Duck, Duck, Otter!

Harlequin ducks scoop up food scraps from messy sea otters to get through the tough winter.

When it comes to finding a meal, some birds have learned to be opportunistic, such as that squawking gull that grabs your dropped French fry. Clumsy humans are an obvious food source for intrepid gulls. But Hakai researchers were scratching their heads a few winters ago when they saw sea ducks eating alongside an unexpected companion.

“Sea otters are carnivores with big teeth and they can be aggressive towards birds,” says Hakai researcher Erin Rechsteiner. “So we were surprised when we saw harlequin ducks following them.”

Harlequins spend British Columbia winters close to the coast, where they fatten up before flying inland in the spring to nest alongside freshwater streams. In the ocean, these ducks feed by dipping their heads into shallow water to pluck crabs and snails from the rocks.

The male ducks have a loud outer appearance—metallic blue and copper plumage with white and black stripes—but, along with the drabber brown females, are usually shy and easily spooked.

“It was such a cold winter in 2014. The beaches were coated in ice and the tide pools had frozen over. This might’ve meant that the ducks had a hard time finding food without resorting to thievery,” says Rechsteiner.

Conveniently for the ducks, their neighbors are notoriously messy eaters. Sea otters are golden-retriever-sized marine weasels with an insatiable appetite. Unlike the shallow-diving harlequin ducks, sea otters regularly dive down more than twenty meters and pluck sea urchins and other creatures off the seafloor. The sea urchins are far too large and far too deep for the harlequin ducks to access themselves, but are full of nutritious fats that the ducks can use to help them get through the winter.

At the surface, sea otters crack open the urchin by ripping it apart with their paws or banging it with a rock that they use like an anvil. Once the urchin is pried open, the otters slurp up the nutritious, mustard-colored gonads as best they can before diving back for more. A mess of uneaten urchin bits floats in their wake. The scavenging ducks are like dumpster divers living next to a wasteful five-star restaurant.

A hungry harlequin will do anything for a meal, even if it means trailing around after a weasel. During the Central Coast winter, harlequin ducks took to snatching urchin scraps like—well, like ducks to water.

“As the winter progressed, some otters would have ten ducks trailing them. The new foraging strategy seemed to really catch on,” says Rechsteiner, who has studied sea otters on the BC Central Coast since 2013.

But the new duck behavior wasn’t universal. The harlequins were only seen scavenging urchin scraps at sites where sea otters had just recolonized following a century-long absence post-fur trade. These are where urchins are the biggest and most plentiful. In other areas, where sea otters had already been feeding for at least two years, ducks didn’t associate with otters.

Unfortunately for the urchin-loving ducks, the sea otters eat most of the urchins at a site within those first two years after arrival. So when there are fewer urchins around, sea otters switch to other food options. The harlequin ducks were uninterested in the other sea otter food scraps, such as clams, octopus, and sea cucumbers.

Rechsteiner and colleague Angeleen Olson published these novel observations in the Canadian Field-Naturalist. While larger birds like bald eagles and gulls are known to scavenge from sea otters in Alaska, this is the first recorded case of a sea duck scavenging from another bird or mammal.

The duck’s usual inclination for otter aversion makes sense considering sea otters have been known to snatch a sea bird or two if they can. But dining at the sea otter cafe is a risk worth taking to survive a rough winter—and get access to the freshest uni on the planet.