Roadkill is a crucial factor in the population decline of migratory monarch butterflies

The charismatic migratory monarch butterfly population has declined dramatically, likely precipitated by loss of its breeding host plants (milkweed). Whether restoring milkweed would allow monarch recovery depends on whether additional factors currently limit the population. We investigated road mortality as one such factor. Monarchs cross thousands of roads during fall migration, and traffic volume has increased sharply while the population has plummeted. Using estimates of pre-migration distribution, flight patterns, and road traffic, we estimate that at least 61% (range 61% to 99.99%) of migrating monarchs are road-killed each fall. Although there is high uncertainty in our estimate, its magnitude suggests that roadkill could inhibit recovery of the population. Recovery planning should not only consider increasing the monarchs host plants, but must also address the reality of roadkill.