The annual migration of North America’s monarch butterfly is a unique and amazing phenomenon. Monarchs are the only butterflies known to make a two-way migration similar to birds. Unlike other butterflies that can overwinter as larvae, pupae, or even as adults in some species, monarchs cannot survive the cold winters of northern climates. Using environmental cues, monarchs know when it’s time to travel south for the winter. They use a combination of air currents and thermals to travel long distances, with some flying as far as 3,000 miles to reach their winter homes.

 

Eastern Monarchs

The eastern population of North America’s monarchs overwinters in the same 11 to 12 mountain areas in the States of Mexico and Michoacán from October to late March. Monarchs roost for the winter in oyamel fir forests at an elevation of 2,400 to 3,600 meters (nearly 2 miles above sea level). The mountain hillsides of the oyamel forest provide an ideal microclimate for the butterflies. Here, temperatures range from 32 to 59 degrees Fahrenheit (0 to 15 degrees Celsius). If the temperature drops lower, the monarchs will be forced to use their fat reserves. The humidity in the oyamel forest ensures the monarchs won’t dry out, allowing them to conserve their energy.

 

Monarchs cluster together to stay warm. Tens of thousands of monarchs can cluster on a single tree. Although individual monarchs weigh less than a gram, tens of thousands of them together weigh a lot. Oyamel trees are generally able to support the clustering butterflies, but sometimes branches break under the weight.

 

Western Monarchs

Monarchs living west of the Rocky Mountain range in North America overwinter in California along the Pacific coast near Santa Cruz and San Diego. The microclimatic conditions here are very similar to those in central Mexico. Monarchs roost in eucalyptus, Monterey pines, and Monterey cypresses in California. In February and March, the monarchs head inland in search of milkweed.

 

Generations

It’s incredible to think that monarch butterflies are long-distance migrants, but what’s truly mind-blowing is that no single butterfly completes the full migration! Each year, there are four generations of monarch butterflies.

 

Generation 1

These are the offspring of the overwintering monarchs, laid from late March through April in the southern United States and northern Mexico. They fly north as adults.

 

Generation 2

These are the grandchildren of the overwintering monarchs, laid throughout much of eastern North America from late April through June.

 

Generations 3 & 4

These are the great and great-great-grandchildren of the overwintering monarchs. Generation 3 is laid from late May through July, and Generation 4 from late June through August. Some Generation 3 monarchs reproduce in the northern breeding range, but those emerging late in August enter diapause (delayed reproductive maturity) and migrate to Mexico, as do most Generation 4 monarchs.

 

The first three generations live 2-6 weeks over the summer, while the fourth generation lives 6-9 months. During September to early November, migratory adults fly to overwintering sites in central Mexico, remaining there from November to March. In March, they journey north, laying eggs that become the new Generation 1 along the way.

GENERATIONS Life Cycle

Migration map

migration cycle

migration by latitude

*Latitude for Chattanooga is 35