We've long passed the peak migration period for the Monarch butterflies in our area (mid-September), even though we saw one in the garden earlier this week. The straggler had better get on his way before daytime temperatures become too cold. Monarchs can't fly in temperatures below 50° F, which is why they flock in trees overnight until it's warm enough to continue on their long journey to Mexico.
Just about now, the first Monarchs are arriving at their winter roosts in central Mexico, completing one of the most amazing migration trips in the insect world.
In case you've never heard the story of the Monarch's life cycle and how they deal with northern winters, Nova has done a wonderful piece for PBS called The Incredible Journey of the Butterflies. It's shown periodically, but you can watch the 50-minute program online at your own convenience.
I urge you to take the time to see it: