Southern right whales grow about 25 times bigger and 60,000 times heavier than an average kelp gull. Even newborn whales dwarf adult gulls by a factor of seven.

But that doesn't stop the gulls from hunting right whales off the coast of Argentina. And although these unlikely attacks have been going on for years, the problem seems to be getting worse: Whales near Valdes Peninsula have been dying in droves lately, with gulls as a leading suspect. Last year's death toll of 116 whales marks a 100 percent jump from 2011, according to the AFP, and 88 percent of the 605 whales killed since 2003 were calves. It remains unclear if birds are killing all these whales, but scientists have noticed the giant mammals changing their natural behavior to avoid the onslaught.

How can 2-pound seagulls prey on live whales? Their strategy is essentially "death by a thousand cuts," although their success doesn't actually depend on whether the whales die. They just swoop down when a whale surfaces to breathe, then use their beaks to slash open its rubbery skin, creating wounds up to 4 inches deep and 5 feet long. Each time the whale resurfaces, gulls dive toward its wounds to tear off pieces of blubber.

"The seagulls attack the skin first, and then the fat of the whales," a tour guide on Argentina's Valdes Peninsula tells the AFP. "They peck several times in each attack. Then the whale submerges, and when it comes back to the surface, the gull is there waiting."

These attacks hit calves hardest, partly because of how whales nurse their young. Since the lipless behemoths can't suckle, mothers release milk into the water for their calves, who need more than 25 gallons daily. Combined with the calves' softer skin and tendency to stay near shore, this makes them especially vulnerable to aerial assault: Mother-calf pairs are attacked five times more often than juvenile whales, researchers report, endangering the animals at a crucial point in their life cycles.

"The attacks are very painful and cause large, deep lesions, particularly on the backs of young 2-6 week-old calves," wrote researchers Vicky Rowntree and Mariano Sironi in a recent press release. "The whales flinch violently and swim away to flee from the attacking gulls. This harassment can last for hours at a time. As a result, right whale mothers and their calves are expending much precious energy during a time of year when mothers are fasting and at a site where little to no food is available to replenish fat reserves."

This was first documented in the 1970s, when only about 1 percent of Argentine right whales had gull-inflicted wounds. That number had swelled to 38 percent by 1990, 68 percent by 2000 and 77 percent by 2008, thanks to a booming gull population that increasingly sees whales as food. This may sound like an odd quirk of nature, but experts say people are partly to blame: Local kelp gulls may be thriving on "refuse available from fishing boats operating at sea and at fishery and urban landfills," according to a recent study (PDF). Companies that are supposed to bury rotting fish parts often just dump them at landfills, potentially helping the gluttonous gulls multiply to unnatural densities.

The whales aren't taking this lying down, but their response is sapping valuable time and energy. Adult right whales near Valdes Peninsula have developed arched postures to keep their backs underwater while they're near the surface, thus reducing the risk of gull bites. Newborn calves and other populations of right whales don't do this, suggesting it's a learned behavior, but it hasn't stopped the rising rate of gull attacks. Mother-calf pairs return to normal behavior 30 to 60 minutes after an attack, researchers say, yet still spend 24 percent of their daylight hours "under states of gull-induced disturbance."

Scientists can't definitively link gulls to the whale deaths, and although malnutrition is also a possible culprit, hundreds of tissue-sample tests have yet to implicate toxins or disease. Southern right whales aren't an endangered species, but only about 7,000 exist worldwide — far below their historical population of 60,000 thanks to centuries of industrial whaling.

The die-off poses economic as well as ecological risks, since right whales are the main tourist attraction on Valdes Peninsula. The number of whale-watching tourists rose 548 percent from 1991 to 2007, and up to 200,000 visitors now swarm the area every year, generating more than $40 million per year in towns like Puerto Piramides.

"The southern right whale population is still only a small fraction of its original size, and now we have reason to worry about its recovery," Rowntree says. "Our long-term data indicate that the Peninsula Valdes whales were increasing steadily at close to 7 percent per year until recently. Elevated calf mortality is reducing that growth rate substantially. If this continues, we just don't know what will happen."

Related whale stories on MNN:

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.

Are seagulls killing whales in Argentina?
The birds' relentless nibbling at live whales is a sign of ecological imbalance, scientists say, and it's forcing the whales to change their natural behavior.