Driven from Arctic sea ice earlier because of global warming, polar bears hunted for duck eggs on land but did so with less-than-optimal movement and awareness of resource scarcity, according to a new study, raising questions on their ability to adapt to living off the ice.
A team of Canadian biologists, who published their findings April 6 in Royal Society Open Science, suggested that the polar bears' inefficient hunting means that seabird eggs might not play as large a role in their future diets as expected — despite predictions that the endangered bears will rely on the eggs for sustenance more heavily as melting sea ice gives them fewer opportunities to hunt seals.
Lead author Patrick Jagielski, an ecology researcher at the University of Windsor, said that for more than two decades his research team has been studying the stressors of common eider ducks on Mitivik Island, a 0.1 square mile island just outside the Arctic Circle in Canada's Hudson Bay. In recent years, the researchers noticed that polar bears were arriving on the island earlier in the year than usual and were eating common eider eggs.
"Given the increasing perturbation by bears here (and at other avian colonies), it was important to determine if polar bears are gaining appreciable calories from eggs ... and if they are foraging efficiently," said Jagielski, who was a master's student when the study was conducted.
Polar bears have been known to eat seabird eggs, but not usually in significant amounts because their primary prey are seals, whose fat-dense blubber contains large amounts of calories. But rapid warming in the Arctic is forcing polar bears to travel farther and expend more energy by swimming to find food. Seabird eggs are considered part of an emerging alternative diet for polar bears.
In 2017, Jagielski and his co-authors — who included University of Windsor, Canadian government and Inuit researchers and partners — tracked polar bears on video with drones and camera traps as they traveled through Mitivik Island and ate eggs from the nests of common eiders. Up to 20 bears were observed between July 10 and 20, about halfway into the average incubation period of the birds' eggs.
The observed polar bears deviated from optimal hunting behavior in a few ways. They could not distinguish a full nest from one that had already been hunted, and their movement was inefficient as nests with eggs became more scarce; they made a large number of turns, for example.
The bears also found and ignored some groups of eggs, known as clutches, which contradicted the authors' predictions.
"The most surprising thing that we found is that bears ignore clutches, which we hypothesized was a result of being covered by eider faeces," Jagielski said, referring to the ducks' defense mechanism against predators. "We naturally assumed that they would eat every clutch they came across, but that was not the case."
Aspects of the flawed hunting could be attributed to inexperience with hunting for bird eggs or already being "full" before arriving on the island, the researchers said. They also noted that polar bears have been known to ignore or walk by full seabird nests directly in their foraging path, according to previous research.
The bears did get some parts of egg-hunting right. They foraged more nests when they picked up on visual cues, such as fleeing common-eider hens, though they didn't use the strategy consistently, according to the researchers. They also became less picky as fewer untouched nests remained on the island.
The study has a limited sample with only one year included, Jagielski said, but it provides a baseline for polar bears' hunting abilities as well as rare high-quality footage of their hunting behaviors.
In a paper published in January, the same group of researchers found that polar bears can satisfy or even exceed their daily energy usage by eating common eider eggs early in the season. But if they are worse at hunting for eggs than expected, the benefit the endangered species can derive from the eggs is limited.
"Poor foraging performance may influence the energy bears gain from eggs, which may impact the overall contribution they make to their diet," Jagielski said.
Furthermore, it is unclear whether common eider eggs can play a sustainable role in the polar bear's diet. The biologists suggested that polar bears could become more effective egg hunters over time as they become more experienced over several hunting seasons. But the duck population could face "devastating" consequences if more polar bears hunted their eggs, they said, and the eggs could become an unreliable food source as the bears look to replace the seal-shaped hole in their diet.
Jagielski and his research team are now investigating how common eiders are responding to the polar bears' presence and whether they may use strategies to delay predation of their eggs long enough for their chicks to hatch.
The study, "Polar bears are inefficient predators of seabird eggs," published April 6 in Royal Society Open Science, was authored by Patrick Jagielski, Cody Dey, Oliver Love and Christina Semeniuk, University of Windsor; and H. Grant Gilchrist and Evan Richardson, Environment and Climate Change Canada.