Birds without natural immunity that migrate or roost together in large flocks, such as geese, are most likely to die in large numbers. A recent bird flu outbreak among migratory eared grebes in Utah killed between 15,000 and 25,000 birds near Great Salt Lake, state wildlife officials said in early February.

The U.S. reported the first outbreak of the deadly H7N9 bird flu on a poultry farm since 2017, as the country continues to grapple with another strain.