This past December (2013) the overwintering Monarch butterflies covered only 1.65 acres of forest in Mexico, compared to 7.14 acres in 2011. This is a record low since the monitoring of the overwintering colony began in 1993, which covered a record 45 acres in 1996. The steep decline in the population size over the last decade is due to both unusual weather events that impede migration & reproduction, as well as declining availability of the milkweed host plant due to increased herbicide use in the midwest.

Read more from the New York Times.