The first monarch butterflies have arrived at their wintering grounds in the mountains of central Mexico almost a week later than usual, Mexico’s Environment Department said Wednesday.
Millions of monarchs make the 3,400-mile (5,500-kilometer) migration from the United States and Canada each year.
The butterflies usually arrive by Mexico’s Nov. 1 Day of the Dead holiday, and local residents long associated their return with the souls of the dead returning to their homes.
But this year, the Environment Department said the first butterflies weren’t seen in the mountain reserve west of Mexico City until Nov. 6. It said the butterflies were delayed because they waited out rainy weather around the U.S.-Mexico border.
The monarchs spend the winter clumped together in fir and pine trees. Last year the monarchs covered about 6.12 acres (2.48 hectares), down about 14.7 percent from the previous winter. As recently as 1996-1997 they covered 44 acres (18 hectares).
There have been several rebound years, but each has generally been less than the preceding upswing.
Increased use of herbicides in the United States have hurt the prevalence of milkweed, which monarch caterpillars feed on, risking their survival. Loss of tree cover in Mexico due to drought, storms and logging has also affected the butterflies’ population.