Authorities say the number of monarch butterflies reaching their wintering grounds in central Mexico appears to be rising this year.

Environment Secretary Rafael Pacchiano says initial reports suggest the butterfly population may be three or four times that of 2014.

He did not explain how the government made the calculation. Authorities conduct informal tracking of monarchs as they enter Mexico from the U.S.

The population of orange-and-black butterflies making the 3,400-mile (5,500-kilometer) migration from the U.S. and Canada has declined in recent years before recovering slightly last year, when they covered about 2.79 acres (1.13 hectares).

The monarchs cluster so closely in trees that their numbers are measured by the area they cover.

Pacchiano spoke Thursday during a visit to a reserve by U.S. Interior Secretary Sally Jewell.