Fall Migration: Too Cold to Fly Today?

(Slideshow: Too Cold to Fly?)

Monarch butterflies are in a race against time during fall migration. They must leave the north before they're trapped by the cold. In order to fly, a monarch's muscles must be warm enough. In fact, if a monarch's thoracic (body) temperature is below 55° F (13° C) it can't fly at all.

Observe: Watch how temperatures fall in the fall!
 This map shows how quickly temperatures drop as the fall season progresses. The days are getting colder (and shorter, too)! With each passing day, monarchs have a smaller and smaller window of time in which they can fly.
Try This! Can a Monarch Fly Today?

Migrating monarchs don't fly at in the dark or if they're too cold.

1. Predict: For how many hours could monarchs fly today in your hometown?
2. Measure the daylight hours: What time in the morning could the monarchs lift off? What time in the evening must they come down to rest? How many hours do you predict monarchs could fly between sunrise and sunset today? Now calculate today's day length using sunrise/sunset data.
3. Measure the outside temperature periodically during the school day. For how many hours could a monarch fly today? Is it sunny or cloudy? As a general rule, monarchs need air temperatures of at least 50°F on a sunny day (or 60°F on a cloudy day).
4. Think! Between sunrise and sunset, how many hours could monarchs have flown? During how many hours was it too cold for them to fly? How did these results compare with your predictions?

Extensions

• Continue gathering data as the season continues. Each day, check the previous day's temperatures. Go to weather.com. Put in your zip code. Choose "yesterday" from the menu and then choose "hourly reports." (You'll find times and temperatures in the lefthand columns.) Make a graph that shows how many hours in each 24-hour day the temperature is above (or below) 55 F.
• Predict where in the country monarchs will be able to fly today or this week. Use this interactive map.
Explore . . . There's more to it!
Monarchs can raise their temperatures above air temperature by 1) basking in the sun and 2) shivering. Explore more!