Eagle Pass Cluster Sites --
a self-guided tour map

Eagle Pass and Maverick County occupy a unique position on the international migration route of  the Monarch butterfly.  

This is the site of some of the last roosting places in the U. S. before the intrepid travelers enter their largely uncharted paths through Mexico, headed for the Preserves in the Transvolcanic Mountains of Michoacan in the central highlands some 650 miles south of the Texas border.  

The Central Flyway of the Eastern Population of Monarchs funnels down from the broad sweep of the Canadian border and the entire region surrounding the Great Lakes , narrowing, narrowing, until it arrives in Maverick County and measures less than 100 miles across.

Eastern migratory populations vary from 180 to 250 million monarchs. These numbers stream into this region beginning as early as October12th and continue to seek nightly cluster sites over the next 4 weeks. At these numbers Maverick County will regularly host 100s of thousands of Monarchs each night of the first 2 weeks of the migration.

The Monarchs passing through Maverick County have already flown between 800 and 1000 miles from the milkweed patches in Canada, Minnesota, Kansas, Iowa.

The Rio Bravo Nature Center Foundation has spent the past 30 years observing and documenting the passage of these marvelous creatures. From these studies 4 factors have been noted to determine the selection of the insect’s nightly roosts: wind shelter, tall trees, moisture and nectar sources nearby.         

These clusters will be found on the side of tall trees away from the prevailing SSE winds. Pecan groves, thickets of hackberry, mesquite, and huizache are favored. Clusters begin forming as early as 4 p.m. and will linger on cool cloudy mornings until after 10 a.m.

Clusters can be approached closely for photographs if the observers move very slowly and do not make loud noises. Camera flashes will also disturb clusters.

Intentional disturbance is strongly discouraged because this stresses the energy stores of this fragile organism.


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