In early May, Harry Coker of Tallassee, Alabama, peered into his parked 1955 Chevy and saw a yellow-jacket nest “the size of a spare tire” on the floor of the backseat. Six weeks later, Coker went back—and found the nest had grown to fill the entire car. Yellow jackets normally construct hives no bigger than a basketball, populated by one queen and at most 3,000 workers. But this year, giant nests are cropping up all over Georgia and Alabama. Auburn University entomologist Charles Ray has documented at least 60, some containing 100,000 workers and hundreds of queens. Ray finds the sudden phenomenon puzzling but speculates that warmer lows in winter—January was six degrees above average this year in Alabama—allowed workers usually killed off by cold to survive into spring, altering the behavior of some colonies.