Around 50% of birds migrate (i.e. undertake large-scale routine long distance travel for breeding / feeding).
The distances covered can be huge - e.g. Manx Shearwaters migrate 14,000 km (8,700 mi) each year. The time-triggers, and the methods of route navigation, have been widely investigated, but much of the detail is still unknown.
It is known that birds use a variety of methods - magnetic fields, star positions, odours etc, and some species are 'taught' navigation routes by their parent and peers. But others, e.g. the Marsh Warlbler are hatched with built-in mental 'maps' - and an acute 'diary' (i.e. timing) system. How these systems operate, and the way that this information is somehow encoded into their genes is entirely unknown.
A BBC audio discussion on the phenomenon can be found here : In Our Time, June 2020