Wildlife

Monarch Migration

A large number of Monarch butterflies have been seen in their migratory funnel in Cape May. N.J. instead of across the Texas-Mexico border. This delayed migration is not normal, and it alarmed monarch researchers across the country. The Cape May stragglers were only a sliver of the record number of monarchs reported in the Northeast in November and December — news that sounded good initially to conservationists. But seeing butterflies so far north so late in the year suggested that few of these latecomers would reach their Mexican wintering grounds. Scientists fear that climate change is behind what they’re calling the latest monarch migration ev er recorded in the eastern United States, and they worry that rising temperatures pose a new threat to a species that saw its population hit record lows in recent years.

Known for their complex, improbable migrations, most monarchs embark on 2,000-mile journeys each fall, from breeding grounds as far north as Canada’s maritime provinces to the Sierra Madre mountains in central Mexico. (A separate western population heads mostly to Southern California.) They mate in Mexico, then fly back north to lay their eggs (and die) in the spring.

Because they’re so delicate — each weighs less than a gram — monarchs are particular about the conditions they’ll fly in, and especially vulnerable to extreme weather systems. Major storms, high winds, early freezes — all pose large-scale dangers, and the butterflies faced all of those this year. But more pernicious than that, scientists believe, are the warmer temperatures, probably a sign of climate change, which manipulated the monarchs’ instincts and pushed their migration back.

According to the U.S. Fish and Wildlife Service, the number of monarchs has dropped from a billion in their heyday to about 33 million in 2014 — or more than 80 percent since the mid-1990s.

For decades, scientists have focused on two main causes to explain what was happening to the monarchs: loss of their habitat to development in the United States and in the Mexican winter grounds and widespread agricultural use of pesticides, which destroy milkweed, their favorite plant. But now they are looking at climate change as a new threat to this icon of conservation.