Wind and Migration

The main signal for birds to begin their migration is the wind.
It is much easier for birds to fly with the wind than against it. A wind in the same direction the bird is traveling is called a “tail wind”. A wind blowing against the bird is called a “head wind”.
When birds approach a head winds, they are often forced to land.
Heavy rain and storms can also force birds to land.
These same factors will affect butterfly migrations as well.