HARI SREENIVASAN: Every year, millions of North American monarch butterflies head south for the winter in one of the insect world's most fantastic feats.

But as you've likely heard, their numbers have declined dramatically in the past two decades. Now the federal government is considering new steps to try to reverse the trend.

Scientists are still trying to determine just how many of the butterflies made the long trip to Mexico this winter.

Still hoping that the 90 percent decline that began in the 1990s has finally come to an end.

DIRECTOR MONARCH BUTTERFLY RESERVE, GLORIA TAVERA: Well this 2014 – 2015 season is a critical season for us after the numbers we had last season.

We hope work done jointly by the United States, Canada and Mexico will help to increase the numbers of butterflies arriving in our sanctuaries.

HARI SREENIVASAN: But even as the butterfly count continues in Mexico…in Washington, responding to a petition submitted by conservation organizations, the U.S. Fish and Wildlife Service recently announced a year-long review that could mean the butterflies are placed on the Endangered Species list.

If that happens, there could be ripple effects, the monarch's habitats in the U.S. could become protected under law and that, in turn, could lead to restrictions on the use of herbicides that many farmers depend on to kill weeds without harming their crops.

But the time it takes to make these decisions is consequential.

A report published by the Center for Biological Diversity, a non-profit conservation organization, found that 83 species became extinct between 1974 and 1994, at the very time the government was deliberating about whether or not to include them on the Endangered Species list.