Is it because of a lack of natural resources (e.g. food and water), bad weather conditions (e.g. too hot or too cold weather), the presence of other more aggressive and more fearsome predators that threaten their existence?
If a particular place does not have enough natural resources, such as food and water, for birds to eat, drink and survive on, birds will search for other new places that have more natural resources (more food and water supplies), which are available for them to eat, drink, and survive on for the short term and the long term.
If a particular place has weather/ climate conditions which are either too hot or too cold, birds will move to another place where the weather/ climate has much more moderate, more favorable, more hospitable weather/ climate conditions for them to live in and survive.
If a particular place has a lot of other more aggressive and more formidable predators that prey on birds as food, birds will naturally want to migrate to another safer place, where they are a lot less of these aggressive and intimidating predators that want to hunt them down for food.