Other signs of spring are just as reliable. Starting in late February and early March, monarch butterflies leave the mountainsides of Mexico in a mass migration that may take them as far as 3,000 miles (5,000 kilometers) north.

But this year, just before the monarchs were due to leave their winter homes in central Mexico, reports came of forest floors covered with dead butterflies. By some estimates, over three-fourths of a single colony had been wiped out, and there were reports of dead monarchs piled eight inches (20 centimeters) deep on the ground.

Some Mexican and U.S. news sources reported that loggers were responsible for the deaths of countless butterflies. Newspapers reported that the loggers used pesticides, such as DDT—illegal in Mexico, to kill butterflies living on land protected because of its use by the monarch as an overwintering site so the loggers could harvest the forest.

However, after an investigation by the Mexican government, it seems the blame for the butterfly massacre is to be placed at another door.

"Profepa [the Mexican government environmental protection agency] reported that according to their laboratory analysis, pesticides were not the cause of the die off," according to the World Wildlife Fund (WWF).

Late Winter Storms

"Profepa and WWF Mexico believe that the monarchs died as a result of severe weather that has characterized the overwintering colonies this year," said WWF. The organization noted that logging in the area has diminished the natural cover normally protecting the butterfly colonies from cold snaps.

There is no reason to think the loggers would do something like this, said Orley "Chip" Taylor, director of Monarch Watch and professor of entomology at the University of Kansas. He explained that masses of monarchs have died from late winter storms in previous years.

Destroying large numbers of the monarchs would only bring negative publicity to the area, "which would hurt the local economy by bringing more regulations by the government," said Taylor. "Creating a long-term sustainable forest is best for the local population."

Taylor said the dead butterflies were from the easternmost colonies of overwintering butterflies. "These are smaller colonies only totalling about six percent of the total monarch population."

"The exact [number] killed is not known," said Taylor, but he said he expects the monarch population to recover. "Conditions look good this spring," he said citing good moisture levels in Texas, which will benefit the plants that produce the nectar that butterflies feed on.

A Long Flight North

Taylor predicts that the migrating monarchs will reach the central United States about mid-April, just after the last frost. By that time, the females will have laid eggs for 1,000 miles (1,600 kilometers) as they make their one-time trip. They return to their northern reaches with tattered wings, exhausted from a 3,000-mile (5,000-kilometer) trip.

Not every monarch makes this incredible journey. Two or three summer generations spend their entire lives in northern regions. Butterflies emerging in late fall, with no migration experience, will make the trip to Mexico to survive the winter.

Monarch butterflies are not the only insect to migrate, but their journey is special because they return to colonies at the same overwintering sites year after year.

In these areas, located in mountains west of Mexico City, the monarchs huddle together in densities estimated to be between five and six million butterflies per acre. The weight of the monarchs has been known to break branches off of trees.

Taylor explained that the butterfly stores fat in its body to survive the winter. When they switch into reproductive mode, their energy rate goes up and the butterflies must feed on nectar as they fly north. "They don't feed a great deal when they are down there," said Taylor.

In late February and early March, as if beckoned by warming temperatures or a change in the angle of sunlight, the monarchs begin their long journey north. The monarch "seems to strive to get north to the maximum extent," said Taylor. "They seem to push and push and push right up to the edge of the frost line."