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Monarch butterfly numbers drop by 27 percent in Mexico

FILE - In this Nov. 12, 2015 file photo, a guide holds up a damaged and dying butterfly at the monarch butterfly reserve in Piedra Herrada, Mexico. The number of monarch butterflies wintering in Mexico dropped by 27 percent this year, reversing last year’s recovery from historically low numbers, according to a study by government and independent experts released Thursday, Feb. 9, 2017. (AP Photo/Rebecca Blackwell, File)

FILE - In this Nov. 12, 2015 file photo, a guide holds up a damaged and dying butterfly at the monarch butterfly reserve in Piedra Herrada, Mexico. The number of monarch butterflies wintering in Mexico dropped by 27 percent this year, reversing last year’s recovery from historically low numbers, according to a study by government and independent experts released Thursday, Feb. 9, 2017. (AP Photo/Rebecca Blackwell, File)  (The Associated Press)

A new study has found that the number of monarch butterflies wintering in Mexico dropped by 27 percent this year, reversing last year's recovery from historically low numbers.

Government and independent experts say the decline could be due to late winter storms last year that blew down more than 100 acres (40 hectares) of forests where migrating monarch butterflies spend the winter in central Mexico.

Millions of monarchs make the 3,400-mile (5,500-kilometer) migration from the United States and Canada each year, and they cluster tightly in the pine and fir forests west of Mexico City. They are counted not by individuals, but by the area they cover.