Millions of butterflies travel south into Mexico, from Texas and then follow the Sierra Madre Oriental mountains to the preserve. The butterflies congregate, clustering onto pine and oyamel trees. To many, the trees appear orange and branches sag from the weight. In spring, these butterflies migrate across America, sometimes ending up in Eastern Canada. Over the time it takes them to make this journey, four generations of monarch butterflies are born and die. The monarch butterflies migration patterns are altered by climate change. During migration, monarchs fly north once they are exposed to cooler temperatures. Dense congregations are thought to conserve heat. If warmed by the sun, the butterflies take flight. The beating of their wings has been compared to the sound of a light rain. The reserve is susceptible to lethal, freezing temperatures.