Hunting success of any active predator, including the polar

bear stalking seals and other prey able to escape, depends

largely on minimizing the distance separating the predator

and prey just before an attack. The ability to get close to prey

during stalking is influenced by hiding opportunities (ice,

rocks, vegetation, etc.) and by wind direction (Stirling, 1974,

1988; Stirling and Archibald, 1977). In this observation, the

polar bear appeared to be using the rocks protruding from the

shallow water to move as close as possible to the resting geese

before being detected. As no wind was recorded that day,

wind direction couldn’t have had any importance when the

bear chose the stalking route. Polar bears are well adapted to

hunt for pinnipeds, which at least in some situations use smell

to detect predators (Lønø, 1970). It would be interesting to

know whether bears take wind direction into account even

when stalking seabirds, which generally lack a sense of smell.

In Svalbard, glaucous gulls nest in two types of habitats.

The first one is mainland mountain slopes, where gulls

often form colonies, and their individual territories are

limited to the nest vicinity and do not cover feeding area.

Nests are accessible to arctic foxes, but gulls use efficient

social colony defense. However, breeding in high density

involves cannibalistic practices (eggs, nestlings). This

type of breeding makes it possible to maintain high local

gull population numbers. The second type of habitat used

by glaucous gulls for nesting is inshore rocks separated by

some water from the coast. In the latter case, gulls have

large individual territories that contain their feeding areas

and are safe from arctic foxes (Stempniewicz, 1995; L.

Stempniewicz, unpubl. data). However, these nests appear

completely vulnerable to bears. Polar bears can both climb

the several-meter-high rocks and swim very well, and

therefore they have no trouble collecting gull nestlings

that abandon their nests for the water.

250 • L. STEMPNIEWICZ

Polar bears visit Hornsund regularly throughout the

year, and their numbers recorded in the summer periods

have increased markedly during the last decades (Polish

Polar Station, unpubl. data). The bears are present in the

area during the breeding periods for glaucous gulls and

barnacle geese, when defenseless eggs and chicks are

available. Bears generally explore the coastal area, and

especially the neighborhood of the Polar Station and the

huts dispersed around the fjord, in search of any organic

waste, as well as stranded carrion. They also penetrate

lower parts of the little auk and Brunnich’s guillemot

colonies and take accessible eggs, nestlings, and adult

birds. Barnacle geese and glaucous gulls are common

birds in the Hornsund area, especially in the vicinity of

large seabird colonies, where locally abundant food in-

cludes eggs, chicks, and adult seabirds, as well as vegeta-

tion (Stempniewicz, 2006). As polar bears, barnacle geese,

and glaucous gulls occur together in this area for five to six

months each year, interactions between them can occur

frequently.

As a result of climate warming in the Arctic, the sum-

mer fast-ice range is receding northward (Dickson et al.,

2000; Hurrell, 2002; Stenseth et al., 2002), which may

force an increasing number of polar bears ashore earlier in

summer and for a longer time. These bears are in a poorer

nutritional state because seals are inaccessible (Stirling et

al., 1999). Poor condition may increase the pressure that

polar bears exert on alternative terrestrial food sources,

such as organic waste left by people and large colonies of

seabirds and waterfowl. Hunting for birds may concern

only individual polar bears and is more likely to occur in

southern parts of the Arctic, where summer sea-ice condi-

tions are poorer. However, the regular exploitation of the

little auk colony by polar bears observed on Franz Josef

Land (Stempniewicz, 1993) contradicts this statement.

Even a few land-locked, nutritionally stressed polar bears

may potentially have a huge impact on recruitment in local

populations of birds breeding in colonies.

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ACKNOWLEDGEMENTS

These observations were made while conducting research

supported by grant no. PBZ-KBN-108/P04/2004 from the State

Committee for Scientific Research. The members of our expedition,

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