Faltering under extreme weather and vanishing habitats, the yearly winter migration of monarch butterflies to a handful of forested Mexican mountains dwindled precipitously in December, continuing what scientists said was an increasingly alarming decline.

The migrating population has become so small — perhaps 35 million, experts guess — that the prospects of its rebounding to levels seen even five years ago are diminishing. At worst, scientists said, a migration widely called one of the world’s great natural spectacles is in danger of effectively vanishing.

The Mexican government and the World Wildlife Fund said at a news conference on Wednesday that the span of forest inhabited by the overwintering monarchs shrank last month to a bare 1.65 acres — the equivalent of about one and a quarter football fields. Not only was that a record low, but it was just 56 percent of last year’s total, which was itself a record low.

At their peak in 1996, the monarchs occupied nearly 45 acres of forest.

The acreage covered by monarchs, which has been surveyed annually since 1993, is a rough proxy for the actual number of butterflies that survive the arduous migration to and from the mountains.