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.
"Migration conditions are a Goldilocks sort of thing. Weather, like porridge, can be too hot, too cold or just right," said Chip Taylor, who heads the University of Kansas's Monarch Watch, the country's most comprehensive monarch research program. "What a warm fall does is often delay the migration in various ways. Late monarchs just don't get to Mexico as well as early monarchs do. The difference is quite striking."
"A lot of people were very concerned" with the late flight, said Mark Garland, director of the Monarch Monitoring Project, which operates through New Jersey Audubon's Cape May Bird Observatory.
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.
"It was the warmest fall we've ever had, at least in the last 123 years," Taylor said. Average high temperatures in the Northeast exceeded 60 degrees in September and October for the first time, according to the National Oceanic and Atmospheric Administration.
Those temperatures were part of a 30-year trend of increasing heat that has more or less paralleled a precipitous decline in monarch populations. 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.
The tangible nature of these threats has long helped turn monarchs into one of our most accessible and unifying symbols of conservation. Thousands of volunteers are attracted to monarch conservation efforts each year, according to the Fish & Wildlife Service, in part because of the butterflies' aesthetic but also because the ways to help them — by planting milkweed, for instance — tend to dovetail with popular hobbies such as gardening.
Saving threatened whale species requires a herculean effort in far-off and inaccessible places. Helping monarchs, for many people, means planting milkweed and then noticing if one fluttered into your living room.
"But nobody ever looks at the weather part of the system. Unless you understand the nuance created by the weather, your data will be completely full of noise," Taylor said. And "if you look ahead to 2040 — oh my God, you don't want to look ahead to 2040," given the projections for future temperature rises.
At first, things were looking promising for monarchs in the autumn of 2017. The butterflies appeared to breed well in the Northeast. Then they arrived en masse along their Atlantic Coast flyways in substantially higher numbers than in the Midwest, where milkweed is being lost more rapidly. Don Riepe, who has counted monarchs along New York's barrier beaches for nearly four decades, reported seeing 35,000 butterflies on Oct. 10.
"It reminded me of a huge flight in the late 1970s," he said. "Back then, they would stream across the highway in enormous numbers. You couldn't help but accidentally kill dozens while you drove."
But Riepe observed this year's flight on New York's barrier beaches two weeks later than the butterflies normally show up there. And along the South Carolina coast, where a little-known population winters, migrants pulsed through in unusual numbers in November, also two weeks tardy. Taylor called it the latest fall migration since Monarch Watch began tracking in 1992.
Some watchers wonder if the tardiness has another, more hopeful explanation: that the warmer temperatures this summer spawned a "bonus generation" of monarchs. In that case, even a few surviving stragglers would bolster the overall number of butterflies that made it to Mexico.
"We had our best numbers since 2012. Of course, that was skewed a little because of the late surge," Garland said. "But the bonus generation may have had very little success. There are always more questions than answers."
Sometime this spring, the annual census of monarchs overwintering in Mexico will be released. The Fish & Wildlife Service will use that and other data to decide by 2019 whether monarchs warrant federal protection under the Endangered Species Act. "Among the threats to the monarch we're looking at is the impact of climate change," said Georgia Parham, a spokeswoman for the service. "It's a question all modern scientists have: How do these different factors impact now what is looking like a declining trend?"