Butterflies in the high forests of Mexico

The two dark spots at the base of the butterfly’s wings indicate that this one is a male.   (Terri Colby/Chicago Tribune )

The monarchs cluster in one specific type of tree, the oyamel fir, that grows on high elevations in the forested mountains west of Mexico City.   (Terri Colby/Chicago Tribune )

The monarchs cluster in such huge numbers that they weigh down the branches on the oyamel firs.   (Terri Colby/Chicago Tribune )

The butterflies spend their winters in forested mountains, west of Mexico City, like this land near the El Rosario sanctuary in the state of Michoacan.   (Terri Colby/Chicago Tribune )

This monarch butterfly was in a Chicago backyard this past July, meaning it most likely descended from one or more butterflies who wintered in Mexico earlier in the year.   (Terri Colby/Chicago Tribune )

The monarch butterflies that spend the winter in Mexico are ones that were born in late summer and early fall in the northern U.S. and Canada, and made the long migration south to a place they had never been.   (Terri Colby/Chicago Tribune )

El Rosario butterfly reserve, a bit more than 100 miles west of Mexico City, is open November through March, and tends to be busiest on weekends.   (Terri Colby/Chicago Tribune )

When you reach the top of the reserve, you’re in a sunny open field. But the magic of the monarchs is most apparent when you go back into the dark and quiet forest.   (Terri Colby/Chicago Tribune )

To see the butterflies clustered in trees at El Rosario, you can walk about an hour uphill from the entrance or you can pay about $5 to ride a horse to the top. Go for the horse if you think the altitude might be a problem for you.   (Terri Colby/Chicago Tribune )

Paula Zito, a nurse from Sarasota, Fla., takes a selfie as the butterflies surround her.   (Terri Colby/Chicago Tribune )

Visitors to El Rosario sanctuary notice signs asking for quiet as they head into the woods where the butterflies cluster on tree trunks and branches. You need to watch where you’re walking because they’re on the forest floor, as well.   (Terri Colby/Chicago Tribune )

It’s not clear how they know where to go, but monarchs born in the northern U.S. and Canada make the long trip to the mountains west of Mexico City to spend the winter.   (Terri Colby/Chicago Tribune )