A very interesting program on NPR Science Friday regarding the annual migration of Monarch butterflies from their overwintering site in Mexico. The program documents the Monarch's journey northward and discusses habitat loss and milkweed destruction due to herbicides as primary causes for the rapidly declining Monarch population.
Here are some interesting links:
Mapping the Monarch Migration
Milkweed for Monarch
Milkweed loss hurts Monarchs
Spring Migration Monarch Map - Add your Monarch photo to the migration map (need to login with Google ID)