National Geographic News

Brian Handwerk

National Geographic

Published October 21, 2013

When the sea ice ebbs during the Arctic summer, some hungry polar bears snack on a bounty provided by seasonal visitors—nests of snow goose eggs.

Now scientists are studying whether the big eggs might become a more regular polar bear menu item, if a warming Arctic lands them on solid ground more often.

David Iles, a Ph.D. candidate at Utah State University, studies what geese eat and, in this case, what eats geese. For several years Iles has been studying polar bear predation on birds and eggs in Western Hudson Bay near Churchill, Manitoba, with colleagues from Utah State and the American Museum of Natural History.

"Sea ice is melting sooner on average, and polar bears are coming back to shore earlier," said Iles, who worked with Utah State's David Koons, who received funding from the National Geographic Committee on Research and Exploration. "So what we're seeing is an increased overlap with nesting waterfowl. That's what we're trying to study.”

“In terms of snow geese there's 50,000 pairs out there, and that could be quite a substantial benefit to polar bears that do happen to take advantage of them,” he continued. “But what we don't yet know is how often that overlap happens, what types of bears are taking advantage, and what it could mean for both polar bears and waterfowl."

Each year Hudson Bay's sea ice melts completely in the summer, and the region's polar bears traditionally spend a few months on shore. (Related: "Longest Polar Bear Swim Recorded—426 Miles Straight.")

Meanwhile, snow geese and other migratory birds that winter in warmer southern climes arrive at their Arctic breeding grounds around the end of May. Millions of them breed, nest, and lay eggs there through August—and the birds and their eggs are easy prey.

Iles and colleagues used remote cameras to catch animals preying on nests. They recorded bald eagles, ravens, arctic foxes, and wolves enjoying egg meals, as expected, but they also saw numerous feasting bears.

The bears are opportunistic eaters, and when ashore they are also known to dine on whale carcasses, berries, grasses, and anything else they come across, including live geese. (See "Pictures: 80 Polar Bears Throng Village in Search of Whale.") It's been observed for decades that the bears occasionally make a meal of the goose eggs, but Iles and colleagues may be the first group to have studied the predation in depth.

High-Calorie Snack

"Whenever you find polar bear scat on the landscape in these bird colonies, [it] will be full of eggshells, egg membranes, along with parts of whatever [other] types of foods they've been eating," Iles said.

During the later part of the summer, when bears are more likely to be off the sea ice, the eggs may be half developed or even contain a mostly formed gosling close to hatching, which seems just fine with the bears.

A typical snow goose nest has four or five eggs, each perhaps twice the size of a typical chicken egg and packed full of four or five times as many calories. Iles's co-researcher Robert Rockwell, of the American Museum of Natural History, previously estimated that about 88 goose eggs would give a bear the caloric equivalent of eating a seal. And the eggs offer much easier pickings.

"The nests don't run away from them like a caribou might," Iles said. "It's a ton of calories just basically sitting there that the bears can take advantage of."

But will they take advantage more often, or do eggs represent merely the odd snack? Arctic sea ice has been shrinking in recent years, causing problems for the polar bears that live most of their lives on it hunting seals. (This summer, Arctic sea ice rebounded from 2012's record low, but is still trending down.)

Snow geese populations have boomed in the Arctic for 40 years or so, thanks to agriculture and increased food sources in the birds' southern winter habitats.

"These snow goose populations are way out of control, and they are degrading parts of the tundra," Iles explained. Increased bear predation on the eggs could conceivably push those populations back into a more sustainable state, he added, while feeding those bears in need of some additional nourishment.

But will it happen?

"What we want to know is, over time, will the bears' [time off the ice] start overlapping more and more with the nesting period of geese?” Iles said. “Are the bears that are eating eggs in poor body condition in general? And what we really don't know yet is, What does this mean for polar bears?"

“We need to do more research,” he said, “to figure out how much benefit this will really be to polar bears, the effects that this will really have on snow goose populations, and how sustainable this type of predation will be, depending on how severe it is."

The team's most recent study appeared in the September issue of Polar Biology. Another recent paper by some of the same researchers was published in the journal Ecology and Evolution.

No "Golden Goose" Can Save Polar Bears If Sea Ice Melts

While goose eggs may help some bears add calories, Steven Amstrup, chief scientist for Polar Bears International, cautioned that they can't save the species if a warming Arctic ecosystem loses its sea ice.

Amstrup said opportunistic bears have been known for decades to occasionally eat nesting birds or their eggs when on land during the summer. It may happen a bit more frequently if bears spend more time on land, he added, especially time coinciding with the easy availability of nesting birds.

"Unfortunately, we have no evidence to suggest that eating geese, or eating char, or eating anything else has any ability to offset the loss of the sea ice and the [primary] foraging opportunities it presents for polar bears," Amstrup said.

"If you read the scientific studies, [Iles and co-authors are] careful to say that this could benefit some individual bears, could offset some of that energy loss from lost feeding opportunities, but that this is not some kind of salvation for polar bears.”

“Unfortunately, in the past few years some media reports have suggested that this might mean polar bears could just come ashore and eat terrestrial foods and somehow do fine without the sea ice,” Amstrup said. We have absolutely no evidence that they have the ability to do this." (Read "On Thin Ice" in National Geographic magazine.)

In fact, he added, evidence from some existing ecosystems suggests that bears eating eggs is not likely to be a recipe for success. In northern Alaska, landlubber grizzlies live adjacent to polar bears on sea ice in a coastal region rich with all types of shorebirds and other terrestrial foods like caribou and vegetation.

But those grizzlies may be the smallest brown bears in the world except the tiny, endangered population of Gobi bears in the food-poor desert, Amstrup noted. Grizzlies are not only relatively small, but are also spread thinly across Alaska's North Slope.

For scientists, the challenge will be to figure out how a whole population of polar bears—whose average size makes them the largest bears in the world—could make it in a place that currently supports only small numbers of small bears.

For the bears, the challenge will be survival, Amstrup says.

"So what logic would lead us to think that you could dump a whole population of the world's largest bears into that same kind of environment, with the same resources, and expect everything to be OK?"

 

Jason Kurtis contributed to this report.

3 comments
Rer Slovak
Rer Slovak

Yet again, Amstrup displays his bias, lack of open-mindedness and sheer refusal to accept any information that contradicts HIS previous publications or anyone of the others in the "Polar Bear Mafia." This select group of scientists have for years dictated and manipulated all info given to the media regarding PBs. Now that bears are displaying their phenotypic plasticity, the Mafia looks foolish and they'll die before they admit their doom & gloom predictions are totally wrong. Pathetic behaviour from so-called educated men.

John Donohue
John Donohue

On this: "...polar bears could just come ashore and eat terrestrial foods and somehow do fine without the sea ice,” Amstrup said. We have absolutely no evidence that they have the ability to do this." the writer of this piece must respond to this:

During the warmest part of this interglacial, around 8000-10000 years ago, the North was completely free, or nearly free, from ice year round, excepting Greenland. This iceless North lasted 1000 years or so. How did the bears, and the geese, survive?

THAT is what they should be investigating.

Share

Feed the World

See blogs, stories, photos, and news »

Latest From Nat Geo

See more photos »