Please explain in brief language: "What is migratory bird migration?"

The wild geese fly south, autumn passes and spring returns. People call these birds migratory birds based on the phenomena related to bird migration and climate change. Because they come and go when the seasons and climate change.

Migratory birds migrate almost at the same time every year. It's not just the onset of warm or cold weather that triggers their migration. What actually controls the start of migration of birds is a complex interaction between external stimuli (including climate) and the internal "biological clock". This effect allows birds to know that it is a certain season of the year and decide whether to start migration. only.