Winged Pirates: Kleptoparasitism as a Lifestyle

Next time you go out birding, watch for pirates. They may not brandish cutlasses, guzzle grog, or walk the plank, but they’re out there, lounging in the leafy shade or plying the winds, just waiting to pounce on passing treasures.

Piracy, or “kleptoparasitism” to use the technical term, is quite common in the animal world, occurring in everything from mollusks to mammals and 197 species of birds (representing 33 families). The slender-winged frigatebirds of tropical seas are so adept that an entire pirate ship has become embedded in their name. Benjamin Franklin cited the Bald Eagle’s habit of stealing fish as a reason not to use it as the national symbol of the United States.

The Common Raven commits larceny of all kinds, including (according to Native American legend) stealing a piece of the sun to bring light to the people of the Pacific Northwest. And gulls don’t even limit their misdeeds to other birds: they nab ice cream cones from beachgoers and shoplift Doritos from convenience stores.

To scientists, kleptoparasitism is a curious, genre-bending kind of behavior. It’s not foraging; it’s not predation; and it’s not even parasitism in the bloodsucking, leechlike way we typically think of it. When kleptoparasites rob a “host” of its energy, they do it before the poor animal has even ingested it.

Birds often win food through harrying, as when Bald Eagles divebomb Ospreys they see carrying fish, or Laughing Gulls hover above pelicans, pecking at their outstretched bills as the big birds try to swallow. But sometimes there’s a hint of mischief. In the Kalahari, Fork-tailed Drongos flock with ground-foraging Pied Babblers, giving out alarm calls to warn of approaching predators. Every so often a drongo fakes an alarm call, then swoops in on whatever the startled babbler drops as it runs for cover. Of course, you can only get away with crying wolf so many times, but a recent study in Behavioral Ecology and Sociobiology found that drongos know which birds are easier to trick: they use the fake alarm on naive juveniles far more often than with adults.

Many birds only dabble in piracy, but some—such as gulls and corvids—are notorious for it. Ecologists working at a refuse dump near León, Spain, recently wrote in The Auk that the effects of kleptoparasitism can ripple through an entire community to affect how several species use a resource.

The daily arrival of garbage attracted Carrion Crows, Rooks, Eurasian Jackdaws, and European Magpies, along with European and Spotless starlings. While most birds combed the rubbish piles, Carrion Crows sat along the fences and attacked birds that had found prize morsels.

Their most frequent victims were starlings. The Carrion Crows synchronized their arrival at the dump with the starlings’ midmorning schedule, which led the jackdaws and magpies to arrive early in the morning and late in the afternoon, when fewer crows meant more peaceful foraging.

If kleptoparasitism seems like the ultimate free lunch, it makes scientists wonder why only certain birds have picked up the habit. Ducks, gulls, hawks, pelicans, and frigatebirds seem especially prone to stealing food, but the behavior seems to be absent from some 180 other families, according to a recent review in Animal Behaviour.

Though piracy can seem like a case of simple bullying, the majority of records involve smaller birds stealing from bigger ones (as with drongos and babblers, gulls and pelicans). The Animal Behaviour review found that brain size (corrected for body size) was a much better predictor of piracy than body size. Kleptoparasitism, it seems, requires more brains than brawn. Or, if you like, more skullduggery than swashbuckling.

—Hugh Powell

References:

Baglione, V., and D. Canestrari. 2009. Kleptoparasitism and temporal segregation of sympatric corvids foraging in a refuse dump. Auk 126:566–578.

Morand-Ferron, J., D. Sol, and L. Lefebvre. 2007. Food stealing in birds: brain or brawn? Animal Behaviour 74:1725–1734.

Ridley, A. R., and M. F. Child. 2009. Specific targeting of host individuals by a kleptoparasitic bird. Behavioral Ecology and Sociobiology 63:1119–1126.