Migratory birds are one of the greatest wonders of the natural world. They have inspired awe and wonder in human cultures globally for millennia. And for a long time, they were a big mystery. Some Europeans and Americans used to think that birds hibernate underwater when they disappear every winter. Others speculated that they transform into different species. Some thought that Barnacle Geese Branta leucopsis grow on trees. One Havard scientist even insisted that birds fly to the moon!
Ornithology has come a long way since then and we now know a good deal about bird migration. When it comes to Africa’s birds, we know a lot about those that move between Eurasia and Africa (the Palearctic migrants). Major routes, wintering grounds and timing of movements have been documented quite well.
We still know comparatively little about the birds that migrate within Africa; the intra-African migrants (but we definitely know they don't go to the moon!)
Bird migration in the eastern hemisphere. Source: National Geographic Society
Why do birds migrate?
Migration is driven by seasonal changes in the environment. Migratory birds move back and forth every year between 'breeding grounds' and 'wintering grounds'. The breeding grounds are very productive, but only at a certain time of year. Wintering grounds are places where they can survive when life gets hard in their breeding grounds. These movements are quite predictable and happen around the same time annually.
Birds that breed in areas where conditions are quite stable year-round are generally sedentary as they have no need to migrate. In these areas, there are no extreme changes in food availability, temperature and other environmental factors that would otherwise cause them to migrate.
The more extreme the seasonal changes, the greater the need to migrate. This is why almost all birds that breed in the arctic are migrants while many that breed along the equator do not migrate.
Little Stint Calidris minuta. A migratory bird that breeds in the high Arctic and winters in Africa.
Hinde's Babbler Turdoides hindei a non-migratory bird of the equator. Endemic to central Kenya.
The African context
Bird migrations within Africa are on a less dramatic scale than those epic inter-continental journeys that the Palearctic birds undertake. Palearctic migrants have to cross vast tracts of inhospitable land or sea, like the Sahara and Arabian deserts, the Mediterranean Sea and the Indian Ocean. In addition to this already risky task, they also have to avoid getting trapped, shot and eaten by people along the way, especially in the Mediterranean region.
Intra-African migrants do not face quite the same problems, although they have their own set of challenges to overcome.
Wahlberg's Eagle Hieraaetus wahlbergi. An intra-African migrant that mainly breeds south of the equator in East and Southern Africa and winters in the Sahel zone of northern Africa. This species' movements are still not fully understood and some birds may be resident in some areas.
Palearctic migration is mainly driven by by changes in temperature and day length. In Africa, these two things do not change very much throughout the year. Intra-African migrations are instead driven mainly by the seasonal switch between wet and dry periods. This is caused by the annual movements of the Inter-tropical Convergence Zone (ITCZ).
The ITCZ is a low-pressure atmospheric belt that oscillates annually between the Tropic of Cancer (northern tropic) and Tropic of Capricon (southern tropic), and is the major driver of rainfall patterns in sub-Saharan Africa. Basically, the rain follows the ITCZ. In July it is in the north and in January it's in the south. For this reason, East Africa experiences two rainy seasons (the ITCZ passes over us twice), while most of the rest of Africa only has one rainy season. You could say that East Africa has two summers!
(Here's a compilation of the best available books on the birds of Africa: Book Recommendations)
Different types of migration in Africa
The interesting thing about Africa's migratory birds is that most of them are only 'partial migrants'. Some populations migrate while others don't. The general trend is for the populations breeding along the equator to be sedentary while those in the northern and southern tropics are migratory.
Examples of this are birds like the White-crested Helmet-shrike Prionops plumatus, African Pygmy Kingfisher Ispidina pictus, Diederik Cuckoo Chrysococcyx caprius and Black Kite Milvus migrans (this one actually has both Palearctic and African races, which some people think should be split into different species).
Apart from the several species of migrants and partial migrants in Africa, there are also birds that are simply nomadic. This means their movements are not predictable or regular. They move randomly in response to environmental conditions like food availability and rainfall. Birds in this category are generally those that depend on food that is erratic in nature, hard to predict and prone to "boom and bust".
Examples are birds like the Black-shouldered Kite Elanus caeruleus and Secretarybird Sagittarius serpentarius (which feed on rodents and large insects like locusts). These birds may be more sedentary in some places than others, depending on the local conditions.