SIERRA CHINCUA, Mexico, Dec 4 (Reuters) - From a distance, they appear like autumn foliage: millions of endangered monarch butterflies blanketing trees in a kaleidoscope of brown, orange and black.As the crisp mountain air warms, they flutter above dazzled visitors who have come to see an annual tradition that persists despite the environmental and human pressures threatening it.Sierra Chincua in 1986 was brought into the protected Monarch Butterfly Biosphere Reserve, which is now a UNESCO World Heritage Site spanning some 138,000 acres (56,000 hectares).The migratory monarch population has fallen between 22% and 72% over the past decade, according to the International Union for the Conservation of Nature, which declared the species endangered in July.Visitor Lizbeth Cerrato said the "unique experience" was like "watching souls flying above.""