Because they like to stick together when they are migrating for the winter, ducks and geese fly in a skein, which is a formation in the shape of a V.

If you have ever witnessed a flock of birds flying the shape of a V, you have witnessed the formation known as a skein.

In a skein formation, there is a leader that takes point at the tip of the V, leading all of his fellow birds to new place to stay for the winter.

While a skein usually refers to the V-formation of migrating geese, it can technically be used to refer to a flock of geese in flight in general.

A flock of swans or geese in flight is called a skein, but most people only call it as such when they are flying in the signature migratory V-formation.