By most indications, the avian influenza outbreak that has washed over U.S. farms is at an ebb. Experts don’t seem hopeful, however, that the problem has taken care of itself.

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.