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 recently released by government and independent experts.

The experts say the decline could be due to late winter storms last year that blew down more than 100 acres of forests where migrating monarch butterflies spend the winter in central Mexico.

Millions of monarchs make the 3,400-mile migration from the United States and Canada each year, and they cluster tightly in the pine and fir forests west of Mexico City. 

Officials estimate the storms in March killed about 6.2 million butterflies, almost 7.4 percent of the estimated 84 million that wintered in Mexico, said Alejandro Del Mazo, Mexico's commissioner for protected areas. The monarchs were preparing to fly back to the U.S. and Canada at the time the storm hit.

While no butterfly lives to make the round trip, a reduction in the number making it out of the wintering grounds often results in a decline among those who return the next year.

In addition, the fight against illegal logging continues. Authorities recently detained a man trying to truck about a dozen huge tree trunks out of the butterfly reserve, using false papers asserting the trees were diseased and were being removed to reduce risk. In fact, investigators found the trees had been healthy.

Get home and garden tips sent to your email inbox

Another problem has been the loss of milkweed — the plant monarchs rely on for food — in the United States and Canada, because of the increased use of herbicides and the conversion of land to development.

Some experts believe that even if Mexico's overwintering sites never lose another tree, without food and habitat along the migration routes, the migration will stop.

The butterfly itself is not endangered, but the migration does appear to be.

Last year, the butterflies covered 10 acres (about 4 hectares), compared to 2.8 acres (1.13 hectares) in 2014 and a record low of 1.66 acres (0.67 hectares) in 2013. That is still well below the 44 acres (18 hectares) they covered 20 years ago.