I saw a swarm of butterflies. What are they, and why are they swarming?

Swarms of butterflies are quite a sight. Different species form swarms in various parts of North America.

When the small, brown American Snouts (Libytheana carinenta) form migratory swarms, they can appear as huge clouds of butterflies that stop people in their tracks. Snout migrations are thought to be related to the intensity and duration of dry periods.

The small Cabbage White Butterfly (Pieris rapae) can appear in large numbers during long periods of hot weather when three to four generations are born in one summer. Parts of Montana and Saskatchewan (see news report) have reported large numbers of these butterflies during the hot summer months.

The charismatic Red Admiral (Vanessa atalanta) undergoes seasonal migrations, and these insects can form huge swarms. For more information on these migrations, go to The Red Admiral and Painted Lady Research Site.