Botanical 'velcro' entraps hummingbirds.

Burrs can snag more than sweaters. Last month, they snared at least four ruby-throated hummingbirds--three of them fatally, according to National Park Service biologists.

One of nature's cleverer feats of engineering, burrs' tough, hooklike fibers securely lock onto anything that brushes against the seed heads. This strategy usually allows them to hitchhike to a new site. Occasionally, however, a snagged animal isn't strong enough to rip a burr free from its parent plant and finds itself locked in a potential death trap.

Four hummingbirds met that fate over a 3-day period while migrating through Rock Creek Park in Washington, D.C., though birders rescued one. The birds were snagged by burdock (Arctium minus), a weed that can grow to 6 feet.

"I've never heard of anything like this," said burdock expert Wayne R. Hawthorn of the University of Waterloo in Ontario. Yet "this is a case of death by misadventure," he says. "Burdock is not some weird, carnivorous plant." Its seed heads stiffen into effective burrs only when the biennial plant dies and dries out.

Should a small bird run into such a plant, however, it's not hard to imagine the outcome, he says. Any thrashing to get loose of one burr will cause its victim to hit neighboring burrs, ensnaring it more. In effect, "this is nature's Velcro," explains Bob Ford, a National Park Service biologist at Rock Creek Park.

Birders, who found the trapped hummingbirds, led park officials to a pair of thickets containing 50 to 60 plants, which park employees have since eradicated. "We haven't historically done a lot to control burdock," says Susan E. Salmons, who is in charge of controlling unwanted plants in the park. From now on, she says, "we probably will."

Though burdock's bird-snaring abilities have garnered little attention over the years. Martin K. McNicholl, a consulting ornithologist based in Burnaby, British Columbia, has accumulated a growing rap sheet on the plant, culled largely from reports in small, regional journals. They indicate this weed sporadically claims the lives of small birds and even brown bats.