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(CBS) Every spring, blue skies in Michoacán, Mexico, turn orange. For centuries, millions of monarchs have gathered in these same ancient forests, traveling from as far away as Maine, North Dakota, and even Canada. They weigh just a fifth of a penny, but so many flapping wings can sound like falling rain.

There are an estimated 250 million monarchs that winter in this preserve. That sounds like a lot - and it certainly looks like a lot, but imagine 15 years ago when there were nearly one billion. The problem is giant fir trees where monarchs huddle for warmth are being cut down.

Without cover, millions of butterflies often freeze to death - in an average year, up to 15 percent. But this year, bad weather killed more than 50 percent.

Right now, we're inside a protected area of about 140,000 acres that the Mexican government set aside as a preserve for the monarch butterflies. Still, inside this zone, every year, more than 100,000 trees are cut down.

Mexico recently lost nearly 7 percent of its forests, an area twice the size of New Jersey, much of it from illegal logging. Deforestation threatens more than butterflies, as it destroys watersheds which can cause mudslides. One five miles from a butterfly sanctuary killed 17 people.