Every fall, millions of monarch butterflies leave Canada and the northern United States and fly south toward the mountains of central Mexico. For many of them, the route runs straight through Texas. Because the monarchs from the eastern half of North America are all heading for the same small area of Mexico, their paths draw closer together as they travel, like sand pouring through a funnel. Texas sits near the narrow end of that funnel. That is why people in places such as Austin and San Antonio sometimes see dozens of monarchs resting in a single tree in October.

The journey is remarkable because no butterfly making it has ever made it before. A monarch born in early summer usually lives only a few weeks. It mates, lays eggs, and dies, and its offspring continue the cycle. The monarchs that hatch at the end of summer are different. Their bodies put off breeding and store energy instead, and they can live for several months. That is long enough to fly thousands of miles, spend the winter in the cool mountain forests of Mexico, and begin the trip north in the spring.

On the way back, Texas matters again. Many of the returning monarchs lay their eggs in Texas in early spring. They lay them only on milkweed, because milkweed is the only kind of plant monarch caterpillars can eat. If a female cannot find milkweed, she cannot lay her eggs where her caterpillars will survive.

Scientists who measure the area the butterflies cover in their winter forests have reported that the eastern monarch population has dropped sharply since the 1990s. The loss of milkweed is considered one of the main causes, along with the loss of other habitat and changes in weather. As a result, many Texas schools, parks, and families now plant native milkweed and other flowering plants. A garden only a few feet wide can give a traveling monarch a place to feed, and a female a place to lay her eggs.