About 50% of all bird species migrate. 2 main patterns – long lived, soaring types e.g. Geese, who go in groups (better aerodynamics), and who learn routes from elders. Else flapping songbird types, who go alone, with no prior knowledge of route.
Enormous physiological changes, weight can almost double, reproductive organs atrophy, flight feathers may renew. Seen in caged birds too.
African spear fragments seen in storks in Germany. Colonists saw familiar species.
Probably a range of different techniques for route finding. Internal clock important, not just for when to leave (some calendar driven but others weather dependent) but also for using sun position over course of the day. Magnetic sense gives compass direction but may also give latitude (v roughly). Visual and olfactory clues. Many birds end up back at same twig.
Not all birds leave. Bigger birds may be able to survive winter and keep best territories. Robins in S Europe migrate! Northern wheatear in Canada flies to Africa via Greenland and Europe but those in Alaska go via Asia (twice as far but less arduous). High mortality seen in some studies but explosion of N European insect populations in spring may be the trade off.
[In Our Time, BBC Radio 4]