Signs of Fall Migration


Monarchs are changing dramatically in the fall, in physiology and in behavior. They are:

1. Becoming Migratory

Watch for signs of migratory monarchs:

  • flying in directional flight
  • clustering in overnight roosts
  • nectaring intensely

2. Responding to Daylength

Declining day length is a central cue that triggers the monarch's migratory state. In the northern breeding range photoperiod is falling by 20 minutes by mid-August. This sends a signal that it's time to migrate.

3. Emerging in Diapause

Beginning in mid-August in the north, adults are in diapause when they emerge from the chrysalis. They are full grown — but they are not reproductively mature. Their reproductive development is on pause. These monarchs will not complete development and begin to mate until next spring in Mexico.

4. Beginning a Long Life

The same hormone deficiency that leads to diapause also leads to increased longevity. Breeding monarchs live only 2-6 weeks; migratory monarchs live up to 8 months.

5. Accumulating Fat

Monarchs are shifting focus now from breeding to intense feeding. They must build body fat to fuel migration and to survive the winter in Mexico.