(CBS)-- A report says the number of Monarch butterflies wintering in Mexico dropped 28.1 percent this year.

The report released Thursday and carried out by the World Wildlife Fund, private donors and Mexico's National Commission on Protected Areas says the Monarchs covered 7.14 acres (2.89 hectares) of forest this year, compared to 9.9 acres (4 hectares) last year and 4.7 acres (1.9 hectares) two years ago.

The drop is due in part to droughts in areas of the United States and Canada where the butterflies breed and migrate. Experts say deforestation in their wintering grounds west of Mexico City is also a factor.

Concern rose two years ago, when the butterflies' numbers dropped by 75 percent in the wintering grounds, the lowest level since 1993.