The melting Arctic ice caps are making it difficult for polar bears to hunt for their main prey, forcing them to turn inland to prey more and more on the eggs of breeding birds.
Birds' eggs have always been part of the polar bear's diet during the summer breeding season, but researchers have found that as the summer sea ice season shortens each year, more and more polar bears and their young show up at coastal areas much earlier to feed on the eggs of the barnacle goose, glaucous gull, and elder duck, as well as the pink-footed goose located further inland.
Ornithologist Jouke Prop of the University of Groningen in the Netherlands says that as the summer sea ice season shortens by 3.5 days each year, polar bears arrive at the colonies three days earlier each year. This is based on information gathered from NASA satellite images of the extent of the melting ice in Greenland and Svalbard, Norway and on-the-ground observations of bird colonies in the areas.
Prop says polar bears also arrive at the coastal areas about a month earlier than they have been since the 1970s, a change that could toll the death knell for Arctic colonial birds, the researchers believe.
"When a new predator enters the scene, the delicate balance between predators and their prey may be perturbed," Prop told the Daily Mail. "Polar bears colonizing coastal areas, from which they have been absent for a long time, is an example of this situation."
Although the several dozens of polar bears that prey on birds' eggs earlier each year comprise a small set of the polar bear population, the researchers call the predation "severe."
In Nordenskioldkysten in Svalbard, where Prop and his team first observed the polar bears feeding on birds' eggs earlier than they used to, Prop says a single polar bear can devour up to 200 unhatched eggs for two hours on average, although they have seen some bears eating as many as 1,000 eggs in the same amount of time, before sleeping for up to 16 hours and waking up for another meal of the same size.
In other locations, such as Belsund, Kongsfjorden, and Hornsund on Svalbard and Traill Island in Greenland, Prop says polar bears fed on all but some 10 percent of the eggs in 2014.
However, while the bears' new feeding habits may currently be detrimental to the seabird populations in the area, other scientists believe the polar bear will crash harder in the long run.
"This paper ties it all together and shows a very clear relationship between the disappearance of sea ice and increasing predation intensity of seabirds," said Arctic ecologist and polar bear scientist Andrew Derocher of the University of Alberta. "What we're seeing is probably a very transient effect on these seabird populations."
The results of the new study are published in the Frontiers in Ecology and Evolution journal.