Usually, birds and other migrating animals move to wherever the conditions are most favourable at that time of year. That means wherever their is enough food and resources for them to survive. As it gets colder, the things birds feed on get harder to find, so they fly south to where it’s warmer and there is more food available. Sometimes birds and other animals also migrate to where their is more safety to have their babies. Like when penguins all group together into huge inland colonies to keep chicks safe from predators.
Birds that nest in the Northern Hemisphere migrate northward in the spring to exploit expanding insect populations, budding plants and an abundance of nesting locations. As winter approaches and food availability drops, the birds move south again. This website gives a pretty good overview: