West Nile virus appears to be the latest affliction hitting American white pelican chicks still nesting at Chase Lake National Wildlife Refuge.

Researchers visited the nesting islands Wednesday and observed several medium to large chicks that were exhibiting signs of the virus, Ken Torkelson, a spokesman for the U.S. Fish and Wildlife Service, said Thursday. The chicks were walking clumsily, and their wings were drooping.

How many chicks are showing symptoms is unknown, Torkelson added.

Samples were taken from seven chicks and sent to the U.S. Geological Survey's National Wildlife Health Center in Madison, Wis., where researchers already are testing other samples from Chase Lake to determine what caused a massive chick die-off at the colony in early July.

An estimated 500 chicks survived at the nesting site, which had the potential to produce as many as 9,000 young. Without chicks to tend, many of the adult birds scattered from the refuge north of Medina after the die-off. An estimated 2,000 adults remained from a late-May population estimated at 18,850.