As climate change melts sea ice earlier in the year, polar bears have begun to hunt for meals on land, like bird eggs. They’re not very good at it, a new drone-based study reveals. That could be another bad omen for the bears’ ability to survive a changing world, scientists say.
“This is a really elegant study,” says Robert Rockwell, an ecologist at the American Museum of Natural History who has studied polar bears for 50 years. He calls the team’s use of drones an “excellent” way to observe the animals’ behavior.
To make the find, University of Windsor biologist Patrick Jagielski and colleagues visited Mitivik Island. Located in Canada’s Hudson Bay, this spit of land—just a bit bigger than New York City’s Grand Central Station—hosts up to 8000 sea ducks, or eiders, every summer. (The island’s name means “the duck place” in Inuit.)
The island has also become a favorite of polar bears. For the past 10 years, the animals—which typically hunt seals and other marine animals on sea ice—have been visiting Mitivik at the same time the seabirds nest, during spring and early summer. “Some stop onto the island to snack on eggs,” Jagielski says. “Polar bears are like teenagers,” Rockwell adds. “They are always hungry.”
From behind the safety of an electric fence, Jagielski and his team launched multiple drones over the island in July 2017, just before the eider eggs were supposed to hatch. After 11 days, they had more than 16 hours of footage from about 20 polar bears.
At first, the bears were pickier about which eggs they ate. They may have been avoiding those covered by feces, a strategy many eiders use to drive away predators. But eventually, the bears overcame their distaste and decimated the nests, leading to almost the complete depletion of the colony.
Still, the bears were not efficient hunters, the team reports today in Royal Society Open Science. The drone footage revealed that, as the season progressed and fewer eggs were left untouched by the bears, the animals often wandered fruitlessly from empty nest to empty nest—suggesting they were wasting precious energy because they couldn’t tell from far away whether a nest had eggs in it.
The ultimate impact on the bears and the birds is unclear. On larger areas of land, the bears may be expending even more critical energy with their inefficient egg hunts, Jagielski says.
Rockwell counters that his own work in the Hudson Bay Lowlands, a wetland 800 kilometers south of Mitivik Island, has shown polar bears there seem to be very effective egg foragers. Perhaps that’s because they have been dealing with ice melt longer and have learned how to find these morsels. He recalls seeing one bear eat 270 eggs in just 96 hours. “Our experience is that they are very methodical,” he says. “They eat one nest, then stand up, look around, and walk directly to the next one.”
The impact on seabirds is also unclear. Even if polar bears aren’t great egg hunters, they could eventually endanger eiders and other birds if more of them shift to an egg diet and attack more bird colonies. For now, only 30% of the eider colonies in Canada are visited by polar bears.
However Mark Mallory, a biologist at Acadia University, says that as pressure on bird colonies keeps increasing, the birds will just move to other areas. In Frobisher Bay in the Canadian Arctic, he has already noticed that eiders have shifted their nesting places to escape the hungry bears. “In the short term, the bears are definitely a problem,” he says. “But if you look into it for 20 years, the eiders will probably move around.”
Source: Science Mag