Fall Migration is Underway!

August 23, 2018 by Elizabeth Howard

Report your sightings and share the story of the monarch's journey to Mexico.

“On August 20th there were at least 200 gathered in our yard, and on August 21st I stopped counting at 300.” Julie Baumberger, Dell Rapids, SD.

Monarchs Are on the Move

The season is off to an impressive start. Overnight roosts - a classic sign of fall migration - have already been reported from 6 states and 1 province. The first was seen on August 11th (one of our earliest dates ever), 17 sites have been found,  and three sites have contained 1,000 butterflies. Numbers this high and this early are a promising sign of a successful breeding season. Get ready for an eventful fall migration!

Monarchs Are Changing

Monarchs are changing dramatically at this time of year, 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 daylength (photoperiod) is a central cue that triggers the monarch’s migratory state. In the northern breeding range, photoperiod is falling by 20 minutes this week.

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 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.

Journal: Documenting the Journey