When Worms Fly: Insect larvae can survive bird guts

Researchers in Spain have shown a novel way for insects to travel—as larval stowaways in the guts of migrating birds.

“It’s the first time insects have been shown to be carried inside birds,” reports Andy J. Green of the Doñana Biological Station in Seville. He and a colleague examined droppings in a salt marsh roosting site of waterbirds called black-tailed godwits. The birds were feeding by the thousands on their way from northern breeding grounds to their winter homes in Africa.

The droppings held bloodworms, which grow up to be midges that look like small mosquitoes. The hitchhiking larvae raise new possibilities for ways that aquatic insects move through ecosystems, Green and Marta Sánchez propose in an upcoming Biology Letters.

Among stowaways, seeds have gotten much more attention than invertebrates, says Green. Darwin did propose birds as a means of invertebrate spread, and he observed snails stuck to a duck’s foot. In 2003, Green and several colleagues showed that some of the tough eggs of Daphnia, a crustacean, indeed travel in bird guts. In the March Limnology and Oceanography, Green, Sánchez, and their coworkers reported that birds were spreading the eggs of an American brine shrimp that is threatening Europe’s native brine shrimp.

In the course of such studies, Green and Sánchez came across godwit droppings that had the bright-red larvae of the midge Chironomus salinarius wriggling within them. The researchers report that of six collections of droppings, three contained at least one live larva. Out of the 95 intact larvae picked out of the samples, 12 showed no ill effects from the birds’ digestive processes.

When birds gorge themselves, digestion becomes less efficient and larvae become more likely to survive, says Green. A lot of gorging goes on during bird migrations.

Darold Batzer of the University of Georgia in Athens says that he’s never before heard of insect larvae surviving digestion but rates the scenario as “plausible.”

He finds the impact of the finding less clear. “For a flying insect like a midge, I would think routine aerial dispersal on wind would cause more dispersal than the chance survival of ingested larvae,” Batzer says. However, for insect species in which the adults don’t fly, “dispersal in bird guts might be more important in the grand scheme of things.”

“How the midge larvae survive is certainly the more interesting question to many of us,” says Vincent Resh of the University of California, Berkeley. He notes that some midge larvae endure sojourns in absolute alcohol and then recover normal function when returned to water.

---