Monarch butterflies overwintering

Thanks to mr. Wiki:

In colder winter areas Monarch Butterflies become sluggish, and they do not reproduce. Because they are cold blooded, when the air temperatures become cooler difficult butterfly habitat challenges are presented. The butterfly stores fat in its stomach for the winter period of lessened activity. Although Monarch butterflies are very susceptible to damage during freezing temperatures, if their delicate wings remain dry they can withstand brief freezing periods. While residing at their winter migratory locations they tend to cluster together roosting in trees, but during the summer season they spread out over very large areas.

Little is known about monarch over-wintering behaviour in New Zealand, but it appears that they only form over-wintering clusters in areas where the temperature regularly falls below 10 0 C. On warm winter days the butterflies will fly from their clusters to feed on nectar. This is the best time to find the clusters ñ look for large numbers flying around a tree.

In Christchurch, the butterflies do not seem to go far afield and, as the weather gets colder, they head for local parks and gardens. During late autumn and early winter, you can sometimes see thousands of butterflies on one tree.