You answered your own question actually. They migrate because it's cold. Many migratory birds fly to North America for our summer and to South America for their summer. That way they're always in summer. (When it's winter in North America, it's summer in South America and vice versa.) So when it starts getting cold, that's their signal to start migrating to the warmer climate.
still summer
If it is summer in South America then it is winter in north america
If it is summer in South America then it is winter in north america
If it is summer in South America then it is winter in north america
when it summer in north america its winter in south america and when its winter in north america its summer in south america.
If it is summer in South America then it is winter in north america
It is Winter in Zimbabwe when North America has its summer.
It's very temperate. Cold and snowy in winter and warm and sunny in the summer.
In July, North America is in summer.
If it is summer in South America then it is winter in north america
it's winter in north america