UPHAM (AP) - Federal biologists have tested ducks at a northern North Dakota wildlife refuge as part of a bird flu surveillance effort.

"The reason they came here is that we can catch a lot of ducks, and they can get a lot of samples," said Gary Erickson, assistant manager of the J. Clark Salyer National Wildlife Refuge.

Erickson and his staff captured several hundred migratory ducks on Friday with the use of cannon nets. The ducks were outfitted with leg bands and then tested.

Bob Dusek, a biologist with the National Wildlife Health Center in Madison, Wis., said testing was done for different types of avian flu, including H5N1.

The H5N1 virus has killed at least 199 people worldwide, according to the World Health Organization, and led to the slaughter of more than 200 million birds since 2003.

It is hard for humans to catch, but experts fear it could mutate into a form that spreads easily among people, potentially sparking a global pandemic. To date, most human cases have been traced to contact with infected birds.

"H5N1 has not been found in North America (but) it is important to keep monitoring it," Dusek said. "It could be fairly devastating to the poultry industry and to agriculture."

Biologists also are working to refine testing techniques, to hopefully lead to earlier detection of bird flu. Dusek said test results from the ducks at the J. Clark Salyer refuge will be compared with similar samples taken in Canada.

It likely will be several months before test results from the refuge are known.