Monarch Butterfly Migration

monarch butterfly migration
Photo by Sonia Carolina Madrigal Loyola

Monarch Butterfly Migration

Did you know that some Monarch butterflies fly over 2000 miles to get to their winter homes? Some come from Canada and fly all the way to Central Mexico. West of the Rocky Mountains, the butterflies spend the winter months in sunny Southern California. How can these insects fly so far?

In August, butterflies from all over Canada and USA fly south. This generation of butterflies is special: the butterflies are non-reproductive and can live as long as 7 months. In contrast, spring & summer Monarch butterflies live 4 to 5 weeks only.

Flying pass natural dangers and sometimes harsh weather, the butterflies arrive at their overwintering site in droves. The butterflies rest and wait - they hibernate until spring.

In late January, the butterflies begin their journey back north. Along the way, they mate, lay eggs, and die. Each generation lives for about 5 weeks and it will take 3 to 4 generations for the butterflies to reach their final northern homes.

By that time, it is August again and the Monarch butterfly migration cycle begins again.

monarch butterfly migration

Photo by Mila Zinkova

monarch butterfly migration
Photo Monarch Alert

The Mystery The flight of the Monarch butterfly has been a mystery to scientists and researchers for many years. How do those butterflies manage to fly for thousands of miles to reach their overwintering sites? Even more surprising, how do the offsprings (3 or 4 generations later) know to come back to the same overwintering sites? Do they follow the sun? Is there magnetic field that directs them? Do they follow certain geological landmarks? No one knows.

Overwintering Sites in San Diego

There are over 300 overwintering sites from north of San Fransisco (Sonoma County) to south of Ensenada (Baja California) and there's only a handful of sites San Diego. Some sites are on private property while others are in public spaces.

As shown on this chart, some years have thousands of Monarch butterflies whereas other years there are none (see rest of chart here). Experts speculate as to why the Monarch butterfly migration into San Diego fluctuates:it could be global warming, the lack of rain, the lack of Santa Ana winds, or any combination of the above.

Interest groups track, record, and tabulate Monarch sightings (example). Some organizations will tag Monarchs with stickers on their wings. If/when the tagged butterflies are found, the information is relayed back to the organization to determine the butterflies flight pattern. In general, mid-November to mid-December is the best time to look for the butterflies.

monarch butterfly migration
Photo by Derek Ramsey

According to this web site, there are 3 sites within San Diego area:
  • Presidio Park; 900 butterflies in 1997; Monarchs roost in the Canary Island Pine trees along each side of Cosoy Way. In November the butterflies often roost in the eucalyptus trees between Serra Historical Museum and the pine trees.
  • UCSD Campus; 4500 butterflies in 1997; numerous overwintering sites:
    1) near the Mandelville Performing Arts Center: on eucalyptus trees along the blue screen art sculpture.
    2) near the UCSD Coast apartments: located in the eucalyptus grove off Azule St.
    3) near the Che cafe
    4) near Salk Institute
    5) near Weiss Theatre
  • Hosp Grove Park in Carlsbad; 900 butterflies in 1997; was once a large overwintering site before urban development and tree cutting. Walk to the end of the eucalyptus grove at the end of Grove Way in Carlsbad. Monarch are in the trees above the gully.