The once-thriving population of monarch butterflies that migrate from the northern U.S. and Canada to Mexico each winter is dwindling, researchers said this week. The orange-and-black butterflies would fill up to 45 acres of Mexican forest each year, according to records kept over the past 20 years. But as of December, they filled only 1.6 acres, the smallest area ever recorded. Environmentalists say the decline could be due to a number of factors, including illegal logging and the use of herbicides that have killed off milkweed plants where the butterflies lay their eggs.

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