Estimates are that tree boughs contain 15,000, possibly as many as 20,000, individuals. Colonies are estimated to contain between 5 and 6 million butterflies per acre and have been as large as 10 acres. The phenomenon of millions flying at once in the air is indescribably awesome. On warm days in late February or March, millions begin to fly simultaneously, forming what is clearly one of the biological wonders of the world. The Migration Through Texas Because it is situated between the principal breeding grounds in the north and the overwintering area in Mexico, Texas is perhaps the most important state in the migration.
Monarchs funnel through Texas both in the fall and the spring.
During the fall, they use two principal flyways. One traverses Texas through its center. It is about 200 miles wide and stretches from Wichita Falls to Eagle Pass. Monarchs enter the Texas portion of this flyway during the last days of September. By the third week of October, most have passed through into Mexico.