North America is the primary home of monarch butterflies.  The majority of the population lives east of the Rocky Mountains, with a much smaller population west of the same.  They are all the same species, just split by those magnificent Rocky Mountains, which funnel the fall migration in different directions:  western monarchs head to the California Coast; eastern monarchs head to the high mountain areas of Mexico.

Monarch butterflies spend their summers searching for healthy breeding habitat (milkweed, nectar sources and water), which brings them north and northeast across the US, with four (sometimes five) generations. Monarchs reach as far as British Columbia in the west, and Ontario, Canada in the east. 

The first new generation of the year reaches Utah in May, and their newly laid eggs hatch about 30 days later. This 2nd generation of the year stays in Utah, breeding and laying eggs for 3-5 weeks.  Generations 3, 4, and sometimes 5 follow depending on the weather.

In August through September, those emerging from chrysalis across the US are the great migrating generation.  They hit the pause button on breeding to save their energy for a journey of up to 3,000 miles.  They must head to warmer climates to stay warm enough to survive the winters. They must survive on nectar from pollinator plants they find along the way, living off their body fat over the winter. Based on the data reported, we know that monarchs in Utah go to both overwintering sites - Mexico and California. 

Beginning in March, these monarchs sense it's time to breed and start migrating back to the US to find milkweed to lay their eggs. 30 or so days later, the first new generation of the year is born, and the migrating generation ends its extra-long lifespan of 7-9 months; all other generations live just 3-5 weeks.

Monarchs grow from egg to caterpillar, chrysalis to butterfly.  This process can go as quickly as 22 days in the heat of the summer, or take months during the colder seasons. See graphic to the left.