Research maps
MONARCH MIGRATION along Texas highways
story by Mamie Hertel
Texas A & M AgriLife Extension
Each fall, millions of monarch butterflies fly across Texas skies on their 3,000-mile journey to the mountains of central Mexico. The state’ s position in the center of the migratory route makes it critical to the species’ success, both in the spring, when monarchs move north to reproduce, and in the fall, when they travel south to overwinter.
The migration – one of the most remarkable natural events in North America – depends on the habitat, wind patterns and vegetation as guides for the butterflies. At the Texas A & M College of Agriculture and Life Sciences, scientists Robert Coulson, Ph. D., professor in the Department of Entomology, and James Tracy, Ph. D., postdoctoral research associate, are studying how geography, wind and roadside habitat shape those journeys. The goal is to keep migration routes viable, supporting the recovery of monarch populations which have declined by more than 80 % since 1990, according to the U. S. Fish and Wildlife Service.
Their research, in partnership with the Texas A & M Transportation Institute and the Texas Department of Transportation, TxDOT, uses science-based mapping and modeling to reduce risks to monarchs during migration and support pollinator health across Texas.
“ When you see how many butterflies don’ t make the migration, it gives you perspective,” Tracy said.“ It shows how closely our daily lives overlap with the natural world, and we are researching some small adjustments that could make a real difference.”
Mapping migration risks
In 2016, with funding from the Texas Comptroller’ s Office, Coulson and Tracy began documenting monarch activity along major Texas highways, identifying another major factor in the decline of the monarch: vehicle collisions. From 2018 to 2021, with additional funding from TxDOT, the research team drove thousands of miles each fall, stopping every 30-50 miles to collect butterfly remains and record GPS coordinates.
By combining these field observations with topographic, vegetation and wind data, they created maps showing how geography influences monarch flight in addition to pinpointing where and why collisions were happening.
“ It was eyeopening,” Coulson said.“ We learned that monarch deaths were concentrated in very specific areas where landforms and wind patterns funneled them.”
Two primary flyways emerged from the data. One runs through central Texas between Wichita Falls and Del Rio, and another along the Gulf Coast. In some areas, food sources like wildflowers and roosting trees along migration routes drew the butterflies closer to roadways. Bridges, causeways and water features often concentrated their flight paths, bringing them even nearer to traffic.
Initial modeling suggested that monarch-vehicle collisions in Texas may account for a significant portion of the species’ overwintering losses, underscoring the need for more refined migration data.
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