Thanksgiving was right around the corner, and a sizable number of one of America’s most famous migrants could be seen still sputtering south. Not across the Texas-Mexico border, where most monarch butterflies should be by that time of year. These fluttered tardily through the migratory funnel that is Cape May, N.J.

This delayed migration alarmed monarch researchers nationwide. 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 ever recorded in the eastern U.S., 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’ 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,” said Mark Garland, director of the Monarch Monitoring Project.

Most monarchs embark each fall on 2,000-mile journeys from breeding grounds as far north as Canada down to central Mexico. They mate in Mexico, then fly back north to lay their eggs (and die) in the spring.

This spring, the annual census of monarchs overwintering in Mexico will be released. The Fish & Wildlife Service will decide by 2019 whether monarchs warrant federal protection under the Endangered Species Act.