Polar bears, as is well known, are creatures of the sea ice. Until the height of the summer melt in the Arctic forces them ashore, they make use of every last minute that they can, prowling the floes in search of the ringed seals that are their favored prey.
When they expand their horizons, it is mostly to take advantage of opportunities afforded by the presence of other marine mammals: bearded seals, belugas or, more dangerously, walruses. When ashore, they will munch on algae, grasses, and carrion, and have been known to occasionally kill birds or even caribou – but such terrestrial predation has been widely considered purely opportunistic. There have been virtually no records of polar bears actively seeking out prey during their time on land.
In a new study in the journal Polar Biology, Paul Smith of Environment Canada and colleagues document four cases of polar bears eating snow geese eggs, and the eggs and chicks of thick-billed murres, in Arctic Canada.
In 2004, researchers saw a solitary female bear walking from nest to nest in a snow goose colony on Southampton Island, consuming the entire contents of each nest.Two years later, between three and five bears exhibited similar behavior on nearby Coats Island.
Perhaps more remarkable, in 2000 and then again in 2003, a solitary bear actually climbed onto cliff ledges to consume murre chicks and eggs, also on Coats Island.
Prior to 2000, no such behavior had been observed during 17 years of study of the Coats Island murre colony, but Smith and his co-authors suggest that the incidents they witnessed may not be isolated ones:
They suggest the reason for the apparent increase in such predation is timing. The first polar bear sighting of summer on Coats Island advanced by approximately 20 days between 1985 and 2007, which correlates almost exactly with observed increases in break-up of summer sea ice in Hudson Bay over that same period.
Conversely, the nesting date of snow geese on Southampton Island has advanced by only two days per decade since 1968, while that of thick-billed murres on Coats Island has advanced by five days since 1988. In other words, the birds' nesting season now increasingly overlaps with the polar bears' presence on land, whereas previously it generally did not do so.
The consequences for the targeted bird colonies were catastrophic: on three of the four occasions, every available egg was eaten, and on the other occasion, the great majority of eggs was consumed.
The authors conclude:
Photographs copyright Kerry Woo