Cases ebbed in many places this summer, but the virus never completely disappeared. Now, it’s on the move. Blue-winged teal, a type of duck, are among the first species to fly south in the fall.
In mid-September, three hunter-harvested teal tested positive for the virus in Mississippi, marking the first detections in that state. The finding affirms that, “Yes, the virus persisted in northern latitudes through the summer,” Mr. Richards said. “And it’s coming back on the wings of wild waterfowl.”
Wisconsin began to see a spike in wild bird infections around Labor Day, Dr. Poulsen said, and recently documented its first poultry outbreaks since May. “We were really hoping that we were out of the woods,” he said.
The virus appears to be making a comeback in other northern regions, including Minnesota, North Dakota and Alberta, Canada. So far, experts have said, the surge has been smaller than it was in the spring. “But there’s lots more geese and ducks to come down out of the Arctic,” said Margo Pybus, a provincial wildlife disease specialist at Alberta’s Fish and Wildlife Division.
Farther south, wildlife facilities are preparing for a flood of fall migrants. Southern California was spared in the spring, but Dr. Hendrik Nollens, the vice president of wildlife health at the San Diego Zoo Wildlife Alliance, does not expect to be so lucky this fall.
The San Diego Zoo and its sister facility, the San Diego Zoo Safari Park, covered their outdoor aviaries and removed food and water sources that might attract avian interlopers, even going so far as to drain a pond popular with wild ducks. They also moved 900 birds into more protected habitats — “a herculean effort” that involved relocating hundreds of flamingos, Dr. Nollens said.
To pull off the feat, employees guided many of the flamingos into enclosed trailers, which delivered the animals to covered or indoor habitats. Staff members modified these new spaces to suit the leggy wading birds, adding feeding pools, trimming low-hanging tree branches and raising the sprinklers designed to keep their feathers in fine fettle.
