One of nature's most colorful annual displays has begun in central Mexico.

Millions of Monarch butterflies have started arriving at a sanctuary in the Mexican state of Michoacan on their annual migration.

After their 2,000-mile journey, the butterflies are crowding hundreds of trees in the 32,000-acre Monarch reserve.

Scientists expect this year's migration to be smaller than last year's because of the drought that hit Texas and parts of Mexico, making the flowers they need to survive hard to come by.